Intermolecular forces
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Why does water have a high melting point and boiling point compared to other small molecules?
Water molecules are relatively small, containing only 10 electrons, so the van der Waals forces in water are weak compared with molecules which possess larger numbers of electrons.
However, the “V-shaped” water molecule is extremely polar, and since it contains hydrogen bonded to the highly electronegative element oxygen, it is capable of forming hydrogen bonds with neighbouring molecules. Hydrogen bonds are about ten times stronger than other intermolecular forces.
In addition to this, since the oxygen atom has two lone pairs of electrons and is joined to two hydrogen atoms, each molecule can form four hydrogen bonds with other molecules. This means that the hydrogen bonding in water is more extensive than it is in, say, ammonia (NH3) or hydrogen fluoride (HF). For these reasons water has unusually high melting and boiling points.
You will find a good description of all this in Topic 9 of the Nuffield Students’ Book, particularly on pages 215 and 216, and the diagrams here should help you to understand these ideas better.
Igloo
Risk assessment
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updated: 25 August 2003
