Which is an exothermic process?

Question

1. water evaporating from liquid to vapor
2. water freezing to ice
3. dry ice sublimating from solid to gas
4. water boiled from liquid to steam
5. ice melting from solid to water

Answer 1

An exothermic process is one in which the "system" loses energy. water evaporation requires the water to gain energy to separate from the other water molecules. The same is true for boiling water or melting ice. The new state of matter requires more energy to transition from it's original state. Dry ice sublimation also absorbs energy to go directly into the gas phase from the solid phase. No liquid CO2 is involved. For water to freeze into ice, energy must be removed so that the water molecules will orient into a shape that allows crystallization. That's an exothermic process - the water is giving up/releasing energy.

Bob