NCERT Solutions Class 7 Science Chapter 6 Physical and Chemical Changes

 

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

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 aluminiun 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 aluminiun 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)

Corrected statement: Cutting of log of wood into pieces is a physical change.

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

Corrected statement: Formation of manure from leaves 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)

Corrected statement: Iron and rust are different substances.

(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 hydrogencarbonate.

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

When baking soda (sodium hydrogencarbonate) is mixed with lemon juice (citric acid), bubbles are produced due to the formation of carbon dioxide and other substances. It is a chemical change. Therefore, we cannot get back the original substances, i.e., lemon juice and baking soda.

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 burns, both physical and chemical changes take place.

When we burn a candle, the wax melts which can be converted back into solid wax on cooling. This shows that melting of wax is a physical change.

Burning of a candle also produces light along with some gases like carbon dioxide. Hence, burning of the wick of a candle is a chemical change.

Cooking food is another example in which both physical and chemical changes take place.

Physical change: appearance of ingredients Chemical change: new substances are formed

Question.6

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

Answer.

Setting of curd is type of chemical change as new substance is formed which has different properties from milk. Once curd is formed, milk cannot be re-obtained from it.

Question.7

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

Answer.

Cutting of wood into small pieces is a physical change because no new substance is formed and only the shape and size of wood changes.

When we burn wood, carbon dioxide along with some other new substances is formed. Therefore, burning of wood is a type of chemical change.

Question.8

Describe how crystals of copper sulphate are prepared.

Answer.

Crystal of copper sulphate is obtained by the process of crystallisation. For the preparation of crystals of copper sulphate take some water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling add copper sulphate powder slowly while stirring continuously. Continue adding copper sulphate powder till it stops dissolving. Filter the solution into a china dish and allow it to cool. Do not disturb the solution while it cools. Slowly, the crystals of copper sulphate separate out.

Question.9

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

Answer.

The presence of both oxygen (air) and water (moisture) is essential for rusting to take place.

When an iron gate is painted, oxygen and water do not come in contact with iron. Hence, rusting is prevented.

Question.10

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

Answer.

The presence of both oxygen and water is essential for rusting. Greater humidity indicates presence of more amount of water vapours in air, therefore, faster rate of rusting.

In coastal areas humidity is high as compared to deserts. Moreover, the presence of salty water also speeds up the process of rusting in coastal areas. Thus, the iron objects rust faster in coastal areas.

Question.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 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.

The correct option is (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.

The correct option is (iii). Both processes A and B are chemical changes.