solubility
/ sol-yoo-BIL-ih-tee /
Keep stirring sugar into a fixed glass of water and at some point no more will dissolve — the extra just settles on the bottom. You have hit a ceiling. Solubility is the name for that ceiling: the maximum amount of a substance that will dissolve in a given amount of solvent under set conditions. Past that point the solution is saturated and cannot hold any more.
Solubility is usually quoted as a quantity per amount of solvent at a stated temperature — for instance, grams of salt per 100 grams of water at 20°C. It is not all-or-nothing: substances span the range from freely soluble to nearly insoluble, and the same solute can be far more soluble in one solvent than another. The familiar guide is "like dissolves like" — polar solvents favour polar solutes, oily solvents favour oily ones.
Temperature usually changes the picture: most solids dissolve more readily in hot water, while gases dissolve less. This is why a cooling cup of hot tea can throw out sugar crystals, and why warming a soda makes it go flat. Solubility sets the practical limits on everything from formulating medicines to predicting which minerals the sea can carry.
About 36 g of table salt will dissolve in 100 g of water at 20°C. Add a 37th gram and it just sits on the bottom — the solution is saturated.
Solubility = the most that will dissolve before the solution saturates.
A saturated solution that is cooled very carefully can briefly hold more than its limit — a supersaturated state. It is unstable: a single seed crystal or a jolt makes the excess crash out at once, the trick behind some hand warmers and rock-candy kits.