Solubility and Saturated Solutions
Solubility is the maximum amount of a solute that dissolves in a given amount of solvent at a given temperature, and a saturated solution is one that already contains that maximum amount. In brief: at 20 °C about 36 g of sodium chloride dissolves in 100 g of water; adding more salt to that saturated solution leaves undissolved crystals at the bottom.
The idea of a limit on dissolving is the key to understanding many everyday phenomena, from sugar sinking to the bottom of a cup of tea to crystals growing from a cooling solution. This article explains saturated, unsaturated and supersaturated solutions, how solubility changes with temperature, how to read a solubility table and how to calculate whether crystals will form.
What is solubility?
Solubility is usually stated as grams of solute that dissolve in 100 g of solvent (often water) at a specified temperature. Because it depends on temperature, the value is meaningless without it. Solubility also depends on the nature of the solute and the solvent. Table salt dissolves well in water, while oil does not.
Chemists use descriptive terms in addition to numbers. A substance is soluble if a considerable amount dissolves, slightly soluble if only a little dissolves and insoluble if practically none dissolves. In reality no substance is completely insoluble, because even a tiny quantity dissolves.
Unsaturated, saturated and supersaturated solutions
- Unsaturated: contains less solute than the maximum. More solute added will dissolve.
- Saturated: contains exactly the maximum amount at that temperature. Added solute stays undissolved, while dissolving and crystallizing occur at the same rate.
- Supersaturated: contains more solute than the normal maximum. It is unstable and can form crystals suddenly if a seed crystal or dust is added.
A supersaturated solution is made by dissolving solute in hot solvent and cooling it carefully without disturbance. Hand warmers made from sodium acetate use this idea: clicking a metal disc starts crystallization, which releases heat.
How does temperature affect solubility?
For most solid solutes, solubility increases as temperature rises, but the size of the effect varies greatly. Potassium nitrate becomes much more soluble when heated, whereas sodium chloride is only slightly affected. A few solids, such as calcium hydroxide, become less soluble when heated.
Solubility data for common substances
| Substance | 0 °C | 20 °C | 40 °C | 60 °C | 80 °C |
|---|---|---|---|---|---|
| Sodium chloride | 35.7 | 35.9 | 36.4 | 37.1 | 38.0 |
| Potassium nitrate | 13 | 32 | 64 | 110 | 169 |
| Potassium chloride | 28 | 34 | 40 | 46 | 51 |
Values are rounded, and different sources differ slightly in the last digit.
Worked example: will crystals form?
A student dissolves 100 g of potassium nitrate in 100 g of water at 60 °C and then cools the solution to 20 °C.
- Is it dissolved at 60 °C? The solubility at 60 °C is about 110 g per 100 g of water. Since 100 g is less than 110 g, the solution is unsaturated and everything dissolves.
- What is the limit at 20 °C? About 32 g per 100 g of water.
- How much crystallizes? 100 g − 32 g = 68 g of potassium nitrate comes out of solution as crystals.
- Check with a different amount of water. If the solution had 200 g of water, the limit at 20 °C would be 64 g, and 100 g − 64 g = 36 g would crystallize.
This calculation is the basis of purification by recrystallization: the substance is dissolved in a hot solvent and crystallizes on cooling, while soluble impurities stay in solution.
What about gases?
The solubility of gases in liquids behaves differently. It decreases as temperature rises and increases as the pressure of the gas rises. A bottle of soda is filled with carbon dioxide at high pressure. When the bottle is opened, the pressure drops and the gas bubbles out. Warm soda loses its fizz faster than cold soda for the same reason. Lakes that warm up in summer hold less dissolved oxygen, which can stress fish.
Where does solubility matter?
- Medicine: a drug must dissolve to be absorbed.
- Industry: salts are purified by crystallization.
- Environment: dissolved oxygen levels determine whether aquatic life thrives.
- Cooking: sugar dissolves better in hot tea than in iced tea.
How was solubility studied?
Chemists in the 1700s and 1800s compiled tables of solubility for many salts, which were essential for analysis and manufacturing. The English chemist William Henry formulated a law in 1803 that connects the amount of dissolved gas to its pressure. Later, the concept of solubility equilibrium, in which dissolving and crystallizing are in balance, became part of the study of chemical equilibrium.
Which mistakes are common?
Mistake 1: Mixing up saturated and concentrated
A saturated solution of a weakly soluble substance can be quite dilute. Saturated means at the limit, not strong.
Mistake 2: Stating solubility without a temperature
Solubility has no meaning unless the temperature is given.
Mistake 3: Assuming that heat always increases solubility
This is true for most solids but false for gases and a few solids.
Mistake 4: Thinking that undissolved solid means the solution is weak
If solid remains in the bottom, the liquid above it is saturated, regardless of how much solid is left.