Separation uses a physical difference between the components

1. The substances in a mixture are not chemically bonded, so each keeps its own physical properties.
2. A separation exploits one of those differences: particle size, solubility or boiling point.
3. A solution forms when a solute dissolves in a solvent, as salt does in water.
4. No new substance is made, so every separation here is a physical process.
5. Choosing a method starts with two questions: which property differs, and which component you want to keep.

Distillation separates by boiling point

1. Simple distillation recovers the solvent from a solution, such as water from salt water.
2. The mixture is heated, the component with the lower boiling point evaporates first, and its vapour passes into a condenser.
3. The condenser cools that vapour back to a liquid, which runs into a receiving flask as the distillate.
4. The dissolved solid stays behind in the distillation flask, because it does not evaporate.
5. Simple distillation also separates two liquids whose boiling points are far apart.
6. When the liquids are miscible and their boiling points sit close together, fractional distillation takes over.
7. A fractionating column sits between the flask and the condenser, and vapours condense and evaporate repeatedly as they rise through it.
8. Each of those steps enriches the vapour in the lower-boiling liquid, which makes the separation far sharper.

Filtration and crystallisation separate by particle size and solubility

1. Filtration separates an insoluble solid from a liquid, because those solid particles are too large to pass through the filter paper.
2. The liquid that passes through is the filtrate, and the solid held on the paper is the residue.
3. Dissolved particles are small enough to pass straight through, so filtration cannot remove a solute.
4. Crystallisation recovers a soluble solid from its solution.
5. The solution is heated gently so that some solvent evaporates and the solution becomes more concentrated.
6. Heating stops well before the solution is dry, and the concentrated solution is left to cool.
7. Crystals form as it cools, because less solute can stay dissolved at the lower temperature.
8. The crystals are then filtered off and dried between filter papers.

Chromatography separates by how far each component travels

1. Paper chromatography separates substances that dissolve in the same solvent, such as the dyes in an ink.
2. A pencil line is drawn near the bottom of the paper, because pencil does not dissolve and will not run.
3. A small spot of the mixture goes on that line, and the paper stands in solvent kept below it.
4. The solvent rises through the paper and carries the dissolved substances up with it.
5. Each substance travels its own distance, because they differ in solubility and in how strongly the paper holds them.
6. The separated spots left behind make a chromatogram.

Choosing the right technique

1. An insoluble solid in a liquid calls for filtration.
2. A soluble solid you want to keep calls for crystallisation.
3. A solvent you want to recover from a solution calls for simple distillation.
4. Miscible liquids with close boiling points call for fractional distillation.
5. Soluble substances to be separated or identified call for paper chromatography.
6. Sand, salt and water needs two stages: filter off the sand, then crystallise the salt from the filtrate.
7. The same mixture is handled differently depending on which component the question asks for.