Solutions

A solution is a clear, even mixture. When sugar dissolves in water, the tiny sugar particles spread out between the water particles so the mixture looks the same everywhere.

Key terms

• Solute: the substance that dissolves (e.g. sugar).
• Solvent: the liquid that does the dissolving (e.g. water).
• Solution: solute + solvent, evenly mixed.
• Aqueous: dissolved in water (e.g. aqueous sodium chloride).
• Soluble/insoluble: can/cannot dissolve in a solvent.
• Saturated solution: a solution that cannot dissolve more solute at that temperature.

How dissolving works

Solvent particles surround solute particles and pull them away from the solid. Think of friends spreading out in a room until no clumps remain. The mixture becomes homogeneous (the same throughout).

Making and separating solutions (practical)

• Choose a suitable solvent (water for many salts; other solvents for oily substances).
• Measure mass with a balance and volume with a measuring cylinder or volumetric flask.
• Stir to speed up dissolving; warming helps many solids dissolve faster.
• To get the solute back: use evaporation/crystallisation.
• To get the solvent back: use simple distillation.
• Filtration does not separate a true solution (particles are too small).

Concentration

How much solute is in a set volume of solution. In g/dm³:

$$\text{concentration} = \frac{\text{mass of solute (g)}}{\text{volume of solution (dm}^3)}$$

Conversions: 1 dm³ = 1000 cm³ = 1 litre. In moles: $c = \frac{n}{V}$ where $c$ is in mol/dm³, $n$ in mol, $V$ in dm³.

Factors affecting solubility

• Temperature: most solids dissolve more at higher temperatures; gases dissolve less when warm.
• Stirring and smaller particles speed up the rate of dissolving (but do not change how much can eventually dissolve).

Common misconceptions

• “Dissolved means gone.” False: particles are still present and can be recovered.
• “Filtering removes dissolved salt.” False: use evaporation or distillation.
• “Adding more solvent always dissolves more.” Only if the solution is not already saturated.