Illustrated Physics
physics, drawn
Concept

Water activity — where it appears

The ratio of a solution's vapour pressure to that of pure water at the same temperature, a direct measure of water's chemical potential in it. It equals 1 for pure water and about 0.75 for saturated salt, and it decides which way water moves between a solution and the air around it.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Salt that takes in water from the air at three-quarters humidity. The water held by a crystal of common salt, in grams per gram of salt, against the relative humidity of the air around it at 25 °C, with the humidity rising (solid) and falling (dashed). Rising, the crystal stays dry until 75.2 per cent and then dissolves at once into a drop holding 2.79 g of water per gram — the saturated solution — which goes on drinking as the air gets damper: 4.24 g at 85 per cent, 11.5 g at 95. Falling, the drop does not crystallise at 75 per cent. It sheds water along the same curve, becomes supersaturated, and stays liquid down to about 45 per cent before a crystal finally nucleates and it dries out at once.

The salt that dissolves in the air's own water

Leave a dish of salt in a bathroom and it does not just clump; on a humid enough day it turns into a puddle, though no water was poured on it. Above 75 per cent humidity, common salt pulls water out of the air until it has dissolved itself. The number is not a property of salt's attraction for water in any vague sense. It is the vapour pressure of a saturated salt solution, read as a fraction of pure water's: wherever the air holds more water than that, the solution is the state with the lower chemical potential. Bring the humidity back down, and the puddle does not turn back into salt until the air is far drier — about 45 per cent.

thermodynamics · Chemical potential Colder until what comes in equals what goes out. Heat flowing into a wet surface from warmer air, and heat carried off as vapour, against the surface's temperature, for air at 35 °C and 40 per cent humidity. The first falls linearly as the surface warms. The second climbs along the exponential of the vapour-pressure curve. There is one crossing, 24.0 °C, and it is stable: a warmer surface loses more than it gains and cools; a colder one gains. That crossing is the whole theory of the wet bulb.

The thermometer that reads low when wet

Wrap a wet cloth around a thermometer, and on a dry 45 °C day it reads 23 °C. It stays there for as long as the cloth is wet. Blow on it, and the reading does not change, because wind speeds heat arriving and water leaving by the same factor. Lines of constant wet-bulb temperature have now become some of the most closely watched in climate science, and the one at 35 °C marks a limit no body can sweat past.

thermodynamics · Chemical potential

Named alongside it

The objects these essays reach for when they reach for this one.

Chemical potentialDeliquescenceHysteresisLatent heatLewis numberNucleationRaoults lawRelative humiditySupersaturationVapour pressure

All concepts