Water has the highest heat of vaporization per gram of any known liquid. A comparison of water with a few other common liquids is given below. The reason for the specially high value for water is the network of hydrogen bonds that holds the water molecules together, even in the liquid state.

In the "iceberg" or "flickering cluster" picture of liquid water structure, small clusters of water molecules are held together by hydrogen bonds in a more or less icelike manner (right). Bonds are made and broken continually, and one particular molecule may be bonded to others at one instant and free the next.

Throughout the liquid a certain percentage of the possible hydrogen bonds will be formed at a given moment. These bonds must be broken when the liquid is vaporized, and this requires energy. The result is a higher heat of vaporization than for liquids without hydrogen bonds.