Sodium chloride added to water at 20 °C dissolves at first, but there is a limit. Once 100 g of water holds 35.8 g of sodium chloride, any more stays at the bottom as a solid, however long the mixture is stirred.
A solution that can dissolve no more of a solute at a given temperature is called a saturated solution. In it, the solid at the bottom keeps dissolving, but dissolved particles return to the solid at the same rate, so the amounts do not change.
The mass of solute that dissolves in 100 g of water to give a saturated solution is called its solubility, so the solubility of sodium chloride at 20 °C is 35.8 g per 100 g of water.
Solubility depends on temperature. A graph of solubility against temperature is called a solubility curve, and it shows at a glance how much of a substance a given mass of water can hold at each temperature.
The solubility of potassium nitrate rises steeply, from 31.6 g at 20 °C to 109 g at 60 °C and 245 g at 100 °C. That of sodium chloride hardly changes, from 35.8 g at 20 °C to 37.1 g at 60 °C.
Most solids dissolve better in hot water than in cold, but not all of them. Calcium hydroxide is less soluble in hot water, so saturated limewater turns cloudy when it is warmed.
A saturated solution of potassium nitrate at 60 °C holds 109 g in every 100 g of water. When it is cooled to 20 °C, the water can hold only 31.6 g, so the rest comes out of solution as crystals.
The mass of crystals is the difference between the two solubilities: 109 g - 31.6 g = 77.4 g from 209 g of saturated solution. The crystals grow as long needles while the solution cools.
For any other mass of solution, the masses scale in the same ratio. From 418 g of the saturated solution at 60 °C, which contains 200 g of water, 154.8 g of crystals form on cooling to 20 °C.
Potassium nitrate that contains a little sodium chloride can be purified in the same way. A mixture of 60 g of potassium nitrate and 5 g of sodium chloride is dissolved in 100 g of water at 60 °C, where both dissolve completely.
On cooling to 20 °C, only 31.6 g of potassium nitrate can stay dissolved, so 28.4 g crystallizes. Sodium chloride could dissolve up to 35.8 g, so all 5 g of it stays in solution.
Filtering separates the crystals of almost pure potassium nitrate from the solution, called the filtrate, which keeps the sodium chloride. Purifying a solid by dissolving it and letting it crystallize again is called recrystallization.
Gases behave differently from most solids, because they dissolve less as the temperature rises. Bubbles appear in water as it warms, and a fizzy drink goes flat faster when it is warm.
At a fixed temperature, the amount of a gas that dissolves in a given volume of liquid is proportional to the pressure of the gas. This is Henry's law. It holds for gases that dissolve only slightly and do not react with water.
At 20 °C and 1.013 × \mathrm{10^5} Pa, 1 L of water dissolves 1.38 × \mathrm{10^{-3}} mol of oxygen, and twice the pressure dissolves twice as much, 2.76 × \mathrm{10^{-3}} mol. Opening a fizzy drink lowers the pressure, so dissolved carbon dioxide forms bubbles.