How does the water kept in an earthen pot (matka) become cool during summer?
Hint: Consider latent heat of vaporization of water. It is the amount of heat energy needed to change the state of one mole (or one kilogram) of water, from liquid to gas, without changing the temperature. For water, the latent heat of vaporization is 40.8 kJ/mol.
Complete answer: During evaporation, water changes its physical state from liquid to gas. Some heat energy must be supplied for the evaporation of water. An earthen pot (matka) is made from mud particles and has minuscule pores in it. There are a large number of these extremely small pores. Through these pores, water can evaporate. This process of evaporation of water is continuous process and water keeps on evaporating slowly. The latent heat energy required for this evaporation comes from the earthen pot and the water present in the earthen pot. In other words, water present in an earthen pot loses some heat energy by evaporation. Loss of heat energy results in a cooling effect. Hence, in summer, to drink cold water, people keep drinking water in earthen pots.
Note: Do not simply write the answer that water placed in an earthen pot simply loses heat. Hence, it gets cooled. You need to explain about the evaporation process. Also, do not mix the concept of latent heat of vaporization with that of latent heat of fusion. Latent heat of vaporization is associated with change of state from liquid to gas, whereas, latent heat of fusion is associated with the change of state from solid to liquid.