Hydrogen bonds in water

This shows a number of molecules of water - initially 4 molecules around a central one .

Hydrogen bonds can form between >O and -H of different water molecules - so one water molecule can have up to 4 of these, if surrounding water molecules are close enough.
Show ... hide H bonds ... Zoom out to show 2 extra molecules at different distances. ... Zoom back in.

These bonds cause water molecules to be attracted to one another, resulting in a number of physical effects that are significant in Biological contexts: high heat capacity and large latent heat of vaporisation, cohesive properties - important in movement of water in plants and surface tension - as well as the expansion as water freezes into ice - which therefore floats on water.

Increasing the temperature of water enables these electrostatic interactions to break so that individual molecules can move apart. Ice has the maximum number (4) of hydrogen bonds per H₂O molecule, and this number decreases below 4 as ice melts and becomes liquid. With extra heat the number presumably decreases steadily so when water becomes vapour or steam the value is 0.