A water molecule is polar. Due to the uneven sharing of electrons in the hydrogen-oxygen covalent bond, the oxygen molecule is slightly negatively charged and the hydrogen atoms are slightly positively charged. Due to this, intermolecular forces exist between water molecules, named hydrogen bonds. Hydrogen bonds require more energy to break than regular vander waals forces, therefore when heating a sample of water up, each hydrogen bond must be broken to vaporise the sample, which requires more energy, resulting in a higher heat of vaporisation, compared to other molecules of similar molecular mass.