Mixture separation Techniques

Before you can separate a mixture, you need the vocabulary of dissolving. These four terms appear as short definition questions almost every series.

• Solvent — the liquid that does the dissolving (e.g. water).
• Solute — the substance (usually a solid) that gets dissolved in the solvent (e.g. salt).
• Solution — the mixture formed when a solute dissolves in a solvent (e.g. salt water). A solution is clear (see-through) — not necessarily colourless, but you cannot see separate particles.
• Saturated solution — a solution in which no more solute will dissolve at that temperature. Any extra solute just sits on the bottom undissolved.

The relationship in one line: solute + solvent → solution.

A common everyday example: stir salt (the solute) into water (the solvent) and you make salt water (the solution). Keep adding salt and eventually it stops dissolving — now the solution is saturated.

Why "at that temperature" matters. Most solids dissolve more in hot solvent than cold. So a hot saturated solution holds more solute than a cold one — this is the whole idea behind crystallisation, which you'll meet below.

Filtration. Separates an insoluble solid from a liquid (or from a solution).

• Apparatus: filter paper folded into a filter funnel, sitting above a clean conical flask.
• Pour the mixture through. The insoluble solid is trapped on the filter paper — this is the residue. The liquid that passes through is the filtrate.
• Example: sand + water. Sand is the residue; the water is the filtrate.

Crystallisation. Recovers a soluble solid (the solute) from its solution.

1. Pour the solution into an evaporating basin.
2. Heat gently (a water bath gives the best crystals and stops spitting). The solvent evaporates, so the solution becomes more concentrated.
3. Stop heating when the solution is just saturated — test by dipping a cold glass rod in: if small crystals form on it, it is ready. Do NOT evaporate to dryness.
4. Leave to cool. As the solution cools it can hold less solute, so the solute comes out of solution as crystals.
5. Filter off (or pour away) the remaining liquid, then dry the crystals between filter paper or in a warm oven.

Why not boil dry? If you heat all the way to dryness on a Bunsen flame, the crystals can scorch or decompose (and you lose the regular crystal shape). Stop at the point of crystallisation and let cooling do the rest.

Putting them together — salt + sand. This classic question needs both techniques because salt dissolves but sand does not:

1. Add water → the salt dissolves, the sand does not.
2. Filter → sand is the residue; salt solution is the filtrate.
3. Crystallise the filtrate → recover pure, dry salt.

Simple distillation recovers the pure solvent (the liquid) from a solution, when the dissolved solute has a much higher boiling point and stays behind.

• Apparatus: a flask of the solution (with anti-bumping granules to stop violent boiling), a Bunsen below, a thermometer at the side-arm, a Liebig condenser (cold water flows through the outer jacket) and a collection beaker.
• Mechanism: the solvent boils, the vapour passes into the condenser, the cold water cools it so it condenses back to a liquid, and the pure liquid is collected. The non-volatile solute stays in the flask.
• Example: getting pure water from salt water. The water boils at 100 °C, travels through the condenser and is collected as pure (distilled) water; the salt is left behind in the flask.

Watch the detail: the thermometer bulb is placed level with the side-arm, so it reads the temperature of the vapour passing over — not the boiling liquid below. Cold water enters the condenser at the bottom and leaves at the top (countercurrent), so the vapour meets the coldest glass last and condenses fully.

Fractional distillation separates a mixture of miscible liquids — liquids that mix completely — that have different boiling points.

• Apparatus: the same as simple distillation but with a fractionating column (packed with glass beads or plates) between the flask and the condenser. The thermometer sits at the top of the column.
• Mechanism: the column has a temperature gradient — hot at the bottom, cooler near the top. Both liquids evaporate, but the one with the higher boiling point keeps condensing back onto the cool glass beads and trickles back down to the flask. Only the liquid with the lowest boiling point reaches the top, passes into the condenser and is collected first.
• Example: separating ethanol (bp 78 °C) from water (bp 100 °C). When the thermometer reads about 78 °C, ethanol is coming over and is collected. Once the thermometer climbs above 78 °C, water is starting to come too — so you change the receiver.

Simple vs fractional — how to choose:

	Simple distillation	Fractional distillation
Separates	solvent from a solution	two+ miscible liquids
Solute boiling point	very high (stays behind)	both liquids boil and come over
Extra apparatus	none	a fractionating column
Example	pure water from salt water	ethanol from water

Industrial use. The same idea separates crude oil into useful fractions (petrol, kerosene, diesel) and separates the gases in liquefied air (nitrogen, oxygen, argon), using tall columns with a temperature gradient.

Most exam questions give you a mixture and ask which technique recovers a named substance. Work through these questions in order:

1. Is the thing you want an insoluble solid in a liquid? → Filtration (e.g. sand from water).
2. Is it a soluble solid dissolved in a liquid, and you want the solid? → Crystallisation (e.g. salt from salt water).
3. Do you want the pure liquid (solvent) back from a solution? → Simple distillation (e.g. pure water from salt water).
4. Are there two (or more) liquids that mix, with different boiling points? → Fractional distillation (e.g. ethanol from water).

The most useful test: what is the difference in property you are exploiting?

• Filtration uses solubility / particle size (insoluble vs liquid).
• Crystallisation and distillation use boiling point (the solvent boils away or is condensed).
• Fractional distillation uses a difference in boiling points between two liquids.

If a mixture has both a soluble solid and an insoluble solid (e.g. salt + sand), you usually need two techniques in sequence: filter first, then crystallise the filtrate.