Hydrogen bonds are permanent dipole-dipole interactions between the hydrogen in a H-F, H-N or H-O bond and an electronegative atom containing a lone pair such as oxygen, nitrogen and fluorine and are the strongest type of intermolecular interactions having some covalent character. The shape around the hydrogen atom is linear.Water has anomalous properties due to the hydrogen bonding between its molecules. In ice, hydrogen bonds hold the water molecules in an open lattice structure so the water molecules are further apart than they are in the liquid state and ice is less dense than water.Water has relatively high melting and boiling points due to the hydrogen bonding. The boiling point of water without hydrogen bonds would be about -75°C. Other properties of water that are due to hydrogen bonding are high surface tension and high viscosity.Hydrogen bonding between nitrogenous bases on the DNA molecule is also responsible for holding the two strands of a DNA molecule together in a double helix.