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

Q. 19. Why is ozone layer getting depleted at the higher levels of the atmosphere?

Ans. The depletion of ozone layer is due to the use of chemicals called chlorofluorocarbons widely used in refrigeration as coolant.

Q. 20. What kind of mirror — concave, convex or plane — would be best suited for use in a solar cooker? Why?

Ans. A concave mirror would be best suited for use in a solar cooker because a concave mirror converges a large amount of sun’s heat rays at its focus due to which a high temperature is produced in the focus area which is suitable for baking and frying also.

Q.21. List any four changes you would like to incorporate in your life-style in a move towards a sustainable use of available resources in our country.

Ans. Four changes I would like to incorporate in my life-style as 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 electrical appliances.
(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. 22. “Variations that confer an advantage to an individual organism only will survive in a population.” Justify.

Ans. The great advantage of variation to a species is that it increases the chances of its survival in a changing environment. For example the accumulation of heat resistant variation in some bacteria will ensure its survival even when the temperature in its environment rises too much due to a heat wave. On the other hand, the bacteria which did not have this variation to withstand heat would not survive.

Q.23. “The sex of the children is determined by what they inherit from their father and not the mother.” Justify.

Ans. The sex of the children is determined by what they inherit from their father and not the mother because the father has one X chromosome and one Y chromosome. Therefore half the male gametes will have X chromosomes and the other will have Y chromosomes.

a. The mother has two X chromosomes. Thus all the female gametes will have only X chromosomes.
b. If a sperm carrying X chromosome fertilises the egg then the child born will be a girl (XX).
c. If a sperm carrying Y chromosome fertilises an egg then the child born will be a boy (XY).

Q. 24. Explain the cause of shoots of the plant bending towards light.

Ans. The plant stem responds to light and bends towards it due to the action of auxin hormone.
(i) When sunlight comes from above, the auxin hormone present in the tip of the stem spreads uniformly down the stem. So the stem grows straight up.

(ii) When the sunlight falls on one side, the auxin hormone prefers to stay in shade as shown in the diagram.

Science 2008 Question Papers Class X
Delhi		Outside Delhi		Compartment Delhi		Compartment Outside Delhi
	All Sets (PDF)				Set 1		Set 1
					Set 2		Set 2
					Set 3		Set 3