Problem? ?18RQ The temperature of boiling water doesn’t increase with continued energy input. Why is this evidence that boiling is a cooling process?
Solution 18RQ When water gets boiled, the heat moves out of the system. This is the reason we say boiling as a cooling process. When any substance starts boiling, it goes through a phase change. Here the phase change is from liquid to vapour. The water vapours which became hot are continuously leaving the system. So, it does not matter how much heat we supply, it will not increase the temperature of water anymore. So, when we leave the system after giving it the boiling temperature, it will become colder as the water vapour is moving out by carrying lot of energy. As energy is flowing out, heat is flowing out, so it’s called a cooling process. But when we continuously supply heat, it stays in equilibrium. The moment we stop supplying heat, it will get cold. Eventually all water will be converted into vapour and the pot will be empty.