Pure substances and separation techniques

Pure vs mixture

In chemistry, a pure substance is a single element or compound — a fixed composition with a sharp melting and boiling point. A mixture contains two or more substances not chemically joined; it can be separated by physical means and melts/boils over a range.

Filtration

Used to separate an insoluble solid from a liquid (e.g. sand from water).

• Pour through filter paper in a funnel.
• Solid (residue) stays in the paper; liquid (filtrate) passes through.

Crystallisation

Used to obtain a soluble solid from its solution (e.g. salt from salt water).

• Heat the solution to evaporate most of the water.
• Leave to cool; crystals form as the solution becomes saturated.
• Filter and dry the crystals.

Simple distillation

Separates a solvent from a solution (e.g. pure water from salt water).

• Boil the solution; vapour rises into a cooled condenser; condenses to pure liquid.
• Solute (salt) stays in the flask.

Fractional distillation

Separates two or more liquids with different boiling points (e.g. ethanol from water).

• A fractionating column lets the liquid with the lower bp pass through first.
• Different fractions collect at different temperatures.

Chromatography

Separates substances in a mixture by how strongly they interact with the paper (stationary phase) and the solvent (mobile phase).

Procedure:

1. Pencil baseline drawn 1 cm from the bottom (pencil so it doesn't dissolve).
2. Spot of mixture placed on baseline.
3. Paper dipped in solvent below the line.
4. Solvent rises by capillary action, carrying components at different rates.

R_f value = distance moved by spot / distance moved by solvent (always between 0 and 1).

WJEC exam tip

When choosing a method, identify what you're separating: solid + liquid (filter or crystallise depending on solubility), liquid + solute (distil), two liquids (fractional distillation), several dissolved colours (chromatography). State your reasoning, not just the method.