CBSE Guess > Papers > Question Papers > Class X > 2008 >Science > Science Theory

Q. 22. Why should we conserve forests? Suggest any two ways to conserve forests.

Ans. Forests are biodiverse hot spots. The main aim of conservation of forests is to try and preserve the biodiversity we have inherited. Loss of diversity may lead to loss of ecological stability. Two ways to conserve forests
(i) By silviculture. It means reforesting those forest lands from where large number of trees have been cut in a planned way.
(ii) By taking the help of the local people in conserving the forests.

Q. 23. What are fossils? What do they tell about the process of evolution?

Ans. See Q.6, Textbook Questions, Chapter 9. [Page 98

Q. 24. List four changes you would incorporate in your life-style in a move towards sustainable use of available resources.

Ans. Four changes I would like to incorporate in my life-style in a move towards a sustainable use of available resources –
(i) I will reduce the use of petrol by walking or cycling for short distances.
(ii) I will reduce the wastage of electricity by switching off unnecessary lights and fans.
(iii) I will send the discarded items of paper, plastic, glass and metal to the respective industries for making fresh paper, plastic, glass and metal objects.
(iv) I will ensure a continuous use of books by passing them on to my juniors.

Q. 25. Answer the following questions on the basis of the diagram of a biogas plant given below :
(a) What is biomass? How is biogas obtained from biomass?
(b) Why is biogas considered an ideal fuel?
(c) Name the parts labelled A, B and C in the diagram.

Ans. (a) The dead parts of plants and trees and the waste material obtained from animals e.g., cattle dung, sewage, crop residue etc. are called biomass. Biomass is the organic matter which is used as a fuel to produce energy. Biomass contains chemical energy in the form of carbon compounds. Biogas is produced by the anaerobic degradation of biomass in the presence of water but in the absence of air.

(b) Biogas is considered an ideal fuel because

(i) Biogas burns without smoke and hence does not cause air pollution.
(ii) Biogas has a high calorific value.
(iii) Biogas burns completely without leaving behind any residue. So, it is a clean fuel.
(iv) There is no storage problem for biogas as it is supplied by pipes directly from the gas plant.
(v) Biogas is cheaper than most other common fuels.

(c) A → Slurry, B → Manure, C → Gas tank

Science 2008 Question Papers Class X
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