Class 7 Science

Physical Chemical Changes

NCERT Solution

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, c and f are chemical changes, while b, d and e are physical changes

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.
(b) Formation of manure from leaves is a physical change.
(c) Iron pipes coated with zinc do not get rusted easily.
(d) Iron and rust are the same substances.
(e) Condensation of steam is not a chemical change.

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) Calcium carbonate, (b) sodium hydrogen carbonate, (c) polishing, galvanization, (d) physical, (e) chemical

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: When baking soda is mixed with lemon juice, bubbles formed because of evolution of carbon dioxide gas. This is a chemical change, because in this reaction new substances are formed.

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: In burning of candle some of the wax melts, which is physical change, while most of the wax burnt, which is a chemical change.

In the burning of wood, water present in wood changes into vapour, this is physical change, while burning of wood is a chemical change.



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

Answer: In the setting of curd, milk changes into new substance, curd. This is an irreversible process that means milk cannot be made from curd. Formation of new substance and irreversible process shows that setting of curd is a chemical change.

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

Answer: In burning of wood, new substances are formed and hence it is a chemical change. While cutting the wood into small pieces no new substance is formed, thus it is a physical change.

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

Question 8: Describe how crystals of copper sulphate are prepared.

Answer: Process:

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


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

Answer: Iron get rusted because of reaction with oxygen present in moist air. By painting iron gate, iron is prevented to come into contact with oxygen present in moist air, which prevent it be getting rusted.

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

Answer: Iron gets rusted because of reaction with oxygen present in moist air. In coastal areas, air contains more moisture because of sea or ocean, while in deserts air is dry and hot. Thus because of getting more moist air iron objects getting rusted 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 exists 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.