NCERT Solutions for class 7 science Chapter 6

Physical and Chemical Changes

NCERT Solutions for Class 7 Science chapter 6 Exercise Question

Question 1.

Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminium foil

(f ) Digestion of food

Answer

a) Chemical change

b) Physical Change

c) Chemical change

d) Physical Change

e) Physical Change

f) Chemical change

Question 2

State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

(a) Cutting a log of wood into pieces is a chemical change. (True/False)

(b) Formation of manure from leaves is a physical change. (True/False)

(c) Iron pipes coated with zinc do not get rusted easily. (True/False)

(d) Iron and rust are the same substances. (True/False)

(e) Condensation of steam is not a chemical change. (True/False)

Answer

a) False

b) False

c) True

d) False

e) True

Question 3

Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _________.

(b) The chemical name of baking soda is _________.

(c) Two methods by which rusting of iron can be prevented are _________ and _________.

(d) Changes in which only _________ properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called _________ changes.

Answer

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate.

(b) The chemical name of baking soda is Sodium hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanization.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) Changes in which new substances are formed are called Chemical changes.

Question 4

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

Answer

Carbon dioxide gas release When baking soda is mixed with lemon juice . This is a chemical change.

Lemon juice + Baking soda → Carbon dioxide + other substances

Question 5

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Answer

Physical change is melting of wax and the chemical change is Burning of vapours of wax to give carbon dioxide, heat and light. Burning of wood is a example of both physical and chemical change

Question 6

How would you show that setting of curd is a chemical change?

Answer

The setting of curd is a chemical change because we can not get the original substance. As properties of curd is different than milk.

Question 7

Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer

Cutting of wood is a physical change as no new substance is formed. On the other hand, burning of wood is a chemical change as a new substance is formed like heat and CO₂ which is has different properties than original wood

Question 8

Describe how crystals of copper sulphate are prepared.

Answer

Take a cupful of water in a beaker. Add a few drops of dilute sulphuric acid to this. Heat the water and when it starts boiling, add copper sulphate powder while still stirring. Add the copper sulphate powder till no more powder can be dissolved. Filter the solution and leave it cool Look it after some time, you can see the crystals of copper sulphate.

Question 9

Explain how painting of an iron gate prevents it from rusting

Answer

Rusting of iron occure due to presence of moisture and oxygen. Painting prevents the iron gate from coming in contact with oxygen and moisture

Question 10

Explain why rusting of iron objects is faster in coastal areas than in deserts.

Answer

As present of moisture in the air in coastal areas is higher than in the air in deserts . As we know Rusting of iron occure due to presence of moisture and oxygen. hence, rusting of iron objects is faster in coastal areas than in deserts.

Question 11

The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change

Answer

(ii) Process – B is a chemical change.

Question 12

Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change

Answer

(iii) Both processes A and B are chemical changes.