NCERT Solutions for Class 7 Science - Physical and Chemical Changes

Exercise

Q1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
Ans: Chemical change

(b) Dissolving sugar in water
Ans: Physical change

(c) Burning of coal
Ans: Chemical change

(d) Melting of wax
Ans: Physical change

(e) Beating aluminium to make aluminium foil
Ans: Physical change

(f) Digestion of food
Ans: Chemical change

Q2. 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)
Ans: False

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

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

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

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

Q3. 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 _________.
Ans: Calcium carbonate.

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

(c) Two methods by which rusting of iron can be prevented are _________ and _________.
Ans: Polishing, Galvanisation.

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

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

Q4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Ans: When baking soda is mixed with lemon juice, bubbles form because of the evolution of carbon dioxide gas. This is a chemical change because, in this reaction, new substances are formed.
Lemon juice + Baking soda → Carbon dioxide + other substances

Q5. 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.
Ans: In the burning of a candle, some of the wax melts, which is a physical change, while most of the wax burns, which is a chemical change.
In the burning of wood, water present in wood changes into a vapour; this is a physical change, while burning of wood is a chemical change.

Q6. How would you show that the setting of curd is a chemical change?
Ans: In the setting of curd, milk changes into a new substance, curd. This is an irreversible process that means milk cannot be retrieved from curd. The process of curd setting does involve chemical changes due to fermentation.

Q7. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Ans: In the burning of wood, new substances are formed and hence, there is a chemical change. While cutting the wood into small pieces no new substance is formed, thus it is a physical change.
Hence, burning wood and cutting it into small pieces are considered two different types of changes.

Q8. Describe how crystals of copper sulphate are prepared.
Ans: Process:

• Water is taken in a beaker.
• A few drops of sulphuric acid are added to the water.
• Water is let to boil.
• When water starts boiling, copper sulphate powder is added to the water with continuous stirring of the solution.
• When copper sulphate starts to be deposited into the bottom of the beaker, the solution is filtered and left for cooling.
• After cooling, the crystals of copper sulphate is prepared.

Q9. Explain how painting of an iron gate prevents it from rusting.
Ans: Iron gets rusted because of a reaction with oxygen present in moist air. By painting an iron gate, iron is prevented from coming into contact with oxygen present in moist air, which prevents it from getting rusted.

Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: Iron gets rusted because of a reaction with oxygen present in moist air. In coastal areas, air contains more moisture because of the sea or ocean, while in deserts, air is dry and hot. Thus, because of getting more moist air, iron objects get rusted faster in coastal areas than in deserts.

Q11. 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.
Ans: (ii) Process – B is a chemical change.

Q12. 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.
Ans: (iii) Both processes A and B are chemical changes.