1.3 - Solubility & Solubility Curves

Solute, solvent, solution and saturated solution

• When a solid dissolves in a liquid, the liquid is the solvent and the dissolved solid is the solute.
• Together they form a solution - a mixture of solute and solvent that doesn't separate back out on its own.
• Soluble describes a substance that dissolves in a given solvent; insoluble describes one that won't. Common salt is soluble in water, but sand is not.

Saturated solutions

A saturated solution contains the maximum amount of dissolved solute possible at that temperature; no more solute will dissolve.

Solubility and solubility curves

• The solubility of a solid is the mass of it that dissolves in 100 g of water at a stated temperature to form a saturated solution.
• A solubility curve is a graph plotting solubility against temperature.

How temperature affects solubility

• For most solids, solubility increases as temperature increases.
• For gases, solubility decreases as temperature increases.

Calculating solubility from experimental data

1. Add excess solid to a known mass of water at a set temperature and stir until no more dissolves.
2. Filter the mixture, then dry and weigh the undissolved solid left on the filter paper.
3. Mass of solute dissolved = mass added − mass of dried excess.

---

Formula for solubility

Where:

• Solubility is in grams of solute per 100 g of water
• Mass of solute dissolved is in grams (g)
• Mass of water is in grams (g)

Worked example - Solubility from experimental data

30 g of sodium nitrate is added to 25 g of water at 20 °C and stirred well. The mixture is filtered, and the solid left on the filter paper is dried and weighed at 8 g. Calculate the solubility of sodium nitrate at 20 °C.