Water stored in a mud pot remains cool because of a phenomenon called evaporative cooling.
Mud pots by their very nature include a number of very minute pores through which water can slowly ooze out. In effect, these pores increase the surface area of water and consequently increase evaporation. For water to evaporate it requires some energy which it takes as heat from the water itself. This results in lowering the temperature of the water in the pot. Liquid water can turn into water vapour in two ways – boiling and evaporation. Boiling takes place only at a particular temperature called boiling point, but evaporation takes place at all temperatures. While foe boiling heat is supplied from outside, for evaporation heat is taken from the water itself.
It is for the same reason, we feel cool when a wind blows and takes away the sweat from us.
The molecules of water or for that matter any liquid are in constant motion but all of them do not move with the same speed (kinetic energy). Their energies vary over a small range and the temperature we measure represents only the average kinetic energy of all the molecules.
Even when left undisturbed, the fast moving molecules escape from the surface and vanish into thin air. As a result, the mean kinetic energy or the temperature of the water is lowered. This process is facilitated by a large surface area as more number of molecules comes into contact with air. (That’s why coffee in a wide saucer cools faster than when kept in a narrow cup.) The pore in mud pots provide a large surface area for evaporation and thus bring about a significant cooling of water in the pot, especially when the outside temperature is hot as in summer.
An interesting observation could be the fact that the water in the pot can never become ice. This is because of the fact that the pitcher is not a closed system as it can also take up heat from its surroundings. Thus an equilibrium temperature is reached when the process of heat loss and gain are balanced. Further conversion of water (even at zero degrees Celsius) into ice would require a further removal of heat (80 calories/gram) to bring about a phase change (from solid to heat). This will only be possible by using another liquid as a refrigerant.
With the same principle Double layer earthen pots are also used for preserving food and vegetables as illustrated in the labelled diagram.
