When the heat is raised (for instance, as water is boiled), the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas. We observe this gas as water vapor or steam.
The more thermal energy added, the greater the attraction between molecules. The molecules gain kinetic energy, move farther apart, and become a gas. The water molecules become packed closer together, and their temperature increases. Each water molecule gives up kinetic energy and eventually stops moving.
