Hydrogen bonds are weaker than covalent bonds, but in large numbers they require a lot of heat energy to break apart. Water molecules attach to each other at the hydrogen's positive pole and the oxygen's negative pole, forming bonds that are not necessarily stronger than covalent bonds, but instead very stable. The heat energy required to break these bonds is mostly used up in the breaking, and so little heat is left to transfer into the water; the result is that water can absorb a lot of heat before the average temperature begins to rise. This also keeps water in liquid form at higher/lower temperatures than other compounds, specifically because of the stability of the hydrogen bonds. Water vapor forms when water is heated because most of the energy is spent in breaking the hydrogen bond, leaving the covalent bonds intact; the newly gaseous water molecules escape the liquid water and rise as vapor, losing most of the absorbed heat in seconds due to convection with the surrounding atmosphere. This also allows condensation from the atmosphere, where gaseous water molecules lose their heat and collect together through newly-formed hydrogen bonds.
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