Question: how much energy is needed to freeze the water?
Answer: It takes 100 calories to heat 1 g. water from 0˚, the freezing point of water, to 100˚ C, the boiling point. However, 540 calories of energy are required to convert that 1 g of water at 100˚ C to 1 g of water vapor at 100˚ C. This is called the latent heat of vaporization.