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

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

The correct option is (ii). Process – B is a chemical change.

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.

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.

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.

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.

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.

(a) Cutting a log of wood into pieces is a chemical change.    (False)

Corrected statement: Cutting a 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)

(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

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.

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

(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.