Separating Mixtures

Choosing a separation method

• A soluble solid mixed with an insoluble one, such as salt and sand: add the mixture to water in a beaker, stir with a glass rod and heat to dissolve the salt, filter to remove the sand, then crystallise the salt from the filtrate.
• When heating a solution, wear safety spectacles.

Filtration

• Filtration separates an insoluble solid from a liquid or solution.
• Apparatus: a filter funnel containing filter paper, in a conical flask to collect the filtrate. Always draw and name the funnel.
• The sand collects in the filter paper: the solid left in the paper is the residue, the liquid that passes through is the filtrate.
• To get a clean, dry solid: rinse the residue with water to remove the solution of soluble substances left on it, then leave to dry. Warm it to evaporate the water.
• Solid still in the liquid after it has settled: filter it.

Crystallisation

Crystallisation gets a soluble solid from its solution.

Simple distillation

• Simple distillation gets the liquid from a solution, such as pure water from salt solution.
• Apparatus: the solution in a flask over a heat source, a thermometer at the top of the flask, a condenser, and a beaker to collect the liquid.

1. The solution is heated.
2. The water evaporates.
3. The vapour cools in the condenser.
4. The vapour condenses.
5. Pure water collects in the beaker.

• The two changes, in this order: evaporation in the flask, then condensation in the condenser.
• The thermometer reads the boiling point of the liquid being collected: 100 °C for water.

Fractional distillation

Fractional distillation separates liquids with different boiling points: the liquid with the lowest boiling point is collected first, and the liquid with the highest boiling point last. The temperature of the cooling water increases as it passes through the condenser.