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

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Updated at 2021-04-09


NCERT solutions for class 7 Science Chapter 6 Physical And Chemical Changes Exercise

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



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


Answer :

(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)



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


Answer :

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

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

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

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

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

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.



NCERT Solutions for Class 7 Science Chapter 6 Physical And Chemical 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.

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.



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


Answer :

(1) When baking soda (Sodium bicarbonate) is mixed with lemon juice(citric acid), bubbles are formed.
These bubbles are formed due to the evolution of carbon dioxide gas.

(2) The reaction between baking soda and lemon juice can be given as below:

Lemon juice + Baking soda ? Carbon dioxide + other substances

(3) A chemical reaction is taking place which can't be reversed.
Since there is the formation of a new substance in this reaction, it is a chemical change.

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.



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


Answer :

(1) When a candle burns, both physical and chemical changes take place.

· Physical change is the melting of the wax.
· Chemical change is the burning of gas with the evolution of CO2.

(2) Digestion of food is another example where both physical and chemical changes occur simultaneously.

· Physical change is the breakdown of larger food molecules into simpler ones.
· The chemical change is the digestion of food by the action of HCl and enzymes.

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



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


Answer :

(1) The setting of curd is a chemical change because a new substance(lactic acid) is formed.

(2) Once the curd is formed, milk cannot be re-obtained from it. It is an irreversible change.

(3) Also, both milk and curd have different properties. The new substance, i.e. curd is different from the milk in taste, smell, and chemical properties.

(4) Since these are the properties of a chemical change, hence, the 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.



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


Answer :

(1) When we burn wood, a new substance coal is formed.
Wood is converted to charcoal with the liberation of CO2.
Therefore, it is a chemical change.

(2) However, when we cut wood no new substance is formed.
Only the shape and size of the wood are changed. It does not change the nature of the wood.
Therefore, it is a physical change.

Q.8 Describe how crystals of copper sulphate are prepared.



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


Answer :

Crystals of copper sulphate are prepared by the method of crystallization.

The process is as follows:

(1) A cupful of water is taken in a beaker.

(2) Few drops of dilute sulphuric acid are added to this.

(3) The water is then heated and when it starts boiling, copper sulphate powder is added with stirring.

(4) Copper sulphate powder should be added on till the solution becomes saturated.

(5) It is then filtered into a china dish and allowed to cool.

(6) The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out

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



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


Answer :

(1) Rusting is aided by both moisture (water) and air (oxygen).

(2) By painting an iron gate, we prevent the contact between iron, Oxygen, and water (moisture).

(3) Hence, 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.



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


Answer :

(1) Iron gets rusted because of the reaction with oxygen present in moist air.

(2) In coastal areas, air contains more moisture because of the sea or ocean,
while in deserts air is dry and hot.

(3) As the content of moisture in the air in coastal areas is higher than in the air in deserts.

(4) So, the process of 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



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


Answer :

(ii) Process – B 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



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


Answer :

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



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