Solutions and solubility
A solution is a homogeneous mixture. The solute is the substance that dissolves, and the solvent is the substance it dissolves in. In salt water, salt is the solute and water is the solvent. A solution in water is called aqueous, written (aq).
Like dissolves like. Polar solvents dissolve polar and ionic solutes. Nonpolar solvents dissolve nonpolar solutes. Water is polar, so it dissolves salt and sugar but not oil.
When an ionic compound dissolves, water molecules surround the ions and pull them apart:
NaCl(s) → Na+(aq) + Cl−(aq)
Solubility is the maximum amount of solute that dissolves in a given amount of solvent at a given temperature.
- An unsaturated solution can dissolve more solute.
- A saturated solution holds the maximum. Extra solute stays undissolved.
For most solids, solubility rises with temperature. For gases it falls, which is why warm soda goes flat faster.
Worked example: a saturated solution
The solubility of KNO3 at 20 °C is 32 g per 100 g of water. If 40 g is stirred into 100 g of water, what happens?
Only 32 g can dissolve. The solution is saturated and 40 - 32 = 8 g stays undissolved.
Common mistake
Confusing how fast something dissolves with how much dissolves. Stirring and crushing speed up dissolving. They do not change the solubility.