3 Consequence 1: hydrogen bonds

• Because of polarity, water molecules attract one another, and also attract the polar molecules of other substances.
• This kind of bonding is called hydrogen bonding.

Hydrogen bond (between water molecules): an electrostatic attraction between the partially negative oxygen atom (δ⁻) of one water molecule and the partially positive hydrogen atom (δ⁺) of a neighbouring water molecule.

Key points:

• It is an intermolecular attraction (between molecules), much weaker than a covalent bond (roughly 4–5 % of its strength: ≈ 20 kJ/mol versus ≈ 460 kJ/mol for an O–H covalent bond), but there are enormous numbers of them.
• Each molecule of liquid water can be surrounded by up to four hydrogen bonds with neighbouring water molecules (it gives two through its two hydrogens, and receives two at its oxygen lone pairs). This creates a network of hydrogen bonds that constantly breaks and reforms in liquid water.
• In ice the network is a fixed open lattice (important for Module 3).
• The network gives water many of its unique properties: high boiling point, high specific heat, high latent heat of vaporization, cohesion and surface tension, the density behaviour of ice.
• Hydrogen bonds also enable water to dissolve many salts and break them into hydrated ions (ions surrounded by water molecules).