Physical and Chemical Changes

Introduction

Question 1

Change is the law of nature which occurs in everyday life. State when a substance [i.e. matter] undergoes a kind of change.

Answer

A substance undergoes a change when energy is added or removed from the matter.

Question 2

Give a reason why -

(a) Freezing of water is a reversible change while burning of a candle an irreversible change.

(b) Change of seasons is a periodic change while change of weather is a non-periodic change.

Answer

(a) Freezing of water is a reversible change because this change can be reversed by change in conditions i.e., if temperature is lowered freezing occurs and if temperature is raised melting occurs.
On the other hand, burning of a candle is an irreversible change, because it cannot be reversed by change in conditions.

(b) Change of seasons is a periodic change because it occurs periodically at regular intervals whereas change of weather is a non-periodic change because it does not occur at regular intervals.

Physical & chemical changes — term

Question 3

State which of the following pertain to - chemical changes.

(a) Substance retains its identity.

(b) The change is permanent.

(c) No new substance is formed.

(d) Is easily reversible.

(e) Produces one or more new substances by change in its composition.

(f) The change is temporary.

(g) The composition and properties of the original substance are not altered.

Answer

The following points pertain to chemical changes :

(b) The change is permanent.

(e) Produces one or more new substances by change in its composition.

Rest of the points pertain to physical changes.

Question 4

Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.

Answer

Burning of magnesium ribbon in air is considered a chemical change because :

1. It is a permanent change.
2. It is not reversible.
3. New substance (magnesium oxide) is formed.
4. Composition & properties of the original substance has changed.

Question 5

Compare the energy changes involved during a physical and a chemical change.

Answer

Physical changes are not accompanied by a marked evolution or absorption of heat energy whereas chemical changes are accompanied by evolution or absorption of heat energy and sometimes emission of light energy.

Physical & chemical changes — examples

Question 6

Is change of state of matter - a physical or a chemical change. Give reasons.

Answer

Change of state of matter is a physical change because :

• it is a temporary change
• it is a reversible change
• no new substance is formed
• composition and properties of substance do not change.

Question 7

State which of the following terms connected with change of state of matter i.e.

(a) Melting

(b) Vaporisation

(c) Condensation

(d) Freezing

pertain to

(i) Conversion of a liquid into a vapour or gas.

(ii) Conversion of a liquid into a solid.

(iii) Conversion of a solid into a liquid on heating.

Answer

(i) Conversion of a liquid into a vapour or gas — Vaporisation

(ii) Conversion of a liquid into a solid — Freezing

(iii) Conversion of a solid into a liquid — Melting

Question 8

Potassium chloride is added to water and stirred. A salt solution is obtained which is then boiled leaving behind a residue

(a) Is the above change physical or chemical?

(b) Name the residue which remains behind after the salt solution is boiled.

(c) Is the change reversible or not reversible?

(d) Are the composition and properties of the original substance altered.

(e) Give a reason why the above experimentation would not be possible, if calcium carbonate is taken, in place of potassium chloride.

Answer

(a) Physical change

(b) Potassium chloride

(c) Reversible, as salt initially added to water is finally recovered after boiling the water.

(d) No

(e) Calcium carbonate reacts with water to form calcium hydroxide, hence a new substance is formed so the change will be chemical not physical.

Question 9

Give a reason why —

(a) Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.

(b) Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.

(c) Rusting of iron is a chemical change but, magnetization of iron is a physical change.

Answer

(a) Heating a platinum wire is a temporary change that gets reversed on cooling the wire. Moreover, the composition and properties of platinum does not change. Hence, it is a reversible change. On burning magnesium wire, a new product [magnesium oxide] is formed. Hence, we cannot recover magnesium back, so it is an irreversible change.

(b) On adding zinc nitrate to water, it dissolves and no new product is formed. The reaction is reversible, hence it is a physical change.
On reacting zinc with dil. nitric acid, hydrogen gas is released and zinc nitrate is formed. The change is irreversible hence it is a chemical change.

(c) Rusting of iron is a chemical change because a new product iron oxide [rust] is formed after the reaction and the reaction is irreversible.
Magnetization of iron is a physical change because on hammering, the iron nail gets demagnetized and we get the original product back. The reaction is reversible.

Question 10

Select the chemical changes from the following list of changes -

(a) Vaporisation of water into water vapour

(b) Boiling of milk

(c) Respiration in mammals

(d) Rotting of eggs

(e) Drying of a fruit

(f) The carbon cycle

(g) Occurrence of lightening

(h) Breaking of glass

(i) Butter turning rancid

(j) Glowing of an electric bulb

(k) Crystallisation of a salt from its solution

(l) Change of seasons

(m) Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid

Answer

Chemical changes are :

• Respiration in mammals

• Rotting of eggs

• The carbon cycle

• Occurrence of lightening

• Butter turning rancid

• Preparation of carbon dioxide from calcium carbonate & dilute hydrochloric acid

Objective Type Questions

Question 1

Select the correct answer from the choice in bracket to complete each sentence:

1. A change in which a substance retains its identity is a ............... [physical/chemical] change.

2. Matter is added or removed during a ............... [chemical/physical] change.

3. Energy required for completion of a physical change is ............... [released/absorbed] when the change is reversed.

4. Dissolution of lead nitrate in water is deemed as a ............... [physical/chemical] change.

5. A change in which matter undergoes a change but the total mass of substance is unaltered is a ............... [physical/chemical] change.

Answer

1. A change in which a substance retains its identity is a physical change.

2. Matter is added or removed during a chemical change.

3. Energy required for completion of a physical change is released when the change is reversed.

4. Dissolution of lead nitrate in water is deemed as a physical change.

5. A change in which matter undergoes a change but the total mass of substance is unaltered is a physical change.

Question 2

State which of the following 1 to 5 pertain to -

A: Physical Change

B: Chemical change

1. Sublimation.

2. Fermentation.

3. Liquefaction or condensation.

4. Magnetization.

5. Respiration.

Answer

1. Sublimation — Physical Change

2. Fermentation — Chemical change

3. Liquefaction or condensation — Physical Change

4. Magnetization — Physical Change

5. Respiration — Chemical change

Question 3

Give reasons why the following are considered as chemical changes.

1. Copper carbonate on heat gives copper oxide and carbon dioxide.

2. A bright light is seen evolved when a strip of magnesium is heated.

3. Sulphur when burnt in air evolves a gaseous acidic product.

4. An iron nail kept open in the atmosphere rusts.

5. A piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid.

Answer

1. Copper carbonate on heating gives copper oxide and carbon dioxide. As new products are formed and the change is irreversible hence it is a chemical change.

2. When a strip of magnesium is heated, magnesium oxide is produced and the bright light seen is the evolution of energy in the form of light. Hence, it is considered a chemical change.

3. Sulphur burns in air to produce a new gaseous acidic product [sulphur dioxide]. Hence, the change is a chemical change.

4. Iron nail kept in air forms rust [iron [III] oxide]. Iron [III] oxide is a new product and has different chemical composition and properties as compared to that of an iron nail. Hence, the reaction is a chemical change.

5. When a piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid, magnesium chloride is formed and hydrogen gas is liberated. As new products are formed and the change is irreversible hence it is a chemical change.

Question 4

Match the examples of changes in List I with the correct type of change in List II.

Answer

Question 5

Name the following:

1. The salt obtained when a chemical change takes place on addition of iron to dilute sulphuric acid.

2. The product obtained during a physical change when water converts from a liquid into a solid.

3. The product of the chemical change on keeping a polished iron nail exposed to the atmosphere.

4. A form of energy required for a chemical change.

5. The gaseous product of the chemical change which takes place during respiration in living organisms.

Answer

1. Iron sulphate

2. Ice

3. Iron [III] oxide [rust]

4. Heat energy

5. Carbon dioxide