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

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:

• Processes as physical change – (b) Dissolving sugar in water, (d) Melting of wax, (e) beating aluminium to make aluminium foil.
• Processes as chemical change – (a) Photosynthesis, (c) Burning of coal, (f) Digestion of food.

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)

Answer: False. [ Cutting a log of wood is a physical change. ]

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

Answer: False. [ Formation of manure is a chemical change. ]

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

Answer: True.

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

Answer: False. [ Iron and rust are the different substances. Rust is oxide of iron. ]

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

Answer: 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 _________.

Answer: calcium carbonate

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

Answer: sodium hydrogen carbonate

(c) Two methods by which rusting of iron can be prevented are _________ and _________.

Answer: galvanization, painting.

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

Answer: physical

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

Answer: 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: Baking soda when mixed with lemon juice produces bubbles with the evolution of carbon dioxide gas. This change is a chemical change. Lemon juice containing citric acid reacts with baking soda (sodium hydrogen carbonate) forming bubbles. Formation of bubbles is due to evolution of carbon dioxide (CO2)gas. In this reaction some other substances are also formed.

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: Burning of candle shows both physical and chemical changes simultaneously. Melting of candle wax shows physical change, as during melting no new product is formed. On the other hand, burning of wax shows chemical change, as wax is vaporised and carbon dioxide is formed. Another examples showing simultaneous physical and chemical change is eating of food. Here, physical changes occurs when food particles are broken down into smaller particles in mouth. Chemical changes occurs at the time of digestion of food.

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

Answer: Curd is formed from milk. Setting of curd is a chemical change, because once curd is formed it cannot be reversed back to milk. Also, curd shows different properties than milk, like change in odour, taste, etc, thereby, confirming a chemical 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 forms new products ash and smoke, which have different properties than wood. Hence, it is a chemical change.

Cutting wood into small pieces forms no new product. Wood pieces are still a wood and their properties are same. Hence, it is physical change.

8. Describe how crystals of copper sulphate are prepared.

Answer: Crystals of copper sulphate are prepared by crystallisation method. Take cup of water into beaker. Add few drops of dilute sulphuric acid. Heat the water. When it starts boiling stir water and add copper sulphate powder continuously till no more copper sulphate powder can be dissolved in water. Filter the solution and allow it cool. Thus, copper sulphate crystals can be observed after some time.

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

Answer: Iron forms rust or iron oxide, when it comes in contact with air and moisture. When iron gate is painted, paint prevents gate from contact of external moisture and air. Hence, painting of an iron gate prevents it from rusting.

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

Answer: For rusting of iron object, it is necessary to have moisture and air. In coastal areas, the quantity of moisture in air is more, whereas, in desert areas due to lack of water the quantity of moisture in air is lower. Hence, rusting of iron objects is faster in coastal areas than in deserts.

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.

Check other lessons NCERT Solutions for Class 7

Download free NCERT textbook class 7 Science – Chapter 6 Physical and Chemical Changes