CBSE Guess > Papers > Question Papers > Class X > 2009 > Science > Science

Q. 23. Name the major component of biogas. List two main advantages of using biogas over fossil fuels. 2

Ans. The major constituent of biogas is methane.
Advantages of biogas over fossil fuels:
(i) Biogas burns without smoke and hence does not cause air pollution.
(ii) Biogas burns completely without leaving behind any residue. So, it is a clean fuel.

Q. 24. How is ozone formed in the upper atmosphere? Why is the damage of ozone layer a cause of concern to us? State a cause of this damage. 2

Ans. Ozone is formed high up in the atmosphere by the action of ultraviolet radiation on oxygen gas by the following process:
Step 1. The high energy UV radiation running from the sun splits oxygen gas into free oxygen atoms

Step 2. The free oxygen atoms thus produced are very reactive. One oxygen atom reacts with an oxygen molecule to form an ozone molecule.

This layer of ozone prevents the entry of harmful UV radiation to the earth’s surface, hence the damage to ozone layer is a cause of great concern to the mankind.

Chlorofluorocarbons (CFCs), which are used in refrigerants, deodrandts and in fire extinguishers, are one of the major causes of depletion of ozone layer.

Q. 25. What is a dam? Write two main advantages and two ill-effects of constructing a big dam. 3

Ans. In order to make proper use of river water, dams are constructed across the rivers to regulate the flow of water. A dam has a large reservoir to store huge amount of water. This stored water is then allowed to flow downstream at the desired rate.

Science 2009 Question Papers Class X
Delhi		Outside Delhi		Compartment Delhi		Outside Delhi		Foreign
	Set 1 (PDF)		Set 1 (PDF)		Set 1		Set 1		Set 1 (PDF)
					Set 2		Set 2
					Set 3		Set 3