NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes
Exercises
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) 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
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. (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) 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 )
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) 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 or greasing and galvanisation
(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.
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:- The reaction between baking soda and lemon juice can be given as below:
Lemon juice + Baking soda ————-> C02 (bubbles) + Other substances
(Citric acid) (Sodium hydrogen carbonate) —-> (Carbon dioxide)
It is a chemical change
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:- When a candle is lit, it undergoes both physical and chemical changes:
(i) Physical change: wax melting, melted wax vapourisation
(ii) Chemical change: The combustion of wax vapours produces carbon dioxide, heat, and light.
Similarly when LPG flows out of the cylinder and is transformed from liquid to gaseous state it is a physical change occurs, and a chemical change occurs when gas burns in air.
Question:- 6. How would you show that setting of curd is a chemical change?
Answer:- Setting of curd is a chemical change since the original substance, i.e., milk, cannot be recovered and a new substance with different taste, smell, and other chemical features is generated.
Question:- 7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer:- Burning of wood is a chemical change because it results in the formation of new chemicals
Wood + Oxygen ———–> Charcoal + Carbon dioxide + Heat + Light
But Cutting it into small bits, is a physical change because no new substance is generated. Only the size of the wood can be reduced.
Question:- 8. Describe how crystals of copper sulphate are prepared.
Answer:- In a beaker, combine a cup of water and a few drops of dilute sulfuric acid. Warm up the water. When the water begins to boil, gradually add the copper sulphate powder. Continue adding copper sulphate powder until there is no more powder to dissolve. Stir the solution constantly throughout this operation. Filter the mixture. Allow time for it to cool. After some time, you can see the copper sulphate crystals.
Question:- 9. Explain how painting of an iron gate prevents it from rusting.
Answer:- The presence of oxygen and moisture is recognised to be necessary for rusting. The iron gate is painted to keep it from absorbing oxygen and moisture.
Question:- 10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer:- Because the moisture content of the air in coastal places is higher than in deserts. As a result, rusting occurs more quickly in coastal places.
Question:- 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:- (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.