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1) For any physical state of a substance,the temperature remains constant during its change of state.
ANS:Heat is required to change the phase as well as to increase the temperature. During phase transitions, the heat supplied (called the latent heat) is fully utilised to change the state by overcoming the force of attraction between the particles. So, the temperature remains constant during the change of state.The temperature of any substance remain constant during the change of state because the heat energy supplied is used up in changing the state of matter and also this heat is used for breaking of several bonds or attractive forces.This heat, which does not raise the temperature of the body, is called Laten heat.
2)Water kept in an earthen pot becomes cool in Summer.
ANS:The water kept in an earthen pot remain cool even in summer because earthen pot has many small holes. Water seeps out through them and evaporates from the surface of the pot. The energy needed for evaporation is taken from the water kept in the earthen pot. As a result, water kept in earthen pot become cool.The water inside the clay pot is cool due to the evaporation process. The clay pot has tiny pores in the surface and water gets evaporated very quickly through these pores. Due to this evaporation process, the heat of the water inside the pot is lost, which lowers the temperature of the water.
3)We are able to sip hot tea from a saucer rather than from a cup.
ANS:The key to this answer is evaporation and the property of evaporation that it causes cooling. The more the surface area the more is the cooling.A liquid has a larger surface area in a saucer than in a cup. One of the factors affecting evaporation is surface area. Larger is the surface area more is the evaporation. Thus evaporation is faster in a saucer causing more cooling than in a cup. For this reason, we are able to sip hot tea or milk faster from a saucer than a cup.
4)Docter advise to put strips of wet cloth on the forehead of a person having high temperature.
ANS:When strips of wet cloth are put on the forehead of person having fever, the water in the cloth absorbs heat from hot forehead to get vaporise. It helps to decrease the extra heat in the body caused due to high fever consequently helps to reduce body fever.The water evaporates and absorb the heat from the body that causes cooling and lowering the body temperature.
5)The rate of diffusion of gases is faster in gases.
ANS:Rate of diffusion is faster in gases because of the following reasons:
- Intermolecular attraction is very low in gases.Therefore, they can move in random manner in the available space.
- Intermolecular attraction is very low in gases.Therefore, they can move in random manner in the available space.•Compared to the bonding force, the kinetic energy of molecules is quite high
- They quickly get diffused into the environment.
- The rate of diffusion is directly proportional to the kinetic energy of the particles of the diffusing object .The kinetic energy of the particles of gases is very high so rate of diffusion in gases is higher.
5)Water droplets occur on water trumblers containing ice cold water
ANS:Sometimes we see water droplets on the outer surface of the glass containing ice-cold water this is because the water vapour presents in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
6)People prefer to wear cotton clothes in summer.
ANS:We prefer to wear cotton clothes in summer because during summer we prespire more and sweat is released from our body. Thus, cotton being a good absorber of sweat, absorbs the sweat and exposes it to the sun causing it to evaporate and makes our body cool (evaporation causes cooling).
7)On a hot sunny day people sprinkle water on the roof or open ground.
ANS:When people sprinkle water on hot and dry roof on a sunny day, that water evaporates by taking energy from the ground, roof and surroundings. The evaporation of water causes a cooling effect because the large latent heat of vaporization of water helps to cool the hot surface.The particles on the Surface evaporates by gaining energy from the surroundings and this gives the Cooling Effect.
8)Ice at 0° is more effective in cooling their water at 0°.
ANS:Ice is more effective in cooling than water at the same temperature because ice can absorb latent heat as well as the heat energy in order to attain room temperature. Water at 0 C can absorb heat energy only to attain the room temperature.