What is a coolant? Why is water a coolant?
A coolant is a substance that can absorb heat from a heat source. Any substance at a temperature lower than the source of heat can absorb heat and act as a coolant. The heat is absorbed till the temperature of the coolant and the heat source reaches the the same level.
But an ideal coolant is one that has the capacity to absorb a large amount of heat; this allows a small mass of coolant to be used to absorb a lot of heat. The coolant should also be able to give up the heat that has been absorbed relatively easily. These are the characteristics that are found in coolants used in refrigerators and air conditioning units. The coolant which is a gas absorbs heat from the interior and easily gives it up to the exterior when there is a change in the pressure.
Water is used as a coolant as it has a very high specific heat capacity. This implies that a lot of heat is needed for a small increase in the temperature of the water. This allows a small amount of liquid water to be used to absorb a large amount of heat before it turns to vapor.