Science Class X (CBSE)
Sample Paper Set - VII (Solved)

Q.1 How does the conductivity of a semi conductor change when certain impurities are added to it ?
Ans4. Impurities increase the conductivity of the semi-conductors.

Q.2 Describe the sequence of changes that would take place if all the hydrogen present in the sun is converted to helium
Ans2. As the H-2 (deuterium) in the Sun's core is used up, the fusion reaction stops there, but it still continues in the outer shell. The core gets the pressure lowered and it contracts under its own gravity. The burning shell expands, increasing its surface area and reduces the intensity of radiated energy. At this stage, the star A enters the Red Giant phase.

Latter the red-giant star will lose its expanding outershell.The core of red giant star left behind would gradually condense into an exremely dense ball of matter. Due to the contraction of condensation, high temperature would be produced in the core. At this high temperature Helium present in the core would fuse to form higher elements like carbon and liberate a lot of energy.
The energy liberated by these fusion reactions cause the small core glow as White Dwarf Star until helium lasts into it.

Q.3 Mass of the earth is optimum to facilitate existence of life on it. What would have happened if the earth had less mass than at present.
Ans3. If the Earth would have been less heavier than it is now, it would have meant less planetessimals aggregated to make it. Thus there would have been less kinetic enegy liberated and also less radioactive disintegration, to release less heat in the earth's atmosphere. In those conditions earth
would not have melted to become differentiated like the one today. Also a smaller mass would have meant lesser gravitational force to keep the CO2 from eascaping out. In the absence of CO2 the solar radiations would not have been trapped to keep the earth atmosphere warm enough to sustain the origin of life on it. A heavier mass would have made the earths atmosphere too rich in CO2 to make it too hot to permit origin of life.


Q.4) Name the main constituents of liquified petroleum gas. Why is this gas considered a better domestic fuel than others ? (Give any four points) Give the calrofic value of LPG.
Ans4. LPG is a liquified mixture of ethane, propane, and butane. Its main constituent is butane which burns readily, giving a lot of heat.

LPG is a better domestic fuel than others because:
1. It has the highest calorific value after hydrogen.
2. Has an ignition temperature much higher than the room temperature and therefore will not burn spontaneously at the room temperature.
3. The rate of combustion is moderate, and does not explode on burning unlike hydrogen.
4. Has very low content of non-volatile material and does not produce obnoxious soot.
5. Does not produce any toxic gaseous bye-products.
6. Is available in sufficiently high amount.
7. Is easy to store and transport in liquified state throgh pipes and cylinders for domestic and industrial use.

Calorific value of LPG is 55 kJ/g

Q.6) What are the thermosetting polymers ? Give two examples. Why can they be not processed again ?
Ans27. Thermoseting polymers are: irreversible polymers which once set in a given shape on heating cannot be softened or melted on being reheated.
Two examples of thermosetting polymers are:
(1) phenol formaldehyde (bakelite)
(2) melamine formaldehyde (melamine)

They cannot be processed again because the long chains of polymers are cross-linked with each other. The linkages prevent the displacement of individual chains on being heated. Thus thermo- setting plastics do not suffer change in shape once they have been set.

Q.7) A doctor examined a child below the age of five years and fund cracks and scaly skin apart from wasting of muscles. Name the disease the child is suffering from. Mention two causes of its occurance. What diet should be given to over come this disease.
Ans7. The child is suffering from Marasmus.
Two causes of the occurrence of this disease are:
(1) weaning of babies from protein-rich breast milk to other diets which are deficient in protein and energy.
(2) Diet of the child deficient in proteins and carbohydrates.

The disease could be cured by giving the child a diet which is rich in protein: such as wheat, gram, peanuts, soybean, jaggery, or animal proteins.

Q.8) Distinguish between bacteriocidal and bacteriostatic methods of food preservation. Give one example each of a food material preserved by these methods.
Ans8. (1) Bactericidal Methods: The bacteria are eliminated completely by cooking and canning.
(2) Bacteriostatic Methods: The bacteria and their enzymes are rendered inactive by dehydration, salting, and pickling.