Water

Water is a molecule of three atoms, and by the ordinary rules for a molecule that small it should be a gas at room temperature. Hydrogen sulfide, its exact structural analogue one row down the periodic table, boils at −60 °C. Water boils at 100. The 160-degree discrepancy is the single fact from which almost everything else about water follows, and the reason a planet covered in it has a liquid ocean rather than an atmosphere of steam.

It is also the substance industry uses more of than any other, by a margin nothing approaches. It is the solvent every leaching, flotation and electrowinning process runs in, the working fluid of essentially all thermal power generation, the coolant of most of the rest, and about a sixth of a concrete mix by volume.

The oxygen atom pulls the shared electrons hard, and the molecule is bent rather than linear, so one end carries a partial negative charge and the other two a partial positive. Each molecule can therefore hold hands with four neighbours — two through its own hydrogens, two through its lone pairs — and a hydrogen bond is roughly a twentieth the strength of a covalent bond but there are a great many of them.

Every anomaly is a consequence of that network.

It boils far too high, because escaping into vapour means breaking those bonds and not merely overcoming the weak attractions a molecule of its size would otherwise have.

Ice floats. The tetrahedral network in ice is more open than the jostling arrangement in liquid water, so the solid is about 9 per cent less dense than the liquid — and water reaches its maximum density at 4 °C rather than at freezing. Almost every other substance is denser as a solid and sinks. Lakes therefore freeze downward from the surface and do not freeze solid, which is not a small point about how life gets through a winter.

Its heat capacity is enormous, higher per gram than nearly anything else, because heat goes into breaking bonds rather than into motion. This is why the sea moderates climate, why water is the coolant of choice, and why the latent heat of vaporisation is large enough that sweating works.

Its surface tension is the highest of any ordinary liquid except mercury, which is what draws sap up a tree and holds a drop together.

It dissolves more kinds of thing than any other solvent, because the same polarity that lets it bond to itself lets it surround an ion and pull it away from its lattice. That is the property mineral processing is built on, and it is also the reason water is never found pure outside a laboratory.

As a reactant it is consumed rather than borrowed: in cement hydration, which is what makes concrete set and is not drying; in the hydration of ethylene to ethanol; in steam reforming, where it supplies the hydrogen for most of the world's ammonia.

As a solvent it is the medium of every hydrometallurgical route here — froth flotation, cyanidation, solvent extraction and electrowinning, the Bayer liquor, chlor-alkali brine.

As a working fluid it runs the thermal power cycle. Coal, gas, nuclear and concentrated solar all do the same thing at bottom: boil water, expand the steam through a turbine, condense it, repeat. That choice is made on water's latent heat and heat capacity, and no other cheap fluid comes close.

As a coolant it takes heat away from engines, reactors, machine tools and data centres, and its capacity here is why data-centre water consumption became a public argument.

And as the thing itself it is drunk, and drinking water is the least of it by volume: agriculture takes roughly seventy per cent of human freshwater withdrawal and industry most of the rest.

Water is not scarce and fresh water in the right place at the right time very often is, and conflating the two is the commonest error in the subject. About 2.5 per cent of the world's water is fresh, and most of that is locked in ice or deep groundwater.

The industrial questions are three, and they are different from one another.

Withdrawal against consumption. A power station withdraws enormous volumes and returns nearly all of it, warmer. An irrigated field consumes what it takes. A figure quoted without saying which is being measured is not informative, and both are routinely quoted as though they were the same number.

Aquifer depletion, where extraction exceeds recharge on a timescale of decades — the Ogallala, the North China Plain, the Punjab — and the water being used is effectively a mineral deposit rather than a renewable flow.

Contamination, which is where mining and chemicals sit. Acid mine drainage, tailings seepage, tannery chromium and PFAS all take water that was fine and make it not, and the volume affected is generally far larger than the volume used.

Desalination answers scarcity where energy is cheap and coastline is available, at a real energy cost and with a hypersaline brine to dispose of. It is a genuine solution to a local problem and not a general one.