Selina Chemistry Class 7 Solutions

Selina Concise Chemistry Class 7 ICSE Solutions – Physical and Chemical Changes

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Selina Concise Chemistry Class 7 ICSE Solutions – Physical and Chemical Changes

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 2 Physical and Chemical Changes

Points to Remember :

  1. All changes are classified into two types
    (i) Physical change
    (ii) Chemical change.
  2. Physical Change— A physical change is a temporary change in which no new substance is formed and chemical composition remains same. e.g. Melting of ice.
  3. Chemical Change— A chemical change is a permanent change, in which a new substances are formed whose chemical composition and physical properties are different, e.g. Burning of a candle.
  4. Chemical Reaction— Any chemical change in matter involving its transformation into one or more new substances is called a chemical reaction.
  5. Chemical Equations— A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction.
  6. The substances that react with one another are called reactants, and the new substances thus formed are called products.
  7. A balanced chemical reaction is one in which the number of the atoms of each element on the
  8. reactant side is equal to the number of atoms of that element on the product side.
  9. The law of conservation of mass states that mass can neither be created nor be destroyed.
  10. A chemical equation gives both qualitative and quantitative information about reactants aftd products.
  11. The type of chemical reaction in which two substances combine to form a new substance is known as combination reaction.
  12. The type of chemical reaction in which a substance breaks up on heating to form two or more simpler substances,
    which can be either elements or compounds, known as decomposition reaction.

Exercise

Question 1.
(a) Define:
(i) a physical change
(ii) a chemical change
(b) Give two examples for each of the above two changes.
Answer:
(a) (i) Physical chagne : A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.
(ii) Chemical change : A permanent change in which new substances are formed whose composition and properties are completely different from those of the original substances.
(b) Physical change:
(i) Change of water into its vapours.
(ii) Heating of iron rod.
Chemical change:
(i) Burning of wood.
(ii) Breathing.

Question 2.
What are reversible and irreversible changes ? Give one example for each.
Answer:

  1. Reversible change : When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.
    Example : Melting of ghee or wax.
  2. Irreversible change : When a substance can not be brought back to its original state after a change, it is said to be an irreversible change.
    Example : Souring of milk.

Question 3.
Mention a change which is always
Answer:

  1. Desirable : Changes that are useful to man are desirable changes, e.g. change of milk into curd.
  2. Undesirable : Change that brings about destruction is an undesirable change, e.g. floods and epidemics are undesirable changes.
  3. Periodic : Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.

Question 4.
Is burning a physical change or a chemical change? Why?
Answer:
Burning is a chemical change as new substance is formed with new properties and it cannot be reversed.

Question 5.
A burning candle shows both physical and chemical changes. Explain ?
Answer:
A burning candle produces wax vapours which solidify and form wax again is physical change. At the same time wax on burning produces water vapours and carbondioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change.
Hence burning of candle shows both physical and chemcial changes.

Question 6.
State three differences between evaporation and boiling
Answer:

Evaporation Boiling
  1. Evaporation is a slow process.
  2. Evaporation takes place from the surface of the liquid.
  3. Evaporation takes place at all temperature.
  1. Boiling is a fast process.
  2. Bgiling takes place from all parts of the liquid.
  3. Boiling takes place at a fixed temperature on heating.

Question 7.
State four differences between physical and chemical changes.
Answer:
Four differences are :

Physical change Chemical change
  1. No new substances with new properties are formed.
  2. It can be reversed by simple physical methods.
  3. It is temporary change.
  4. Energy is neither needed nor produced.
  1. New substances with new properties are formed.
  2. Change cannot be reversed by simple physical methods.
  3. It is permanent change.
  4. Energy is either needed or evolved.

Question 8.
What do you observe when :

  1. water is boiled
    On boiling water changes into steam (gas) physical change.
  2. a piece of paper is burnt
    On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.
  3. some ice cubes are kept in a glass tumbler
    Ice cubes (solid) turn into water (liquid) only state changes (physical change).
  4. solid ammonium chloride is heated
    Solid ammonium chloride on heating changes into vapours (change of state) is physical change.
  5. an iron nail is kept in tap water for few days
    We observe reddish brown coating on the nail called rust (entirely new substance) is chemical change.
  6. a spoon of sugar is heated in a pan
    When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change.
  7. lighted match stick is brought near the mouth of the test tube containing hydrogen gas.
    We observe that hydrogen bums at the mouth of test tube with blue flame and pop sound is heard. It is chemical change.
  8. quick lime is dissolved in water.
    The following two observations will be observed
    (i) A hissing sound is observed.
    (ii) The mixture starts boiling and lime water is obtained.
  9.  liittle ammount of curd is added to a bowl containing warm milk and kept for five hours.
    When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured.
    The milk will change to curd. On boiling water changes into steam (gas) physical change.

Question 9.
Name a chemical change which takes place in presence of:
Answer:
Heat:

  1. Burning of paper.
  2. Cooking of food need heat.

Light :

  1. Formation of food by plants i.e. photosynthesis need light.
  2. Hydrogen and chlorine react in presence of light

Electricity:

  1. Water breaks into hydrogen and oxygen on passing electricity.
  2. Sodium chloride solution breaks up into its components sodium and Chlorine on passing electric current through sodium chloride solution.

Objective Type Questions

Question 1.
Fill in the blanks.

  1. A new substance is always formed in a chemical change.
  2. Melting of ice is a physical change.
  3. When a candle burns, wax melts. Melting of wax is a physical change.
  4. Chemical change occurs as a result of rearrangement of atoms between two substances.
  5. Burning of a fuel is a chemical change.
  6. Revolution of the earth around the sun is a natural periodic change.
  7. Growing of a seedling into a plant is chemical change.

Question 2.
State whether the following are physical or chemical changes.

  1. glowing of a bulb — physical change,
  2. burning of sugar — chemical change,
  3. heating of water — physical change,
  4. growing of a piglet into a pig — chemical change,
  5. burning of wood — chemical change,
  6. passing electric current through a heater rod — physical change.
  7. water cycle in nature — physical change,
  8. respiration in living beings — chemical change,
  9. shaping a piece of glass — physical change,
  10. lightning — chemical change,
  11. (energy produced) dissolving sugar in water — physical change,
  12. heating a mixture of iron filings and sulphur—chemical change.
  13. mixing oil with water — physical change.
  14. cutting wood into small pieces — physical change,
  15. photosynthesis — chemical change.
  16. Digestion of food — chemical change.
  17. Melting of wax — physical change.
  18. Boiling of an egg — chemical change.
  19. Slaking of lime — chemical change.

Question 3.
Match the following:

Physical and Chemical changes 1
Physical and Chemical changes 3
Answer:
Physical and Chemical changes 4

Question 4.
Write true or false against the following statements :

  1. Cutting of paper into pieces is a chemical change.
    False
  2. Rusting of iron is a chemical change.
    True
  3. Earthquake is a desirable change.
    False
  4. Melting of ice is a physical change.
    True
  5. Burning of sugar is a temporary change.
    False

Multiple Choice Questions

Tick (√) the correct alternative from the choice given for the following statements :
Question 1.
A substance which can not sublime is
Answer:

  1. iodine
  2. camphor
  3. sugar
  4. dry ice

Question 2.
When you put some ice cubes in a glass, droplets of water are formed on the outerwall of the glass. This explains the phenomenon of
Answer:

  1. melting
  2. freezing
  3. condensation
  4. evaporation

Question 3.
Burning is a
Answer:

  1. slow process
  2. fast process
  3. natural process
  4. none of the above

Question 4.
Which one of the following is volatile in nature ?
Answer:

  1. common salt
  2. petrol
  3. water
  4. milk

Question 5.
An example of both physical and chemical change is
Answer:

  1. burning of candle
  2. melting of ice
  3. cooking of food
  4. blowing of bulb

Question 6.
The compound rust is a hydrated oxide of
Answer:

  1. copper
  2. aluminium
  3. iron
  4. gold

Question 7.
When sugar is heated, its colour changes into
Answer:

  1. red
  2. brown
  3. black
  4. grey

Question 8.
A pop sound is heard when a lighted match stick is brought
near the mouth of a jar. This indicates the release of gas.
Answer:

  1. oxygen
  2. hydrogen
  3. nitrogen
  4. water-vapour

Question 9.
When we add water to the following substances, which one will show a chemical change ?
Answer:

  1. salt
  2. sugar
  3. oild
  4. quick lime

Selina Concise Chemistry Class 7 ICSE Solutions – Matter and Its Composition

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Selina Concise Chemistry Class 7 ICSE Solutions – Matter and Its Composition

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 1 Matter and Its Composition

Points to Remember :

  1. Matter has mass and occupies space.
  2. Matter is made up of atoms and molecules.
  3. Atoms are the smallest particles of matter which may or may not have independent existence.
  4. Molecules are capable of independent existence. They are made up of atoms of same kind or different kinds.
  5. The atoms and molecules are in random motion.
  6. There are gaps between the molecules of matter called as intermolecular space.
  7. There exists a force of attraction between the molecules known as intermolecular force of attraction.
  8. Matter exists in three states : solids, liquid and gas.
  9. Matter can change from one state to another on changing temperature and pressure.
  10. The change of state of a matter from one form into another is called interconversion of states of matter.

EXERCISE

Question 1.
Define matter.
Answer:
Anything that has mass and occupies space is called matter.

Question 2.
What is the difference between mass and weight.
Answer:
Mass is the “quantity of matter” and weight is “the force with which the earth pulls a body towards itself’. The mass of a body does not change but its weight changes from place to place.

Question 3.
If an object weighs 6 N on earth what will be its weight on moon. What will be the change in its mass?
Answer:
Weight of body on moon = \(\frac { 1 }{ 6 }\)th of its weight on earth.
∴ Body will weigh \(\frac { 1 }{ 6 }\) of 6 = \(\frac { 1 }{ 6 }\) x 6 = 1 N on moon
Mass of a body does not change with change in gravity. So mass of a body will remain the same on moon.

Question 4.
Write your observation and conclusion for the following:
(a) When few marbles are put in a glass half filled with water.
(b) Ice is kept at room temperature.
Answer:
(a) Take some marbles and put them into the water of glass tumbler one by one. After some time you will notice that water level crosses the mark and rises. This is because the marbles occupy space. Again weigh the glass with the marbles. You will find that the second mass is greater than the first one. This proves that, marbles have mass.
Selina Concise Chemistry Class 7 ICSE Solutions - Matter and Its Composition-img 4
This proves that, matter has mass and occupies space.
(b) Ice when kept at room temperature again changes back into liquid water.

Question 5.
State three main characteristics of the particles of matter.
Answer:
Characteristics of Matter

  1. It can neither be created nor destroyed.
  2. It is composed of a particular material which can either be Homogeneous or Heterogeneous.
  3. Matter has, volume, mass and weight as per their state.

Question 6.
Differentiate between an atom and a molecule.
Answer:

Atom Molecule
  1. It is the smallest part of an element.
  2. It does not have independent existence.
  1. It is the smallest part of a compound.
  2. It has an independent existence.

 Question 7.
Define :
(a) Solid
(b) Liquid
(c) Gas

Answer:
Giving two examples of each type.
(a) Solid : A solid is that state of matter which has a fixed shape, mass and volume. It suffers very small changes in volume by changing the temperature. It can not be compressed,
e.g. – Sand, Wood, Copper, Ice, etc.
(b) Liquid : It has a definite mass and volume but lacks a shape of its own. It takes up the shape of the containing vessels. It can be compressed to an extents,
e.g. – Milk, water, ink, etc.
(c) Gas : It is a state of matter which has only definite mass but no definite shape and volume. It takes up the shape of the container
e.g. – Carbon dioxide, oxygen, etc.

Question 8.
Why are liquids and gases called as fluids.
Answer:
The particles are free to move in any direction i.e. they can
flow because all substances that can flow are called fluids.
Liquids and gases are fluids.

Question 9.
(a) Define interconversion of states of matter.
(b) Why do solids, liquids and gases differ in their physical state?
(c) Under what conditions do solids, liquids and gases change their state.
Answer:
(a) The process by which matter changes from one state to another and back to original state, without any change in its chemical composition.
(b)Intermolecular force of attraction.
Intermolecular spaces are two important properties of matter that account for the different states of matter.
(c) Matter can change from one state to another on changing temperature and pressure.

Question 10.
Give reasons :
(a) When a stone is dipped in a glass containning some water the level of water rises but when a spoon of sugar is added to it and stired, the water level does not rise?
(b) A drop of ink added to water in a glass turns whole water blue.
Answer:
(a) Take half a glass of water. Dip a spoon in it. What do you observe? The water level rises, indicating that spoon occupies space.
Selina Concise Chemistry Class 7 ICSE Solutions - Matter and Its Composition-img 10
Now remove the spoon, water comes down to its original level. Now add a spoon of sugar to it and stir well. The sugar disappears but the level of water in the glass does not rise, that means the volume of water has not increased. But where did the sugar particles disappear?
The sugar particles being smaller get adjusted between the water molecules. This shows that there are intermolecular space in water.
(b) This is because tfie water as well as ink particles (molecules) are in continuous random motion. Due to motion, the blue coloured particles of the ink spreads all over and give blue colour to the water.

Question 11.
Fill in the blanks :

(a) Air is a matter because it has weight, mass and space and it can be compressed.
(b) The molecules are made up of atoms.
(c) The quantity of matter in an object is called its mass.
(d) The state of matter with definite volume and definite shape is called solid.
(e) The substances which can flow are called fluids.

Question 12.
Name the terms for the following :
(a) The change of a solid into liquid.
(b) The force of attraction between the molecules of matter.
(c) The particles of matter which may or may not have independent existence.
(d) The process due to which a solid directly changes into its vapours.
(e) The change of vapour into a liquid.
Answer:
(a) Melting.
(b) Intermolecular force of attraction.
(c) Solid.
(d) Sublimation.
(e) Condensation.

Question 13.
Classify the following into solid, liquid and gas :
Coal, kerosene, wood, oxygen, sugar, blood, water vapour, milk, wax.
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Matter and Its Composition-img 13

 

Selina Concise Chemistry Class 7 ICSE Solutions – Elements, Compounds and Mixtures

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Selina Concise Chemistry Class 7 ICSE Solutions – Elements, Compounds and Mixtures

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 3 Elements, Compounds and Mixtures

Points to Remember :

  1. Every substance is made up of very tiny particles, called molecules. Molecules are formed from even smaller particles called atoms.
  2. Element— (a) Element is the simplest pure substance. It cannot be divided further into simpler substances by any chemical method, e.g. oxygen, hydrogen, sulphur, etc.
    (b) At present 116 elements are known, of which 92 are natural elements.
  3. Based on their properties, elements are classified into : metals, non-metals, metalloids, noble gases.
  4. Metals are ductile, malleable, good conductors of heat and electricity, high melting and boiling points. Metals are sonorous, e.g., Iron, Gold, Silver, etc.
  5. Non-metals are solids and brittle in nature, bad conductor of heat and electricity (exception Graphite) low melting and boiling points, e.g. sulphur, carbon, hydrogen, etc.
  6. Metalloids— These elements show properties of both metals and non-metals. They are hard solids, e.g. Boron, Silicon, Arsenic.
  7. Inert or noble gases— These elements do not react chemically with other elements or compounds are called noble (Inert) gases, e.g., helium, neon, argon, etc.
  8. Symbols of Elements— Each element is denoted by a symbol usually to first letter.
    Examples : Oxygen by O Hydrogen by H.
  9. Atom— “An Atom is the smallest particle of an element that can take part in a chemical reaction but may or may not have independent existence.”
    The atom of an element exhibits all the properties of that element.
  10. Molecule— A molecule is the smallest particle of a pure substance of element or compound which has independent existence. It exhibits all the properties of pure substance.
  11. Atomicity— The number of atoms of an element that join together to form a molecule of that element is known as the atomicity.
  12. Molecular Formula— of an element is the symbolic representation of its molecule. It indicates the number of atoms present in it. e.g. Magnesium oxide – MgO.

EXERCISE – I

Question 1.
Write the symbols of helium, silver, krypton, antimony, barium.
Answer:

Element                   Symbol
Helium                         He
Silver                            Ag
Krypton                       Kr
Antimony                    Sb
Barium                        Ba

Question 2.
Write the names of following elements Na, C, Kr, U, Ra, Fe, Co.
Answer:

Symbol           Element
Na                        Sodium
C                          Carbon
Kr                        Krypton
U                         Uranium
Ra                        Radium
Fe                           Iron
Co                        Cobalt

Question 3.
Define :

  1. Elements : An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions.
  2. Compounds : A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.

Question 4.
Name the main metal present in the following :
Answer:

(a) Haemoglobin                                    Iron
(b) Chalk                                               Calcium
(c) Chlorophyll                                   Magnesium
(d) Chocolate wrappers                    Aluminium

Question 5.
Give four examples of non-metallic elements.
Answer:
Examples : Hydrogen, oxygen, nitrogen, carbon, chlorine, sulphur, phosphorus, etc.

Question 6.
What do you understand by :
Answer:

  1. Metalloids : Metalloids are those substances which have some properties of metals and some of non-metals e.g. boron, silicon.
  2. Noble gases : Noble gases are those which do not react chemically with other elements or compounds e.g. helium, neon, etc.

Question 7.
Select elements and compounds from the following list: Iron, plaster of paris, chalk, common salt, copper, aluminium, calcium oxide, cane sugar, carbon, silica, sodium sulphate, uranium, potassium carbonate, silver, carbon dioxide.
Answer:
Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 3 Elements, Compounds and Mixtures-7

EXERCISE – II

Question 1.
State four difference between compounds and mixtures.
Answer:

Compound Mixture
1. A compound is a pure substance. 1. A mixture is an impure substance.
2. Compounds are always homogeneous. 2. Mixtures may be homogeneous or heterogeneous.
3. A compound has a fixed composition, i.e., it is formed when two or more pure substances chemically combine in a definite ratio by mass. 3. A mixture has no fixed composition, i.e., it is formed by mixing two or more substances in any ratio without any chemical reaction.
4. Formation of a compound involves change in energy. 4. Formation of a mixture does not involve any change in energy.
5. Compounds have specific set of properties. 5. Mixtures do not have any specific set of properties.
6. Components of compounds can be separated only by complex chemical processes. 6. Components of mixtures can be separated by simple physical methods.

 Question 2.
What are the characteristic properties of a pure substance? Why do we need them?
Answer:
Pure substance : Pure substances have a definite set of properties such as boiling point, melting point, density, etc. They are all homogeneous i.e., their composition is uniform throughout the bulk. Both elements and compounds are pure substances.
Pure substances are needed to :

  1. Manufacture medicines.
  2. To prepare chemicals in industry.
  3. For scientific purposes.
  4. To maintain the good health of human beings.

Question 3.
Give two examples for each of the following :
(a) Solid + Solid mixture
(b) Solid + Liquid mixture
(c) Liquid + Liquid mixture
Answer:
(a) Solid + Solid mixture :Sand and sugar,

  • Sand and stone,
  • sand and sugar.

(b) Solid + Liquid mixture :

  • Sand and water,
  • Charcoal and water.

(c) Liquid + Liquid mixture :

  • Oil in water,
  • Alcohol and water.

Question 4.
Define :

  1. Evaporation : Is the process ~of converting a liquid into its vapours state either by exposing it to air or by heating.
  2. Filtration : The process of separating solid particles from liquid by allowing it to pass through a filter paper is called filtration.
  3. Sublimation : The process in which a solid changes directly into its vapours on heating is called sublimation.
  4. Distillation : Distillation is the method of getting a pure liquid from a solution by evaporating and then condensing the vapours.
  5. Miscible liquids : Homogeneous liquid-liquid mixtures are called miscible liquids.
  6. Immiscible liquids : Heterogeneous liquid-liquid mixtures are called immiscible liquids.

Question 5.
Name the process by which the components of following mixtures can be separated.

  1. Iron and sulphur
  2. Ammonium chloride and sand
  3. Common salt from sea water
  4. Chaff and grain
  5. Water and mustard oil
  6. Sugar and water
  7. Cream from milk

Answer:

  1. Magnetic separation.
  2. Sublimation.
  3. Evaporation.
  4. Winnowing separates chaff (lighter) from heavier grains in two different heaps.
  5. Mustard oil and water is liquid-liquid immiscible mixture and is separated by separating funnel. Water being the heavier forms the lower layer.
  6. By evaporation in this process of converting a liquid into its vapour state by heating. Liquid is heated and water evaporate and sugar is obtained.
  7. Centrifugation.

Question 6.
How will you separate a mixture of common salt, chalk powder and powdered camphor? Explain.
Answer:
Comphor with sublimation. Chalk powder by Alteration then the residual left is common salt.

Question 7.
How is distillation more advantageous than evaporation?
Answer:
The advantage of distillation is that both components of the
solid and liquid mixture are obtained. Whereas in evaporation only solid is obtained.

Question 8.

  1. What is chromatography?
  2. Why is it named so?
  3. What are the advantages of chromatography?
  4. Name the simplest type of chromatography?
  5. On what principle is this method based?
  6. What is meant by stationary phase and mobile phase in chromatography?

Answer:

  1. The process of separating different dissolved constituents of a mixture by their absorption on an appropriate material is called chromatography.
  2. It is named so, because earlier it was used to separate mixtures containing coloured components only but these days this technique is applied to colourless substances too.
  3. Advantages of chromatography :
    (i) A very small quantity of the substance can be separated.
    (ii) Components with very similar physical and chemical properties can be separated.
    (iii) It identifies the different constituents of a mixture.
    (iv) lt also helps in quantitative estimation of components of a mixture.
  4. The simplest type of chromatography is “Paper chromatography”.
  5. Chromotography is based on differential affinities of compounds towards two phases i.e. stationary and mobile phase.
  6. The filter paper acts as “stationary phase” while the solvent act as “mobile phase”.

Question 9.
On what principle are the following methods of separation based? Give one example of a mixture for each of the methods mentioned in which they are used
Answer:

  1. Sublimation : Change of solid into vapours directly on heating and change of vapours into solid again on
    Example : Salt from ammonium chloride.
  2. Filtration : The process of separating insoluble solid particles from a liquid by allowing it to pass through a filter is called Alteration. These filters allow liquids to pass through them but not solids. The insoluble solid left on the filter is called the residue, while the liquid which passes through the filter is called the filtrate. Mixtures like chalk and water, clay and water, tea and tea leaves, sawdust and water, etc., are separated by this method.
  3. Sedimentation and decantation : The settling down of suspended, insoluble, heavy, solid particles in a solid- liquid mixture when left undistrubed is called sedimentation.
    The solid which settles at the bottom is called sediment while the clear liquid above it is called supernatant liquid.
    The process of pouring out the clear liquid, without disturbing the sediment, is called decantation.
    Example : A mixture of sand and water.
  4. Solvent extraction method : This method is used when one of the solid components is soluble in a liquid.
    Example : A mixture of sand and salt can be separated by this method. Salt gets dissolved in water while sand settles down in the container. The salt solution is then decanted. Salt is separated from the solution by evaporation. In this way, they can be separated.
  5. Magnetic separation : This method is used when one of the components of the mixture is iron. Iron gets attracted towards a magnet and hence can be separated. Mixtures of iron and sulphur, iron and sand, etc., can be separated by moving a magnet over them. Iron gets attached to the magnet and is separated.
  6. By using a separating funnel : It is a simple device used to separate the components of a liquid-liquid heterogeneous mixture.
    Example : Kerosene oil and water. The mixture is placed in a separating funnel and allowed to stand for sometime. The components form two clear layers. Water being heavier forms the lower layer and oil being lighter forms the upper layer. When the stopper of the funnel is opened, the heavier liquid trickles out slowly and is collected in a vessel. The stopper is closed when the bottom layer is entirely removed the funnel. In this way, the two liquids are separated.
  7. Fractional distillation : The process of distillation is used for separating the components of a homogeneous liquid-liquid mixture, like water and alcohol. This is based on the fact that alcohol boils at a lower temperature than water. The vapour of alcohol are collected and cooled while water is left behind in the original vessel. Thus, two liquids having different boiling points can be separated by distillation provided that difference in their boiling points must be 25 °C or more.

OBJECTIVE TYPE QUESTIONS

Question 1.
Fill in the blanks:
Answer:

  1. Elements are made up of same kind of atoms.
  2. Elements and compounds are pure substances.
  3. In a mixture the substances are not combined chemically.
  4. Clay is separated from water by the method called loading and decantation.
  5. Crystallisation is a process to obtain a very pure form of a solid dissolved in a liquid.
  6. Camphor and ammonium chloride can sublimate.

Question 2.
Give one word answers for the following :
Answer:

  1. The solid particles which remain on the filter paper after the filtration residue.
  2. The liquid which evaporates and then condenses during the process of distillation distillate.
  3. The process of transferring the clean liquid after the solid settles at the bottom of the container decantation.
  4. The process by which two miscible liquids are separated fractional distillation.

MULTIPLE CHOICE QUESTIONS

Select the correct alterative from the choices given for the following statements:
Question 1.
A pure liquid is obtained from a solution by :
Answer:

  1. evaporation
  2. distillation
  3. Alteration
  4.  crystallisation

Question 2.
Components of crude petroleum can be separated by :
Answer:

  1. distillation
  2. evaporation
  3. filtration
  4. fractional distillation

Question 3.
Example of a homogeneous mixture is :
Answer:

  1. tap water 
  2. distilled water
  3.  sand and water
  4. water and oil

Question 4.
In chromatography the filter paper is :
Answer:

  1.  stationary phase 
  2. mobile phase
  3. mixture
  4. none of the above

Question 5.
A set of mixture is :
Answer:

  1.  ink, honey, icecream, milk
  2. tapwater, gold, common salt, alloy
  3.  milk, brass, silver, honey
  4. butter, petroleum, tapwater, iron

 

Selina Concise Chemistry Class 7 ICSE Solutions – Atoms, Molecules and Radicals

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Selina Concise Chemistry Class 7 ICSE Solutions – Atoms, Molecules and Radicals

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 4 Atoms, Molecules and Radicals

Points to Remember :

  1. Atoms of the most elements are composed of still smaller particles known as fundamental particles or subatomic particles. They are protons, electrons and neutrons.
  2. Protons are the positively charged particles present in an atom.
  3. Electrons are the negatively charged particles present in an atom and its mass is 1/1837 the mass of one hydrogen atom.
  4. Neutrons are particles with no electrical charge. Its mass is equal to one atomic mass unit.
  5. The central part of an atom is called nucleus, which contains both protons and neutrons.
  6. Shells or orbits around the nucleus.
  7. Atomic number refers to the number of protons present in an atom. It is denoted by the alphabet Z.
  8. Mass number— Mass number is the sum of number of protons and neutrons present in the nucleus of an atom. Mass number (A) = Number of protons + Number of neutrons.
  9. Atomic mass— The mass of an atom is known as its atomic mass.
  10. Relative atomic mass ip the mass of an atom of an element as a multiple of the standard atomic mass unit.
  11. Atoms of different elements combine to form molecules so as to attain chemical stability.
  12. A positively charged ion is called a cation and a negatively charged ion is called anion.
  13. The valency of an element is the number of electrons donated or accepted by its ‘atom’ during chemical combination.
  14. There are some elements with more than one valency e.g., iron, copper, tin, lead.
  15. Two or more different non-metals that collectively accept or donate one or more electrons and become negatively or positively charged in the process are called radicals.

EXERCISE

Question 1.
Define the following terms :
Answer:

  1. Atom : An atom is the smallest indivisible unit of an
    OR
    Atom is the smallest unit of matter.
  2. Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.
  3. Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.
  4. Valency : It is the number of electrons donated or accepted by the valence shell of an atom during chemical combination.
  5. Periodic table represents the tabular arrangment of elements in horizontal rows called periods and vertical columns called groups in order to classify the elements and their systematic study.

Question 2.
Write the names of the elements present in the following compounds.
Answer:

  1. Common salt : Sodium, chlorine.
  2. Ammonia : Nitrogen, hydrogen.
  3. Sulphuric acid : Hydrogen, sulphur, oxygen.
  4. Glucose : Carbon, hydrogen, oxygen.
  5. Sodium hydroxide : Sodium, oxygen, hydrogen.
  6. Acetic acid : Carbon, hydrogen, oxygen.

Question 3.
What does each of the following represent ?
Answer:

  1. 2C02 = 2 molecules of carbon dioxide.
  2. 2H2S = 2 molecules of hydrogen sulphide.
  3. 5H2S04 = 5 molecules of sulphuric acid.
  4. 6NaNO3 = 6 molecules of sodium nitrate.

Question 4.
Write the symbols and valencies of the following radicals:
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 4
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 4.
Question 5.
Name the following radicals :
Answer:

  1. SO42- = Sulphate
  2. HC03– = Bicarbonate
  3. OH- = Hydroxide
  4. Cr2072-  = Dichromate

Question 6.

  1. Name one ion for  each of the valencies +1, +2  and +3.
  2. Name one ion for  each of the valencies-1, -2  and -3.

Answer:

  1. +1 = Sodium Na+
    +2 = Calcium Ca+2
    +3 = Aluminium Al+3
  2. -1 = Chlorine Cl-
    -2 = Oxygen 0-2
    -3 = Nitrogen N3_

Question 7.
The valency of calcium is 2. Write the valencies of other radical in the following :

  1. CaO
  2. Ca(OH)2
  3. CaC03
  4. CaCl2

Answer:

  1. O= 2
  2. OH = 1
  3. CO3 = 2
  4. Cl = l

Question 8.
Write the names of the following compounds :
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 8

Question 9.
Write the molecular formulae of:
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 9

Question 10.
The valency of sodium is one, write the molecular formula for the following compounds of sodium.

  1. sodium oxide : Na20
  2. sodium sulphate : Na2S04
  3. sodium carbonate : Na2CO3
  4. sodium hydroxide : NaOH
  5. sodium nitrate : NaN03

 

Question 11.
What is variable valency ? Give two examples of elements showing variable valency.
Answer:
There are some elements with more than one valency. They are said to have variable valency, e.g. Iron, copper.

Question 12.
Give the group number of following elements present in periodic table

  1. Magnesium : IIA
  2. Carbon : IVA
  3. Sulphur : VIA
  4. Neon : Zero

 

Question 13.
An element belongs to group VA. What would be its valency? Name two such elements.
Answer:
Elements of group VA has valency 3.
Two elements : Nitrogen and phosphorus.

Question 14.
An element belongs to group II. What would be its valency? Write the formula of molcules of compounds it will form with elements in VA, VIA and VIIA groups.
Answer:
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 14
Selina Concise Chemistry Class 7 ICSE Solutions - Atoms, Molecules and Radicals- 14 a

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
Answer:

  1. Atoms are neutral.
  2. An ion with positive charge is called cation.
  3. An ion with negative charge is called anion.
  4. 2H2 means two atoms of hydrogen.
  5. Ozone is a triatomic molecule.
  6. Metals have variable valency.
  7. Chemical name of caustic soda is sodium hydroxide NaOH.

2. Tick (√) the correct answer.

(a) The valency of iron in Fe203 is

  1. 1
  2. 2
  3. 3
  4. 6

(b) Which of the following has valency 4 ?

  1. aluminium
  2. oxygen
  3. carbon
  4. phosphorus

(c) The sulphate radical is written as S042-. What is the formula of calcium sulphate ?

  1. Ca(S04)2
  2. Ca2(S04)
  3. Ca(S04)3
  4. CaS04

(d) Which of the following exhibit variable valency ?

  1. calcium
  2. copper
  3. carbon
  4. chlorine

3. State the term for the following:

  1. The number of atoms present in a molecule of an element atomicity.
  2. The symbolic representation of a molecule molecular formula.
  3.  A group of atoms that react as a single unit molecule.
  4. The combining capacity of an element valency.
  5. The tabular arrangement of elements in horizontal rows and vertical columns periodic table.

Selina Concise Chemistry Class 7 ICSE Solutions – Metals and Non-metals

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Selina Concise Chemistry Class 7 ICSE Solutions – Metals and Non-metals

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 6 Metals and Non-metals

Points to Remember :

  1. Knowledge of chemistry plays a vital role in the development of human society and civilization.
  2. Metals are known to man from ancient times. Metals are used to make our life comfortable.
  3. Non-metals form another class of elements, e.g. hydrogen, oxygen, carbon, etc. They are used for various purposes.
  4. Alloys are homogeneous solid mixtures containing two or more metals e.g. steel, brass, bronze.
  5. Common salt, hydrocloric acid, carbohydrates, fats, proteins, vitamins, occur naturally and can also be prepared artificially.
  6. Fertilizers are artificially prepared substances, which are necessary for the proper growth of crops.
  7. There are a number of man-made materials that are used in our daily life for various purposes, e.g., cement, plaster of pairs, plastics.
  8. Medicines are used to cure diseases.
  9. Solution is a homogenous mixture of solute and solvent.
  10. Soda water is prepared by dissolving carbon dioxide in water under high pressure.
  11. Syrup is a highly concentrated sugar solutions. It contains a specific flaviour.

EXERCISE

1. Name a metal

  1. that is most malleable : Pure gold
  2. that is brittle : Zinc
  3. as precious as gold : Platinum
  4. that can be cut with knife : Sodium
  5. used in making electric cables : Copper
  6. used as a thermometric liquid : Mercury
  7. that is the best conductor of electricity : Silver

2. Name a non-metal that is :

  1. a good conductor of heat and electricity : Graphite (Carbon)
  2. hardest naturally occurring substance : Diamond (Carbon)
  3. used to kill germs in water : Chlorine
  4. lustrous : Iodine
  5. used for filling into electric bulbs : Argon
  6. used for cancer therapy : Radon
  7. liquid at room temperature : Bromine

3. Mention two uses of the following metals and non-metals

(a) Iron :
It is used to make pipes, tanks, railing, etc.
It is used in the construction of power transmission towers.

(b) Aluminium :
It is used to make electric wires.
It is used to make utensils, cans, window fram’es, etc.

(c) Gold :
It is used for making ornaments and coins.
It is used in the manufacture of electronic devices like computers, telephones, home appliances, etc.

(d) Oxygen :
It is used by all living beings for breathing.
It is important for combustion.

(e) Iodine :
It is used in photographic films in the form of potassium iodide.
It is added to salt to make it iodized salt which is necessary for the growth of human body.

4. Give reasons :

(a) Magnesium is used in fire works.
Ans : Magnesium is used in fire works because it bums with a dazzling light.

(b) Aluminium is used in making aircrafts.
Ans : Aluminium is used in making aircrafts because it is light and strong. It is mixed with other metals to make it stronger.

(c) Copper is used in making electric cables.
Ans : Copper is ductile and a very good conductor of heat and electricity. This is the reason that copper is used in making electric cables.

(d) Graphite is used in the leads of pencils 
Ans : Graphite turns paper black that is why it is used in the leads of pencils.

(e) Impure diamond is used to cut glass
Ans : Impure diamond is used to cut glass becuase it is the hardest substance and can easily exert force required for cutting.

(f) Gold is mixed with copper and nickel.
Ans :  Pure gold is a very soft metal. It cannot be moulded into ornaments so it is mixed with copper and nickel so that it becomes harder and bit cheaper also.

(g) Tungsten is used in electric bulbs.
Ans : It is a shiny grey metal, in solid state at room temperature. It can withstand high temperature because it has highest melting point among metals. Hence, it is used in electric bulbs.

5. Name the metals present in the following alloys

  1. Brass— Copper and zinc
  2. Bronze— Copper and tin
  3. Duralumin— Aluminium and copper
  4. Stainless steel— Iron, chromium, nickel

6. Give four differences between metals and non-metals with reference to their
(a) Melting point and boiling point,
(b) Conductivity of heat and electricity,
(c) Malleability
(d) Solubility

            Metals Non-metals
Melting point and boiling point Metals have both high high melting point and boiling point. Non-metals have both low melting and low boiling point.
Conducti­vity of heat and electricity They are good conductors of heat and electricity. Nofi-metals are bad conductors of heat and electricity.

Malleabi­ lity

 Metals are ususally malleable. All non-metal are non- malleable.

Solubility

 Metals are generally insoluble in water and other organic solvents. They are both soluble and insoluble

7. What are metalloids?
Ans : Metalloids are the elements which show some properties of metals and some properties of non-metals. They all are solids. They are silicon, boron, arsenic, antimony, germanium, tellurium and polonium.

8. Give two uses of

(a) Silicon :

  • Highly pure silicon is used in making microchips for computers, transistors, solar cells, rectifiers and other solid state devices that are used extensively in the electronic and present space age industries.
  • It is used in the manufacture of a waterproof material called “silicone”. Silicone is used to make bags, umbrellas, raincoats, etc.
  • It is an important substance present in steel, an alloy of carbon.

(b) Antimony :

  • Antimony is used in electric industry to make semiconductor devices.
  • It is alloyed with lead to improve its hardness and strength and is used in batteries.
  • It is also used in printing presses as type metal.

(c) Tungsten :

  • It is used in making electrodes.
  • It is used in heating elements.
  • It is used as filaments in electric bulbs and cathode ray tubes.

(d) Germanium:

  • Germanium is used as a semiconductor.
  • It is used as a transistor in many electronic applications when mixed with arsenic, gallium, antiomony, etc.
  • Germanium is also used to form alloys and as a phosphor in fluorescent lamps

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks :

(a) The most ductile metal is silver.
(b) A metal stored in kerosene oil is sodium.
(c) Tungsten metal is a poor conductor of heat.
(d) Pure gold is a soft metal.
(e) Silicon carbide is the hardest compound known to us.
(f) A non-metal used to purify water is phosphorus.
(g) A metal that gives dazzling effect to crackers when they explode is magnesium.
(h) A chemical compound that makes up the striking heads of match sticks is sulphur.

2. Match the following :
Selina Concise Chemistry Class 7 ICSE Solutions - Metals and Non-metals-2
3. Write ‘true’ or ‘false’ for the following statements :
(a) Silver is used to make electric cables : False
(b) Iodine acts as an antiseptic in the form of tincture of iodine : True
(c) Sodium can be cut with a knife : True
(d) Antimony is a metal : False
(e) Sand is an oxide of silicon : True

MULTIPLE CHOICE QUESTIONS

1. The noble gas used in advertising signboards is
(a) Helium
(b) Neon
(c) Argon
(d) Krypton

2. A metal with melting point less than 50°C is
(a) Gallium
(b) Iron
(c) Gold
(d) Aluminium

3. A metal which is neither ductile nor malleable is
(a) Copper
(b) Silver
(c) Zinc
(d) Aluminium

4. Rust is a hydrated oxide of iron which is
(a) Reddish brown
(b) Green
(c) White
(d) Black

5. Aluminium is not used to make :
(a) Foils
(b) Wires
(c) Fireworks
(d) Utensils

6. A metalloid used in the manufacture of microchips used in computer is :
(a) Antimony
(b) Germanium
(c) Silicon
(d) Arsenic

7. A metalloid used to make glass :
(a) Sulphur
(b) Germanium
(c) Silicon
(d) Antimony

 

Selina Concise Chemistry Class 7 ICSE Solutions – Air and Atmosphere

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Selina Concise Chemistry Class 7 ICSE Solutions – Air and Atmosphere

ICSE SolutionsSelina ICSE SolutionsML Aggarwal Solutions

APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 7 Chemistry. You can download the Selina Concise Chemistry ICSE Solutions for Class 7 with Free PDF download option. Selina Publishers Concise Chemistry for Class 7 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 7 Air and Atmosphere

Points to Remember :

  1. Air is a mixture of many gases, mainly Nitrogen = 78.1%, Oxygen = 20.9%, Carbon dioxide = 0.03 – 0.04%, Inert gases = 0.9%, [Water vapours, Dust particles and Impurities = Variable].
  2. Nitrogen is a colourless, an odourless and a tasteless gas. It is slightly lighter than air.
  3. The process of conversion of free atmospheric nitrogen into its compounds is called nitrogen-fixation.
  4. Oxygen constitutes about 21% of air by volume. It supports life on earth.
  5. Carbon dioxide is present in air in a very small quantity, e. 0.03 – 0.04%. It is essential for the process of photosynthesis.
  6. Inert gases like neon, argon do not react with any substance.
  7. The harmful substances added to air are called pollutants.
  8. Some pollutants are suspended particles like pollen grains, oxides of sulphur and nitrogen, oxides of carbon, chlorofluorocarbons etc.
  9. Symbol of oxygen = O ; atomic number = 8, relative mass = 16, molecule formula = O2.
  10. Oxygen is available in free and combined state.
  11. A catalyst is a substance that increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.
  12. Oxides are binary compounds formed by the chemical combination of substance with oxygen.
  13. Rusting is the process in which iron slowly reacts with oxygen in the air and produces a flaky brown substance.
  14. Photosynthesis is a process by which C02 and water are used up by green plants in the presence of sunlight to produce glucose and oxygen gases.

A. AIR : A MIXTURE OF GASES

EXERCISE — I

Question 1.
Give one use for each of the following inert gases :
(a) argon
(b) helium
(c) neon
(d) radon
(e) krypton
(f) xenon

Answer:
(a) Argon— Argon is filled into electric bulbs to prevent the oxidation of their filaments.
(b) Helium— It is used in filling up weather observation balloons.
(c) Neon— Neon is used for making advertisement sign boards.
(d) Radon— It is used for treatment of Cancer.
(e) Krypton— It is used in photography.
(f) Xenon— It is also used in photography.

Question 2.
Answer the questions put against each of the following constituents of air :
(a) Nitrogen : Explain its significance for plants and animals.
(b) Oxygen : What is the percentage proportion of oxygen in air ? Why is oxygen called active air.
(c) Carbon dioxide : “Although carbon dioxide plays no role in respiration, all life would come to an end if there is no carbon dioxide in air.” Support this statement with relevant facts.
(d) Water vapours : Explain their role in modifying the earth’s climate.
Answer:
(a) Plants convert nitrogen into protein. It is an important constituent of proteins, which are necessary for the growth of animals, plants and human beings. Plants convert nitrogen into proteins.
(b) 20.9%, oxygen is called active air because it supports life on earth. It is essential for the process of combustion.
(c) Carbon dioxide is essential for photosynthesis by which green plants prepare their food. It minimises heat loss by radiation. Thus, it balances the temperature on earth.
(d) Water vapour determine the earth’s climate conditions. It causes rain. It controls the rate of evaporation from the bodies of plants and animals.

Question 3.
Define the following terms :
(a) pollutants
(b) acid rain
(c) Global warming
(d) smog

Answer:
(a) Pollutants : Air contains substances which are harmful to plants and animals. These harmful substances are called pollutants.
(b) Acid rain : When sulphur trioxide and nitrogen oxide present in the air mix with rainwater they form sulphuric acid and nitric acid respectively. Rainwater containing these acids is called acid rain.
(c) Global warming : An increase in the percentage of carbon dioxide, methane, nitrous oxide and chlorofluorocarbon traps the heat causing the temperature of the earth and its surroundings to rise. This is known as global warming.
(d) Smog : Oxides of nitrogen form a mixture of smoke and fog known as smog which affects our eyes too.

Question 4.
“Air is a mixture”. Support this statement citing at least three evidences.
Answer:
“Air is a mixture” The following are in evidences which prove that air is a mixture.

  1. The composition of air varies from place to place and from time to time.
  2. The components of air retain their individual properties.
  3. Liquid air has no definite boiling point.
  4. No energy exchange occurs when the components of air are mixed with each other.

Question 5.
What is air pollution ? What are the harmful effects of sulphur dioxide, nitrogen dioxide and hydrogen sulphide present in the air ?
Answer:
Air Pollution : Air is polluted by natural processes like volcanic eruption, crop pollination, etc. mostly it is polluted by human activities like burning of coal, wood, diesel oil, kerosene, petrol etc.
Fossil fuels contain sulphur and nitrogen as impurities. When fuels bum these substances combine with air to produce gasses like sulphur dioxide, nitrogen oxide and hydrogen sulphide. They cause many serious respiratory problems. They can destroy the ozone layer, which protects us from the ultra violet radiations of the Sun. They also cause acid rain, which damages crops and buildings.

Question 6.
(a) What are the causes of air pollution ?
(b) Suggest five measures to prevent air pollution.
Answer:
(a) When fuels bum they produce sulphur dioxide, sulphur trioxide, nitrogen dioxide, hydrogen sulphide when these gases mix with rain water. They produce sulphuric and nitric acid. These acids mix with rain water to form acid rain.
(b) Five measures for the prevention of air pollution are:

  1. By using smokeless sources of energy, like solar energy and electrical energy, in place of conventional fossil fuels.
  2.  By using filters for the. smoke coming out of the chimneys of factories and power plants.
  3. By using internal combustion engines in vehicles for complete and efficient burning of fuel.
  4. By locating industries away from residential areas.
  5. By growing more trees.

Question7.
(a) What is nitrogen-fixation ?
(b) What are the two ways in which nitrogen fixation occurs?
(c) Explain the conversion of nitrogen into nitrates during lightning.
Answer:
(a) Nitrogen fixation : Symbiotic bacteria living in the root nodules of leguminous plants like peas, beans, absorb nitrogen directly from air and convert into nitrates. Thereafter, the plants convert it into proteins. Nitrogen is returned to the soil when plant and animal matter decays. This decomposition work is done by organisms called denitrifying bacteria which reconvert dead organic tissue into its constituent nitrogen.
(b) 1. Natural process.
2. Non-biological fixation.
(c) During lightning, temperatures often reach as high as 3000°C. At such high temperatures, nitrogen and oxygen present in the air combine to form nitric oxide, which further react with oxygen to form nitrogen dioxide
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-7a
Oxygen constitutes about 21% of air by volume. It is the active part of air.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-7b
Nitrogen dioxide then reacts with the water vapour present in air to form nitrous and nitric acids.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-7c
Oxygen constitutes about“21% of air by volume. It is the active part of air.
Nitric acid, so formed, reaches the earth along with rain-water, and reacts with metal carbonates to form metal nitrates.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-7d
Oxygen constitutes about 21% of air by volume. It is the active part of air.

B. OXYGEN

EXERCISE — II

Question 1.
Name :
(a) The most abundant element in the earth’s crust.
Ans. Oxygen.

(b) A chemical called oxygenated water.
Ans. H2O2 (Hydrogen peroxide)

(c) A metal highly resistant to rusting.
Ans. Tin.

(d) A mixture of oxygen and carbon dioxide used for artificial respiration.
Ans. Carbogen

(e) Two substances from which oxygen can be obtained at a large scale.
Ans. Air, water.

(f) An oxide and a carbonate containing oxygen.
Ans. Mercuric oxide and potassium chlorate.

(g) Two substances which undergo rapid oxidation.
Ans. Sodium, carbon.

Question 2.
(a) Taking hydrogen peroxide, how would you prepare oxygen in the laboratory ?
(b) What is the role of manganese dioxide in the preparation of oxygen ?
(c) Write the balanced chemical equation for the above chemical reaction.
(d) Why is hydrogen peroxide preferred in the preparation of oxygen gas ?
(e) Why is oxygen collected by downward displacement of water ?
(f) What happens when a glowing splinter is introduced in a jar containing oxygen ?
(g) What happens when oxygen gas is passed through alkaline pyrogallol solution ?

Answer:
(a) Take manganese dioxide in a round bottom flask and add hydrogen peroxide drop by drop to it, which acts ; a catalyst as shown in the figure. Collect oxygen by downward displacement of water.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-e2

(b) Manganese dioxides acts as a catalyst.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-e3
(d) H2O2 is preferred for lab preparation of oxygen because of following reasons.

  1. No heating is required.
  2. The rate of evolution of oxygen (O2) is moderate and under control.
    H2O2 is a safe chemical.

(e) Since the water is displaced downward by the gas collecting in the jar, the process is called downward displacement of water. The reasons are :

  1. Oxygen is only slightly soluble in water. Therefpre it can be collected over water without fear of excessive dilution.
  2. Oxygen is slightly heavier than air, so it cannot collected over air.

(f) Introduction of glowing splinter in the jar. The glowing splinter rekindles, but the gas does not catch fire.
(g) Alkaline pyrogallol solution turns brown when oxygen is passed through it.

Question 3.
(a) What happens when

  1. mercuric oxide and
  2. potassium nitrate are heated ?

(b) Why is potassium chlorate not used for laboratory preparation of oxygen ?

Answer: (a) 

  1. When mercuric oxide is heated, it decomposes to give mercury and oxygen.
  2. Potassium nitrate on heating gets converted into molten potassium nitrite with the release of oxygen.

(b) Potassium chlorate needs heating for quite sometime (to a high temperature) before it decomposes.

Question 4.
What are oxides ? Give two examples for each of me – tallic and non-metallic oxides.

Answer:
Oxides are binary compounds formed by the chemical combination of a substance metal or a non-metal with oxygen.
Examples :
Metal:

  1. Sodium oxide (Na2O).
  2. Calcium oxide (CaO).

Non-metal:

  1. Sulphur dioxide (SO2).
  2. Carbon dioxide (CO2).

Question 5.
Name the three types of oxidation processes. In which of these large amount of heat and light energy are produced?

Answer:
Oxidation can be categorised into three types :

  1. Spontaneous oxidation
  2. Rapid oxidation
  3. Slow oxidation

Out of the above said three types, rapid oxidation produces large amount of heat and light energy.

Question 6.
What do you observe when the following substances are heated and then tested with moist blue and red litmus – paper?
(a) Sulphur
(b) Phosphorus
(b) Calcium
(d) Magnesium

Answer:
(a) Sulphur : Blue litmus turns red.
(b) Phosphorus : Blue litmus turns red.
(c) Calcium : Red litmus turns blue.
(d) Magnesium : Red litmus turns blue.

Question 7.
Complete and balance the following chemical equations.
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-L

Question 8.
(a) Give four uses of oxygen.
(b) How is oxygen naturally renewed in air ?

Answer:
(a) Uses of oxygen

  1. Oxygen is used by firemen, miners, aviators, sea divers and even by every living being.
  2. Oxygen is necessary for burning of fuels.
  3.  Oxygen mixed with hydrogen as fuel produces.a flame with a very high temperature about 2800°C.
  4. As a fuel in spacecraft.

(b) All living beings use atmospheric oxygen in breathing and burning of fuels and in the formation of oxides of nitrogen. Yet amount of oxygen in the air remains more or less constant. This is because green plants return oxygen to the atmosphere by the process of photosynthesis.

Question 9.
(a) What is rust ?
(b) State at least two ways of prevent rusting.

Answer:
(a) Rust: Rust is hydrated ferric oxide (Fe2O3 . x* H2O), which forms a brownish red coating over iron. (* x can be any number.)

(b) Two ways of prevention of rusting :

  1. Painting with red lead.
  2. Oil paint is applied on doors and windows.
  3. Enamel coating. Enamel is a mixture of iron, and steel with silicates.
  4. Coal tar it is used for coating the lower parts of ships and bridges.

Question 10.
State two differences between : Rusting and burning.

Answer:
Difference between rusting and burning

Rusting Burning
  1. Rusting is the process in which iron slowly reacts with oxygen in the air and produces a flaky substance called rust.
  2. Air and moisture are necessary for rusting.
  1. Burning is fast oxidation process in which large amount of energy is produced.
  2. Only air is necessary for burning.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks :

(a) Argon is the most abundant inert gas present in air.
(b) Oxides of sulphur and nitrogen combine with rain water to form sulphuric acid and nitric acid which cause acid rain.
(c) NO2 and CO are the most common air pollutants.
(d) Joseph Priestly discovered the oxygen gas.
(e) Oxygen occupies about 21% of air by volume.

2. Match the following :
Selina Concise Chemistry Class 7 ICSE Solutions - Air and Atmosphere-o2

MULTIPLE CHOICE QUESTIONS

1. A fuel when used releases least amount of pollutants in the air.
(a) sulphur dioxide
(b) chlorofluorocarbon
(c) smoke
(d) CNG

2. The natural way of adding oxygen to air which involves green plants is called
(a) photosynthesis
(b) respiration
(c) burning
(d) dissolution

3. Which one of the following is most likely to be corroded?
(a) a stainless steel cup-board
(b) a galvanised iron bucket
(c) an iron hammer
(d) a tin plated iron box

4. The process by which oxidation of food in our body takes place is
(a) photosynthesis
(b) respiration
(c) decomposition
(d) combustion

 

Selina Concise Chemistry Class 7 ICSE Solutions – Language of Chemistry

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Selina Concise Chemistry Class 7 ICSE Solutions – Language of Chemistry

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Selina Concise ICSE Solutions for Class 7 Chemistry Chapter 5 Language of Chemistry

Points to Remember :

  1. A chemical reaction involves the transformation of original substance into an altogether new substance(s).
  2. A chemical reaction can be represented with the help of the symbols or the formulae of the elements and the compounds taking part in that reaction. This gives a chemical equation.
  3. Certain necessary conditions for a chemical reaction to happen are  close contact, solution form, heat, light and catalyst.
  4. Characteristics of chemical reactions are change of colour, evolution of a gas, formation of a precipitate, change of state, change of smell and evolution/absorption of heat.
  5. A complete chemical equation symbolically represents the reactants, products and their physical states.
  6. The substances that react with each other are called reactants and they are represented on the left hand side of the equation. The substances that are formed as a result of the reaction are called products. They are represented on the right hand side of the equation.
  7. A chemical equation needs to be balanced to make it follow the law of the conservation of mass.
  8. The law of conservation of mass states that mass can be neither created nor destroyed, it can only be transformed from one form to another.
  9. A chemical equation gives both qualitative and quantitative information about the reactants and products.

EXERCISE

Question 1.
(a) Define chemical reaction.
(b) What is a chemical equation?
(c) Why do we need to balance chemical equations?
Answer:
(a) Chemical reaction : Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.
(b) Chemical equation : A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formula of the substances involved in the reaction.
(c) A chemical equation needs to be balanced so as to make the number of the atoms of the reactants equal to the number of the atoms of the products.

Question 2.
State four conditions necessary for chemical reactions to take place.
Answer:
Conditions necessary for chemical reactions :

  1. Close contact
  2. Solution form
  3. Heat
  4. Light
  5. Catalyst

3. Differentiate between :
(a) Reactants and products.

Reactants Products
  1. The substances that react with one another are called reactants.
  2. Reactants are written on the left hand side of equation.
  1. The new substances formed are called products.
  2. Products are written on the right hand side of equation.

(b) Chemical reaction and chemical equation.

Chemical reaction Chemical Equation
Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction. A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formula of the substances involved in the reaction.

(c) A balanced and a skeletal chemical equation.

Balanced Equation

Skeletal Equation

A balanced chemical equation is one in which the number of atoms each element on the reactant side is equal to the number of atoms of that element on the product side.

In a skeletal equation the number of atoms on reactant side are not equal to number of atoms of product side.

Question 4.
Write word equations for the following skeletal equations:
(a) KClO3 →  KCl + O2
(b) Zn + HCl → ZnCl2 + H2
(c)FeCl2 + Cl2 → FeCl3
(d) CO + O2 → CO2
(e) Ca + O2 → CaO
(f) Na + O2 →  Na2O
(g) NaOH + H2SO4 → Na2SO4 + H2O
(h) AgBr →  Ag + Br2
(i) KNO2 →   KNO2 + O2

Answer:
(a) 2KClO3  →  2KCl+ 3O2
(b) Zn + 2HCl    →   ZnCl2 + H2
(c) 2FeCl2 + Cl2  →  2FeCl3
(d) 2CO + O2   →   2CO2
(e) 2Ca + O2 →  2CaO
(f) 4Na + O2   → 2Na2O
(g) 2NaOH + H2SO4   → Na2SO4 + 2H2O
(h) 2AgBr     → 2Ag + Br2
(i) 2KNO3   → 2KN02 + O2

Question 5.
Balance the following chemical equations :
(a) FeS + HCl →  FeCl2 + H2S
(b) Na2CO3 + HCl  →  NaCI + H2O + CO2
(c) H2 + O2   →  H2O
(d) Na20 + H20  →  NaOH
Answer:
(a) FeS + 2HCl   →   FeCl2 + H2S
(b) Na2CO3 + 2HCl  →  2NaCl + H2O + CO2
(c) 2H2+ O2    →    2H2O
(d) Na2O + H2O   →   2NaOH

Question 6.
What information do you get from the equation H2+ Cl2  →  2HCl ?
Answer:
(a)Hydrogen and chlorine molecules are the reactants.
(b)They are in gaseous form.
(c)The product is hydrogen chloride gas.
(d)Two molecules of hydrogen chloride are formed.

Question 7.
Write your observations for the following chemical reactions and name the products formed :
(a) When sugar is heated.
(b) When manganese dioxide is added to potassium chlorate and heated.
(c) When dilute acetic acid is poured on baking soda.
(d) When an aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate.
(e) When ammonium chloride is heated with sodium hydroxide.
(f) When water is added to quick lime?

Answer:
(a) Black solid mass (charcoal) is formed along with water vapours.
(b) Manganese dioxide acts as a catalyst for the decomposition of potassium chlorate into potassium chloride and oxygen at a lower temperature.
(c) Sodium acetate, CO2 and water is formed.
(d) A white insoluble solid precipitate of silver chloride is formed along with Sodium nitrate.
(e) When solid ammonium chloride is heated with sodium hydroxide solution, a gas ammonia is evolved which is recognised by its strong pungent smell.
Selina Concise Chemistry Class 7 ICSE Solutions - Language of Chemistry-7
(f) When water is added to quick lime, a large amount of heat energy is evolved.
Selina Concise Chemistry Class 7 ICSE Solutions - Language of Chemistry-7f
Question 8.
Write symbolic representation for the following word equations and balance them :
(a) Calcium carbonate → Calcium oxide + Carbon dioxide
(b) Carbon + Oxygen → Carbon dioxide
(c) Calcium oxide + Water → Calcium hydroxide
(d) Aluminium + Chlorine → Aluminium chloride
(e) Iron + Sulphur → Iron sulphide

Answer:

Selina Concise Chemistry Class 7 ICSE Solutions - Language of Chemistry-8

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks:
(a) The substances which undergo chemical change are called reactants.
(b) The substances formed as a result of a chemical reaction are called products.
(c) During a chemical reaction transfer of energy takes place.
(d) The basic conditions necessary for a chemical reaction is close contact.
(e) In some chemical reactions an insoluble precipitate is formed when two solutions are mixed.

2. Write ‘true’ or ‘false’ for the following statements :
(a) No new substance is formed during a chemical reaction : True
(b) Hydrogen sulphide has rotten egg smell : True
(c) When potassium iodide solution is added to lead acetate solution a red precipitate is formed : False
(d) A black residue is formed when sugar is heated : True
(e) When iron and sulphur are heated together a grey mass is formed which is attracted by a magnet : False
(f) A chemical equation gives only qualitative information of a chemical reaction : False

MULTIPLE CHOICE QUESTIONS

1. A chemical equation is a statement that describes a chemical change in terms of
(a) symbols and formulae
(b) energy
(c) number of atoms
(d) colours

2. Balancing a chemical equation is based on
(a) Law of conservation of mass
(b) Mass of reactants and products
(c) Symbols and formulae
(d) None of the above

3. Copper carbonate when heated, it turns :
(a) Blue
(b) Green
(c) Black
(d) Yellow