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Class 7 NCERT Solutions Science Chapter 5 - Physical and Chemical Changes

Physical and Chemical Changes Exercise 54

Solution 1

(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

Concept insight: In physical change, no new substance is formed and in a chemical change, new substances are formed.

Solution 2

(a) False

Concept insight: Cutting a log of wood into pieces is a physical change.

(b) False

Concept insight: Formation of manure from leaves is a chemical change.

(c) True

Concept insight: because zinc forms zinc oxide and prevents iron from coming in contact with air.

(d) False

Concept insight: Iron and rust are different substances. Rust is iron oxide (Fe2O3)

(e) True

Concept insight: In condensation, water vapors changes into water.

Physical and Chemical Changes Exercise 55

Solution 3

(a) Calcium carbonate (CaCO3)
(b) Sodium hydrogen carbonate.
(c) Galvanization and painting
(d) Physical
(e) Chemical

Solution 4

When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. The bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change involving the formation of new substances.
Lemon juice + Baking soda Carbon dioxide + other substances

Concept insight: It is an example of chemical change.

Solution 5

When a candle burns, both physical and chemical changes take place.
Physical change Melting of wax
Chemical change Burning of wax
Eating of food is another example where both physical and chemical changes occur simultaneously.
Physical change Breaking down of larger food particles into smaller particles
Chemical change Digestion of food

Solution 6

Once the curd is formed, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.

Concept insight: When milk is converted into curd, lactose sugar is converted into lactic acid which is a new substance.

Solution 7

When we burn wood, a new substance, coal is formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance is formed. Therefore, it is a physical change.

Concept insight: In cutting of wood, only physical appearance of the wood changes while on burning, new substance is produced.

Solution 8

Crystals of copper sulphate are prepared by the method of crystallization. The process is as follows.
i.Take a cupful of water in a beaker.
ii.Add few drops of dilute sulphuric acid to this.
iii.The water is then heated and when it starts boiling, copper sulphate powder is added with stirring.
iv.Copper sulphate powder should be added on till the solution becomes saturated. It is then filtered into a china dish and allowed to cool.
v..The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
Concept insight:Crystallisation is a technique to obtain pure substance.

Solution 9

Rusting is aided by both moisture (water) and air (oxygen). By painting an iron gate, we prevent its contact from the air and moisture present in the atmosphere. Hence, rusting is prevented.
Concept insight: Important question.

Solution 10

Both air and moisture are required for rusting to take place. In coastal areas, the quantity of moisture present in air is more than that in deserts. In desert areas, the amount of moisture in air is even lower. Therefore, rusting of iron objects is faster in coastal areas than in deserts.
Concept insight: For rusting, both moisture and oxygen are required.

Solution 11

(ii) Process - B is a chemical change.

Solution 12

(iii) Both processes A and B are chemical changes.

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