The Ever-Evolving World of Science — Science STD 7 — Question

Explain how in ancient time a day, a month and a year were measured.

Give two uses each of a convex and a concave mirror.

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

What do you understand by passive smoking?

Distinguish between the following.
Exothermic reaction and Endothermic reaction.

Why do electricians use rubber gloves?

Choose the odd one out from each group and given reasons.
(i) Liver, salivary gland, starch, gall bladder
(ii) Stomach, liver, pancreas, salivary gland
(iii) Tongue, absorption, taste, swallow
(iv) Oesophagus, small intestine, large Intestine, rectum

Define an electric circuit and give symbols of two electrical components.

Rajasthan is a desert state in India. Once while travelling to Rajasthan by train, Boojho observed several streams and rivulets of rain water during the journey but to his surprise he did not see streams of water in the desert region even during rains. Help Boojho find a suitable explanation for this.

Salma had very soft and smooth skin during her childhood. As she entered adolescence, she developed pimples on her face. The skin specialist advised her to wash her face at regular intervals. Can you explain the reasons for the appearance of pimples on her face and suggest ways to prevent them?