There were lot of organisms A in the crop fields in an area. These organisms used to feed on organisms B also present in the crop fields but which damaged the standing crops. One day some people arrived in the fields and killed all the organisms A so that they could remove their C for making fancy items which were in great demand. Due to large scale killing of A, the population of B increased too much damaging all the crops in the area and causing a famine-like situation. Apart from A, there is another organism D which can also kill and eat B. Name A, B, C and D.

Khadins are used in Rajasthan to :

In a village, farmers started cultivating crops all around a lake which was always filled with water. They added lot of materials W to the soil in their fields to increase the yield of crops. Soon they found that the whole surface of water in the lake was covered with a green layer made up of tiny organisms X. After some time, the fish present in the lake started dying in large numbers because they could not get sufficient Y due to a process Z which had occurred in lake water and drained out all the Y present in the water of lake.

(a) What do you think materials W are ?

(b) Name (i) X, and (ii) Y.

(c) What is the process Z known as?

(d) What happens during the process Z which uses up all the Y present in lake water?

(e) What does this example tell us about the use of materials like W?

There are two important fuels A and B both of which are extracted from deep inside the earth. Fuel A is a thick, dark, foul smelling liquid whereas fuel B is a black solid. Combustion of both the fuels produces products, C, D, E and F. The product C makes the rainwater only slightly acidic but it is mainly responsible for causing global warming. The product D is neither acidic nor basic. It is harmless and does not affect the environment in any way. Both E and F attack breathing system of humans and are mainly responsible for causing acid rain. In addition to combustion of fuels A and B, the product F is also formed when lightning occurs in the sky.

(a) What are (i) fuel A, and (ii) fuel B ? (b) Name (i) C (ii) D (iii) E, and (iv) F

(c) What is the process of formation of F during lightning known as ?

(d) Which fuel is supposed to be exhausted sooner : A or B ?

(e) Which fuel is mostly used for generating electricity at thermal power plants ?

A man bought a device X which could cook pulses, vegetables and rice without using any fuel like wood, coal, kerosene or LPG, etc. This device did not work at night. It also took a lot of time for cooking.

(a) Name the device X.

(b) What is the source of energy which cooks food in this device?

(c) What is the name of the process which traps energy in this device?

(d) State one advantage (other than saving on fuel) of using such a device for cooking food.