When a vast number of water molecules are mixing freely in the liquid form, the positive poles are attracted to the negative poles by what amounts to static electricity. This electrostatic attraction is termed a hydrogen bond. It is about 20 times weaker than the H – O bonds within any one water molecule, but still gives rise to considerable, transient adhesion, which packs the water molecules closely together in the liquid state.
In contrast, as the temperature falls below about 4 ° C and ice begins to form, the hydrogen bonds between the molecules become longer lived. They eventually settle into a rigid, rather open framework, comprising a stack of tetrahedrons, in which each oxygen atom is hydrogen-bonded to four surrounding oxygen atoms. The more open, rigid framework of ice fixes the oxygen atoms further apart than they tend to be, on average, in liquid water – which explains why ice is less dense than water.
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Although some water molecules will spontaneously adopt the vapour form at all temperatures, water is nothing like as volatile as other similar molecules. This is fortunate, as it ensures the persistence of surface water in most climate zones typical of the Earth’s surface. It also explains why the wholesale conversion of water to vapour (i.e. by boiling to form steam, which is just the word for hot water vapour) occurs only at temperatures in excess of 100 ° C: far higher than would otherwise be expected for a molecule of its size and shape. Yet again, this proves crucial to the success of carbon-based