Heat

Q1. Define the following 1. Kilocalorie:- Kilocalorie is the quantity of heat required to raise the temperature of 1 kg of water through 10C. from 14.50C to 15.50C. 2. Specific Heat Capacity:- The specific heat capacity of substance is defined as the quantity of heat required to raise the temperature of unit mass of the substance through 10C. 3. Dew Point:- The temperature at which air becomes saturated with water vapour is called dew point. 4. Absolute humidity:- The mass of water vapour present in one cubic meter of air is called absolute humidity. 5. Principle of heat exchange:- If the system of two bodies is kept isolated from the surrounding so that no heat enters or leaves the systems, then Heat lost by hot body = Heat gained by cold body. 6. Relative humidity:- Relative humidity is the ratio of actual mass of water vapour present in a given volume of air to the mass of water vapour required to saturated the same volume at the same temperature. 7. Unit of specific heat capacity:- In M.K.S = K cal / kg 0C, in C.G.S = cal /g 0C. Q2. Give scientific reasons:1. If a piece of ice is dropped into a glass of water, it floats on the surface of the water. Ans. 1. Ice is the solid form of water but is density is much less than that of water. 2. We know as water – cools below 40 C its volume increases. 3. As a results density of ice is less that that of water and so it floats on the surface of the water. 2. In winter the pipeline carrying, water burst in cold countries. Ans. 1. In cold countries during winter, the temperature of the atmosphere fall below 00 C. 2. When the temperature of water falls below 40 C, it expands. Hence, the water in pipes freezes and expands. As there is no room for expansion, water exerts a large pressure on the pipes. Hence, the pipes sometimes burst. 3. In cold countries during winter even through the lakes are frozen, fish and other aquatic animals and plants can survive in it. Ans. 1. In cold countries, during winter, the temperature of the atmosphere falls well below 00 C. 2. As the temperature decreases, the water at the surfaces of lakes and ponds starts contracting.

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3. Hence, its density increases and it sinks to the bottom. This process continues till the temperature of all the water in the lake falls to 40 C. 4. As the water at the surface cools further, its temperature falls below 40 C, it starts expanding instead of contracting. 5. Therefore, its density decreases and it remains at the surface. The temperature of the water at the surface continues to fall to 00 C. 6. Finally, the water at the surface is converted into ice, but the water below the layer of ice is at 40 C. 7. Ice is a bad conductor of heat. Hence, the layer of the ice at the surface does not allow transfer of heat from the water to the atmosphere. 8. As the water below the layer of ice remains at 40 C, fish and other aquatic animals and plants can survive in it. Q 3 . Answer the following questions:1. What is ‘Anomalous behavior of water’? (OR) Describe its role in the survival of the aquatic plants and animals in cold countries. Ans. 1. Generally, liquids expands on heating and contract on cooling. Water, however shows an exceptional behaviors below 40C. 2. If water at room temperature is cooled continuously, it goes on contracting till its temperature falls to 40C. 3. But as its temperature falls below 40C, it starts expanding instead of contracting. 4. Similarly, if water is heated form 00 C to 40C, it contracts instead of expanding. 5. This exceptional behaviors of water is known as the anomalous behaviors of water. 6. In cold countries, during winter, the temperature of the atmosphere falls well below 00 C. 7. As the temperature decreases, the water at the surfaces of lakes and ponds starts contracting. 8. Hence, its density increases and it sinks to the bottom. This process continues till the temperature of all the water in the lake falls to 40 C. 9. As the water at the surface cools further, its temperature falls below 40 C, it starts expanding instead of contracting. 10. Therefore, its density decreases and it remains at the surface. The temperature of the water at the surface continues to fall to 00 C. 11. Finally, the water at the surface is converted into ice, but the water below the layer of ice is at 40 C. 12. Ice is a bad conductor of heat. Hence, the layer of the ice at the surface does not allow transfer of heat from the water to the atmosphere. 13. As the water below the layer of ice remains at 40 C, fish and other aquatic animals and plants can survive in it.

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2. Write the unit of heat in M.K.S and C.G.S system Ans. Unit of heat in M.K.S. system is Kilocalorie. C.G.S system is Calorie. 3. Explain the formation of dew and fog. Ans. During the day time the quantity of water vapour in air is not sufficient to saturate the air. During the cold night temperature may fall to dew point. At this temperature air becomes saturated with water vapour. As the temperature falls below the dew point, water vapour in air condenses on the surface of cold bodies and dew is formed. Sometimes condensation of water vapour occurs on the fine dust particles present in the atmosphere and this gives rise to the formation of fog or moist. 4. Explain Hope’s apparatus with the help of neat and labeled diagram. Ans. 1. Initially, the cylindrical vessel in Hope’s apparatus is filled with water at about 2.

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100C and the trough is filled with a freezing mixture of ice and salt. Initially, both the thermometers show the same temperature. In a short time, the temperature shown by the lower thermometer starts decreasing, while the temperature shown by the upper thermometer remains almost unchanged. This process continues till the temperature shown by the lower thermometer shown by the lower thermometer falls to 40C and remains constant thereafter. This shows that in the temperature range 10 0C to 40C, the density of the water in the central part of the vessel goes on increasing and hence the water sinks to the bottom. It means that water contracts as its temperature falls from 100C to 40C. As the temperature of the water in the central part of the vessel becomes less than 40C, the temperature shown by the upper thermometer begins to fall rapidly to 0 0C. After some time, a thin layer of ice is formed at the surface of the water in the vessel. But the temperature shown by the lower thermometer remains constant (i.e. 40C) In the temperature range 40C to 0 0C, the water moves upward. This shows that the density of water goes on decreasing in this range. It means that water expands as its temperature falls from 40C to 00C. Thus the volume of a given mass of water is minimum at 40C, the density of water is maximum at 40C.

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5. A piece of ice is melted by heating it, what will be the effects on its mass, volume, density and molecular structure. Ans. Mass Remains the same Volume Increases Density Decreases Molecular structure Regular arrangement of the molecule is broken and molecule are free to move inside the liquid. i.e. Ice is converted into (liquid) water. 6. Explain principle of heat exchange. Ans. Principle of heat exchange 1. When a hot body and a cold body are kept together, the temperature of hot body goes on decreasing while that cold body goes on increasing until both bodies attain the uniform temperature. 2. In this process the hot body losses heat and the cold body gains heat. If no heat enters or leaves the system then, Heat lost by hot body = Heat gained by cold body. Application :- Principle of heat exchange is used to determine specific heat capacity of any substance with the help of calorimeter. 7. Give two examples of the anomalous behaviour of water with explanation. (or) State two effects of anomalous behaviour of water observed in nature. Ans. 1. Some times water enters into crevices of the rocks. When the temperature of the atmosphere falls below 40C, water expands. As there is no room for expansion, it exerts a tremendous pressure on the rocks, which breaks up into small pieces. 2. In cold countries, in winter, when the water of lakes freezes, the ice formed floats on the surface of water. Hence, the aquatic animals and plants in the water below the layer of ice are protected from being frozen to death.

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