Here are textbook solutions of the chapter Physical and Chemical Changes in NCERT Class 7 Science. Extra questions from this chapter can be found here.

1. Classify the changes involved in the following processes as physical or chemical changes

(a) photosynthesis – chemical

(b) dissolving sugar in water – physical

(c) burning of coal – chemical

(d) melting of wax – physical

(e) beating aluminium to make aluminium foil – physical

(f) digestion of food – chemical

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. (false; it is a physical change)

(b) Formation of manure from leaves is a physical change. (false; it is a chemical change)

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

(d) Iron and rust are the same substances. (False; they are not the same substances)

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

3. (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of …………………………………….

Calcium Carbonate (CaCO3)

(b) The chemical name of baking soda is ……………………………..

Sodium bicarbonate or sodium hydrogen carbonate (NaHCO3)

(c) Two methods by which rusting of iron can be prevented

Greasing and galvanization

(d) Changes in which only ………………………. properties of a substance change are called physical changes.

Physical

(e) Changes in which new substances are formed are called ………………………. changes.

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.

This is an example of a chemical change. When baking soda is mixed with lemon juice, we get carbon dioxide, salt and water. When acid reacts with carbonates, salt is formed releasing carbon dioxide and water.

Baking soda + lemon juice -> salt + carbon dioxide + water

5. When a candle burns, both physical and chemical changes take place. Identify these changes.

When a candle burns both physical and chemical changes take place

Physical change – the solid candle wax melts into liquid wax

Chemical change – when wax comes into contact with fire, new substances like carbon dioxide, water vapor, carbon soot, heat and light are formed.

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

The setting of curd is an irreversible process. Curd cannot become milk again. Also, milk and curd have different chemical properties. Hence, the 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?

Burning of wood is a chemical change because it leads to the formation of new substances. When wood is burned, it turns into charcoal and during this process it generates heat and light and releases carbon dioxide.

Cutting of wood is a physical change because no new substance is formed.

8. Describe how crystals of copper sulphate are prepared.

Take a cupful of water in a beaker and add a few drops of diluted sulphuric acid into it. Heat the water. When it starts boiling, add copper sulphate powder slowly and stir continuously. Continue to add copper sulphate powder until no more powder can be dissolved in the water. Filter the solution. Leave the solution undisturbed and allow it to cool. After sometime, you will find the crystals of copper sulphate in the beaker.

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

When iron gets exposed to air and moisture, it rusts. The higher the moisture content in the air, the faster the rusting. Applying a coat of paint on iron articles prevents them from coming into contact with air and moisture and protects them from rusting.

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

Iron rusts when it comes into contact with oxygen and water (moisture). The higher the moisture content in the atmosphere, the faster the rate of rusting. Since coastal areas have too much moisture in the air, iron articles rust faster in such places. By contrast, moisture content in the air is considerably low in deserts; hence, iron articles exposed to air in deserts do not rust that easily.

11. Refer textbook page 66

Answer: Process B is a chemical change

12. Refer textbook page 66

Answer: Both processes A and B are chemical changes

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