Science 2 Marks Questions

(i) (a) Colour of the solution in the beaker changes from blue to green.
(b) A brown coloured deposit is found on the surface of the iron nail.
(ii) The changes are chemical in nature as new substances, iron sulphate (green) and copper (brown) are formed.

Physical changes are beating of aluminium metal to make aluminium foil and cutting of a log of wood into pieces. Chemical changes are digestion of food and burning of crackers.

Stretching of rubber band, burning of paper, burning of candle, rotting of food, steams rising from boiling water, making of curd, sublimation of camphor, breaking of a glass, bending of iron pipe, freezing of water.

When an iron blade of a knife is dipped in a copper sulphate solution, then iron blade is coated with reddish brown deposits of copper.
And the blue colour of copper sulphate solution changes to light green due to the formation of new substance iron sulphate. So, it is a chemical change.

The ozone layer protects us from the harmful ultraviolet radiations, which come from the Sun. It absorbs ultraviolet radiations coming from the Sun and breaks down to form oxygen.
In this way, ozone layer absorbs harmful ultraviolet radiations.

When we burn a cracker, it explods. Explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. Many new products are formed. So, it is a chemical change.

During photosynthesis, the plants intake carbon dioxide and water in the presence of chlorophyll and sunlight to form two new substances, i.e. glucose (food) and oxygen gas.
So, photosynthesis is a chemical change.

In the process of digestion, the various food materials breakdown to form new substances, which can be absorbed by the body. So, the process of digestion is a chemical change.

Formation of clouds is a physical change. Clouds are formed by the condensation of water vapours present in the atmosphere. When rainwater goes back on the Earth, no new product is formed.

We know that for rusting, the presence of both oxygen and water (water vapours) is essential. Thus, in coastal areas, the air contains high moisture, which means more humid environment so, rusting becomes faster. Whereas in deserts, moisture in air is less. Hence, rusting of iron is very slow there.

Burning of wood is a chemical change, while cutting of wood is a physical change because during burning, new substances are formed. After burning, we cannot get original substance (i.e. wood) back. Cutting of wood into small pieces is a physical change because no new substance is formed.

Setting of curd is a chemical change because we cannot get the original substance (milk) back. The new substance, i.e. curd is different from the milk in taste, smell and chemical properties.

Physical changes in burning candle On heating, candle's wax melts. Since, it can be turned into solid wax again on cooling. Hence, it is a physical change.
Chemical changes in burning candle The wax near to flame burns and gives new substances like carbon dioxide, carbon soot, water vapours, heat and light. Hence, it is a chemical change.
Cooking of food, boiling of eggs are examples of both physical and chemical changes. In both cases, the physical appearance of the substances change and new substances are formed together.

The formation of curd from milk is a chemical change because:
1.A new substance, curd, is formed from milk.
2.The change is irreversible and cannot be converted back into milk.

When magnesium is burnt in the presence of air, it catches fire and burns with a bright white flame. It reacts with oxygen in the air to form magnesium oxide.