NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes

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

(a) Photosynthesis

Answer:

(a) Photosynthesis is a chemical change.

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

(b) Dissolving sugar in water

Answer:

(b) Dissolving sugar in water is a physical change.

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

(c) Burning of coal

Answer:

(c) Burning of coal is a chemical change.

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

(d) Melting of wax

Answer:

(d) Melting of wax is a physical change.

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

(e) Beating aluminium to make aluminium foil

Answer:

(e) Beating aluminium to make aluminium foil is a physical change.

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

(f ) Digestion of food

Answer:

Digestion of food is a chemical change.

During digestion, enzymes in the stomach and intestines break down food molecules into simpler molecules.

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 because cutting a log of wood into pieces is a physical change.

(b) False because the formation of manure from leaves is a chemical change.

(c) True.

(d) False because Iron and rust are different substances.

(e) True

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 galvanization and painting.

(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.

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:

When baking soda is mixed with lemon juice then it is a chemical change, bubbles are formed with the evolution of a carbon dioxide gas.

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:

When a candle burns, both physical and chemical changes take place. On heating, candle wax gets melted is a physical change since it again turns into wax on cooling.

The wax gives carbon dioxide, heat, light, water vapour and carbon soot on heating with states chemical change.

Another example is the lightening of the bulb using a dry cell.

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

Answer:

The setting of curd is a chemical change because the new substance(curd) formed from milk differs in taste and it cannot be converted into milk again i.e. it is irreversible.

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

Answer:

The burning of wood is a chemical change because wood after burning gives carbon dioxide, ash, heat, water vapour also it is an irreversible process.

Whereas cutting wood into small pieces is a physical change because the original composition of wood remains the same and no new substance is formed.

8. Describe how crystals of copper sulphate are prepared.

Answer:

The crystals of copper sulphate are prepared as follows:

1. A cup of water is taken in a beaker.

2. Add some drops of dilute sulphuric acid in it.

3. Heat it and when it starts boiling add copper sulphate powder slowly with continuous stirring.

4. The copper sulphate powder is added till the saturation level.

5. Filter the solution and cool it.

6. Leave it undisturbed and we will observe copper sulphate crystals after some hours.

9. Explain how painting of an iron gate prevents it from rusting.

Answer:

Painting of an iron gate prevents direct contact of iron with water and air(oxygen) which are required for rusting.

No exposure of water and oxygen with iron prevents iron from rusting.

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

Answer:

Rusting occurs with direct contact of iron with water and oxygen.

In coastal areas,the moisture content is high in the air which states rusting of iron objects is faster in coastal areas because in deserts moisture in the air is absent.

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:

The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). This is a chemical change.

Thus, option (ii) is correct.

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:

Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B).

Both process A and B are a chemical change.

Thus, option (iii) is correct.