NCERT Solutions for Class 7th Science
Chapter 6 – PHYSICAL AND CHEMICAL CHANGES
(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)
Q.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
Ans (a) Photosynthesis – Chemical Change
(b) Dissolving sugar in water – Physical Change
(c) Burning of coal – Chemical Change
(d) Melting of wax – Physical Change
(e) Beating aluminium to make aluminium foil – Physical Change
(f ) Digestion of food – Chemical Change
Q.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)
Ans (a) Cutting a log of wood into pieces is a chemical change. (False)
Corrected statement: Cutting a log of wood into pieces is a physical change.
(b) Formation of manure from leaves is a physical change. (False)
Corrected statement: Formation of manure from leaves is a chemical change.
Q.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.
Q.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.
Q.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.
Q.6 How would you show that setting of curd is a chemical change?
Q.7 Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Ans Burning of wood is a chemical change as new substances are formed in the process along with heat, light etc. It is an irreversible process.
On the other hand, cutting of wood into small pieces is a physical change as there is no change in the properties and no new substance is formed.
Q.8 Describe how crystals of copper sulphate are prepared.
Q.9 Explain how painting of an iron gate prevents it from rusting.
Q.10 Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans Rusting occurs because of the reaction of iron with moisture and air (oxygen). In case of coastal areas the air is more humid i.e. it contains more moisture than the air in the deserts and hence rusting is faster in coastal areas.
Q.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.
Q.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.