Rain comes from evaporated water, much of which comes from the sea. So why isn’t rain salty?
• Recall the classic school science demonstration. Saltwater is heated or left to stand. The water then boils or evaporates, leaving salt crystals in the container.
Thomas Parker, Barry, Vale of Glamorgan, UK
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• A liquid boils when its vapour pressure – the pressure exerted by molecules escaping the liquid into gaseous form – is equal to atmospheric pressure. Pure water boils at 100 °C, molten sodium chloride at 1413 °C.
Evaporation can happen well below boiling point because molecules are constantly escaping from the surface of liquids (and even solids). The reason water evaporates more quickly on a hot day than on a cold day is because its molecules move faster at higher temperatures, so more escape from the surface. In other words, vapour pressure rises with temperature.
Spill some salt on the table and it does not evaporate, however. That’s because its vapour pressure is negligible at room temperature. So no salt will vaporise from the sea, and rain is therefore not salty.
Vapour pressure is affected by the strength of attraction between molecules in a liquid. The attraction between water molecules is due to hydrogen bonding – involving a hydrogen atom in one molecule being attracted to the oxygen atom in another. This effect is relatively weak, so at a given temperature, water has a high vapour pressure compared with salt (sodium chloride), in which there is a strong electrostatic attraction between sodium and chloride ions.
That said, water molecules attract each other more strongly than, say, methane molecules do – hence, liquid methane will boil at a chilly -164 °C.
The sodium and chloride ions in seawater do attract water molecules, making it harder for them to escape and reducing vapour pressure. So salty water boils at a slightly higher temperature than pure water, the size of the effect depending on the salt concentration.
Peter Borrows, Amersham, Buckinghamshire, UK