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Class seven Science NCERT Sol additional Questions

Chapter 6: Physical and Chemical Changes

N C E R T Textbook Questions Solved

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

Answers: (a) Chemical change

(b) Physical change (c) Chemical change

(d) Physical change (e) Physical change

(f) 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)

Answers:

(a) False

Correct statement: Cutting a log of wood into pieces is an irreversible physical change.

(b) False Correct statement: Formation of manure from leaves is a chemical change.

(c) True

(d) False Correct statement: Iron and rust are two different chemical substances.

(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.

Answers:

(a) calcium carbonate

(b) sodium hydrogen carbonate

(c) painting or greasing, galvanisation

(d) physical (e) chemical

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:

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.

Answers: When a candle burns, both physical and chemical changes occur:

(i) Physical change: melting of wax, vapourisation of melted wax.

(ii) Chemical change: Burning of vapours of wax to give carbon dioxide, heat and light. LPG is another example in which physical change occurs when LPG comes out of the cylinder and is converted from liquid to gaseous state and a chemical change occurs when it burns in air.

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

Answer: We can say that the setting of curd is a chemical change because we cannot get the original substance after making the curd from milk. Curd is a new substance with different taste and texture and state and it is a permanent/irreversible change.

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 in burning new substances are formed. We can show the change as follows:


But, cutting it into small pieces is a physical change because no new substance is formed.

We can only reduce the size of wood.

8. Describe how crystals of copper sulphate are prepared.

Answer: We can obtain the crystals of copper sulphate by pouring it into the solution of hot water and dilute sulphuric acid. We mix the water and few drops of dilute

sulphuric acid in a beaker and put the mixture on the burner to boil. While the mixture begins to boil, copper sulphate is added to the boiling mixture slowly till it reaches to the saturation. We keep stirring the solution. When no more copper sulphate can be added to the solution, we put off the mixture from the burner and filter it. Then, we leave the mixture untouched to cool it down. While cooling, copper sulphate starts crystallising.

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

Answer: Painting prevents the iron gate from coming in contact with oxygen and moisture, which are responsible for rusting.

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

Answer: The content of humidity in the air of coastal areas is higher than in the air of deserts. That is why; the rusting of iron objects is faster in coastal areas.

11. The gas we use in the kitchen is called liquefied 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 bums (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 change

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