Specific Heat and Heat of Vaporization

Temperature measures how fast water molecules are moving on average. But heat energy added to water does not always go straight into that motion. Water molecules are held together by hydrogen bonds, weak attractions between the slightly positive hydrogen of one molecule and the slightly negative oxygen of another. Breaking those bonds takes energy. So when you heat water, a lot of the incoming energy gets spent prying molecules apart rather than speeding them up. That is why water has a high specific heat, 4.18 joules per gram per degree C: it takes a large amount of heat to raise its temperature even a little. The ocean absorbs heat all day, but most of that energy breaks and reforms hydrogen bonds instead of raising temperature, so it stays cool while dry sand, which has a much lower specific heat, heats up fast.

Evaporation pushes this idea further. To turn liquid water into vapor, every hydrogen bond holding a molecule to its neighbors has to break completely, not just loosen. That takes a huge amount of energy per gram, water's heat of vaporization, about 2260 joules per gram. The molecules with the most kinetic energy are the ones that escape first, so when a sweat bead evaporates off your skin, it carries away that extra energy as it leaves, and the molecules left behind are cooler on average. That is evaporative cooling.

This also explains why boiling water sits at 100 degrees C. Once water reaches its boiling point, added heat stops raising kinetic energy and instead goes entirely into breaking hydrogen bonds to make vapor, so the thermometer holds steady until all the liquid converts. These same hydrogen bonds, covered right alongside polarity and cohesion, are the reason water behaves unlike almost any other small molecule.