NCERT Science Class-VII Chapter-6: Physical and Chemical Changes Solutions

Exercises

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

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) False

(b) False

(c) True

(d) False

(e) True

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 limewater, it turns milky due to the formation of calcium carbonate (CaCO3).

(b) The chemical name of baking soda is sodium hydrogen carbonate (NaHCO3).

(c) Two methods by which rusting of iron can be prevented are galvanizationand painting.

(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.

 

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, the bubbles which are formed with the evolution of a gas is due to the evolution of carbon dioxide gas. Since, there is formation of a new substance in this reaction, it is a chemical change.

 

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 burns, both physical and chemical changes take place.

Physical Changes: On heating, candle wax gets melted is a physical change. Since it again turns into solid wax on cooling.

Chemical Changes: The wax near to flame burns and gives new substances like Carbon Dioxide, Carbon soot, water vapour, heat and light.

Cooking of food, is another example where both physical and chemical changes occurs.

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

Answer. The curd is formed from milk. Both curd and milk have different properties. Also, once the curd is formed it cannot be reversed back into milk. So, there is formation of new substance with different properties and also an irreversible process, setting of curd is a chemical change.

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

Answer. When we burn wood, it turns into ashes which is a new substance and process is irreversible one, hence is a chemical change. While cutting the wood into small pieces no new substance is formed, hence is a physical change.

8. Describe how crystals of copper sulphate are prepared.

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

The process is as followed:

Step 1: A cupful of water in a beaker is taken.

Step 2: Few drops of dilute sulphuric acid is added to it.

Step 3: Water is heated and when it starts boiling copper sulphate powder is added slowly while    stirring till no more copper sulphate powder dissolved in it.

Step 4: Solution is filtered and let it cool without disturbance. After some time the crystals of copper can be observed in it.

 

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

Answer. For rusting, the presence of both oxygen and water (or water vapour) is required. The coat of the paint prevents direct contact of iron with air and oxygen and thus prevents it from rusting.

 

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

Answer. For rusting, the presence of both oxygen and moisture (water vapour) is required. In coastal areas the quantity of moisture content in air is high (highly humid) as compared to desert regions (low humid), so the rusting becomes faster in coastal areas.

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.

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.

 

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