Water, water, everywhere…

Water exists almost everywhere on the Earth. In the oceans it can be thousands of metres deep. But it also occurs in the form of ponds and puddles (which we shall not dwell on too much), seas and lakes, streams and rivers, and ice caps and glaciers, the subject of our next chapter. Water makes up a significant percentage of the weight of all living creatures.

Only plants and animals that have become specialized in living in dry conditions by long millennia of evolution can last long without a drink. Why is water so important? The answer lies in its physical properties, which would be astounding if they were not so familiar. Across the range of temperatures found at the Earths surface, it can exist as a vapour (water vapour), a liquid (water) or a solid (snow or ice). The changes in phase between water and water vapour, driven by solar energy, are the key driver of the weather, as we saw' in the previous chapter. In addition, a water molecule has positive charge at one end and negative at the other (it is covalent, in chemistry-speak), which makes it a brilliant corrosive and solvent, constantly eroding away any material it meets and transporting it somewhere else. Not for nothing is there a spoof description of the Earth, written by a supposed visiting alien, claiming that the place is totally uninhabitable. Most of its surface is covered by this ferocious solvent while its atmosphere contains fatal amounts of the equally reactive and unpleasant chemical oxygen.

So how much of this desirable substance does the Earth actually contain? One recent estimate puts it at 1385 million cubic kilometres. If the Earth were perfectly smooth, this would provide enough for a sea 2700m deep covering the whole planet. This would make a fair swimming pool, but is trivial compared to the amount of rock that lies beneath, as the Earths radius is just over 6300km.

…Nor any drop to drink

Of this water, the vast majority is in the oceans and about 97 percent of it is saline. That means that it contains about 3.5 percent dissolved material, most of it common salt, sodium chloride. The usual technical word for something salty is saline. But because the salts dissolved in sea water include others beside sodium chloride, scientists prefer the term haline, for salts in general. In addition, a look at the detailed composition of sea water confirms waters powers as a solvent. Pretty wrell every element in the periodic table is there, albeit sometimes at a level of only a few parts per million. The solvents used in processes such as dry cleaning cannot even begin to compete. They are used only because they are better than water at leaving fabrics looking nice after cleaning, and at dissolving some organic chemicals.

Because most of the Earths water is salty, the 3 percent that is not is especially valuable, not least to human beings, for drinking and for other activities that need pure water. But much of this non-saline or “fresh” water is locked up in ice caps and glaciers, so the amount of available fresh water is even less.

The saltiness of sea water is far from constant. The salt and other dissolved material that it contains has mainly got there by river after being eroded from the land, although there are other sources such as undersea volcanoes. Sea areas like the Baltic, which is connected to the rest of the world's oceans by only a narrow channel, are less salty than the ocean at large. The Baltic gets most of its salt from occasional bursts of ocean water that make their way from the North Sea in storm-driven “saline pulses”.

At the other extreme, areas of the ocean that lose water are likely to become steadily more salty. For example, if you freeze salt water, the solid ice that forms is not salty, at least until you freeze the last little bit and the salt has no choice but to follow. This is good news for polar explorers, who can melt ice for drinking water. But it also makes polar waters saltier in the winter. By the same logic, sea water in the tropics is saltier because more water evaporates from the sea at higher temperatures. So salinity is at its lowest in temperate oceans and rises as you travel either north or south. In the same way, river water is less saline than ocean water, so sea water near the mouth of a major river will be less salty than water from mid-ocean. While the average salinity of sea water is about 3.5 percent, it falls to below 3 percent near major rivers. Water in the 1-3 percent salinity range is termed brackish. These lower ranges of salinity tend to occur in tidal estuaries. If the salinity goes over 4 percent the water is termed hypersaline. The best-known case is the Dead Sea, which is filled by rivers but empties only by evaporation, so the salt that finds its way there has no way out.