CBSE Set Qa2 Science Class X 1999 Sample Test Papers For Class 10th for students online

Science Class X (CBSE)
You are on Answers 21 - 33

Q.21 A 100 watt electric bulb is lighted for 2 hours every day and five 40 watt tubes are lighted for four hours every day. Calculate (i) the energy consumed for 60 days and (ii) the cost of electricity consumed at the rate of Rs. 1.50 per kwh.
Ans21. Power of bulb = 100 W
Time of use = 2 h
Power of tube = 40 W
Time of use = 4 h
Cost of electricity = 1.50 per kWh
                                              100 x 2
Electricity consumed per day kWh
                                               1000
                                                   100 x 2
Electricity consumed in 60 days x 60 kWh
                                                    1000
= 12 kWh

                                                              40 x 4
Electricity consumed by 5 tubes per day x 5 kWh
                                                                1000
                                                                     40 x 4
Electricty consumed by the tubes in 60 days x 5 x 60 kWh
                                                                       1000
= 48 kWh
Total consumption of electricity in 60 days = 12 + 48 kWh
= 60 kWh
The cost of electricity = 60 x 1.5 = 90.00 Rs.
Ans. the cost of electricity will be Rs. 90.00

Q.22 Describe the sequence of changes that would take place if all the hydrogen present in the sun is converted to helium
Ans22. 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.23 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.
Ans23. 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.24 List the characteristics which define the orbit of a satelite. Show their position in a diagram. When is a satellite is said to be in a sun synchronous orbit?
Ans24. The three characteristics which describe the orbit of an artificial satallite are:
1. Perigee
2. Apogee
3. Inclination


A satallite is said to be Sun-synchronous when with respect to the elevation of sun from a place on the earth, its position in the space is fixed. That means the position of the sun with respect to the point on the earth remains approximately the same as the satellite passes over it.

Q.25 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.
Ans25. 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.26 Explain how the following metals are obtained from their compounds by the redcution process

a) Metal X which is low in the reactivity series.

b) Metal Y which is in the middle of the series.

c) Metal z which is high up in the reactivity series.
Ans26. (a) A metal low in the reactivity series can be obtained by directly heating their compounds in air.
XS + air (O₂) X + SO₂

(b) A metal in the middle of the reactivity series needs be heated with a reducing agent, which is usually carbon. When metal oxide is heated with carbon, the metal is freed from oxygen.
YO + C Y + CO₂

(c) The metals high up the reactivity series can be obtained by electrolytic reduction. Fused salts of the metals are used as electrolytes and the freed and pure metal is obtained at the cathode.
ZO Z⁺⁺ + O^-2
Z⁺² + e^- Z at cathode
O^-2 1/2 O₂ at anode

Q.27 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.28 Describe with the help of a flow diagram the transfer of energy from sun to top carnivores.
Ans28. Ten percent law: The energy available at each successive level is 10 per cent of the previous level. The sole source of energy on the earth is the sun. Plants trap the solar energy with the help of chlorophyll by the process of photosynthesis and store it as chemical energy as carbohydrate. Plants utilize this energy for their metabolic activities, such as respiration and tissue building. They also use some of the enrgy as unutilized heat.

The Level I consumers, the herbivores consume the plants and the chemical energy stored into the plants is transferred to them. These animals also utilize some of this energy for metabolic activities, and waste some energy as heat.

The Level II consumers, the carnivores eat the herbivores and the chemical energy stored in the herbivores' body gets transferred to the carnivores. Again the same fates of energy follow.



Q.29 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.
Ans29. 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.30 Distinguish between bacteriocidal and bacteriostatic methods of food preservation. Give one example each of a food material preserved by these methods.
Ans30. (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.

Q.31 Explain the working of a four-stroke engine using petrol as fuel. Draw a neat labelled diagram of the ignition stage.
Ans31. The petrol engine works in four strokes, which are:
1. Intake Stroke: The mixture of Petrol fuel and air is let in the chamber through a nozzle.
2. Compression Stroke: The fuel-air mixture is compressed by the piston.
3. Working Stroke: This stroke involves Ignition, and Expansion of the fuel-air mixture. The compressed mixture is ignited by the spark of an electric Spark Plug. The fuel-air mixture burns and the burnt out gases expand out in the compresssion chamber. The expansion of the gases increaes their pressure, and the increased pressure pushes the piston back.The back movement of the piston moves the crank-shaft, which in-turn moves the wheel.
4. Exhaust Stroke: The exhaust valve is opened to let out the by-products of combustion.


     Intake      Compression     Working         Exhaust

Q.32 Name the chief impurity present in iron ore. What are the two requirements to obtain iron from its ore. Draw a labelled diagram of a blast furnace. State the main reactions that take place in the blast furnace. Write the chemical equations involved.
Ans32. Chief ore of iron is Red Haematite (Fe₂O₃), which has sand (SiO₂) as the main impurity in
it.The two raw material required in the extraction of iron from Haematite are:
1. Coke,
2. Lime Stone (CaCO₃)

The two requirements to obtain iron from its oxide ore are:
1. a reducing agent to reduce iron oxide to iron metal, and
2. a suitable solvent to dissolve the silicon oxide

Iron ore, limestone, and coke are put together in a big Blast furnace.
Blasts of preheated air are forced in from the bottom of the furnace, and the exhaust gases
escape out from the top. Powdered charge (Ore + limestone + coke) is fed from the top of the
furnace.

Fig.:- Blast Furnance9

Limestone gets dissociated because of intense heat in the furnace:
CaCO₃ CaO + CO₂
Calcium oxide reacts with sand to form calcium silicate:
CaO + SiO₂ CaSiO₃
Iron oxide thus freed from sand gets reduced by the help of carbon monooxide:
2 C + O₂ 2 CO
Fe2O3 + 3CO 2 Fe + 3 CO₂
The calcium silicate formed keeps the freshly formed iron away from oxygen. Calcium silicate being lighter than molten iron floats on the surface, and is separated as scum.

Q.33 Which nutrient serves as a ready source of energy in our body? Write its chemical formula. Write down the balanced equation for its complete oxidation. What test will you perform for its presence in a food sample ? Where is this nutrient stored in the body when present in excess?
Ans33. Sugars serve as a ready source of energy in our body.
Its chemical formula is C₆H₁₂O₆.
The balanced equation for the complete oxidation of glucose is:
C₆H₁₂O₆ + 6O₂ 6CO₂ + 6H₂O + 3070 kJ/mole

The presence of glucose in the food sample can be detected by heating the crushed water solution of the food with Benedict's solution in a test tube on the spirit lamp flame. The colour would change to brown red if glucose would be present in the food.
Glucose gets stored as glycogen in the Liver.