Separating mixtures
A mixture is two or more substances that are not chemically joined, so they can be separated by physical methods. The skill the exam tests is choosing the right method from the properties of the substances.
Part 1 of 3: Learn it
In short
- Filtration separates an insoluble solid from a liquid.
- Crystallisation gets a soluble solid out of a solution; simple distillation gets the solvent out.
- Fractional distillation separates liquids that mix, using their different boiling points.
Where this is in your specification
Spec points: AQA 4.1.1.2 (Combined Trilogy 5.1.1.2), Edexcel Topic 2, OCR C1.3
| Board | Topic: Atomic structure and the periodic table |
|---|---|
| AQA 8462 | 4.1 (8462); 5.1 (8464) |
| Edexcel 1CH0 | Topics 1, 2 and 6 |
| OCR J248 | C1 and C2.1 |
| Eduqas C410 | 1, 2, 4 |
| Cambridge IGCSE 0620 | 1.1, 1.2, 2.1 to 2.3, 8.1 to 8.5 |
| National 5 C813 75 | 1(b) Atomic structure and bonding related to properties |
Choosing a method
| You want | Method | Example |
|---|---|---|
| an insoluble solid from a liquid | filtration | sand from water |
| a soluble solid from its solution | crystallisation | copper sulfate crystals from copper sulfate solution |
| the solvent from a solution | simple distillation | pure water from sea water |
| liquids that mix but boil at different temperatures | fractional distillation | ethanol from water; fractions from crude oil |
| the dissolved substances in a mixture such as ink | chromatography | the dyes in a felt-tip pen |
Crystallisation
- Heat the solution gently in an evaporating basin to drive off some of the water.
- Stop when crystals start to form at the edge (or a drop on a cold glass rod crystallises).
- Leave it to cool so more crystals form, then filter them off and dry them.
Distillation
In simple distillation, the solution is heated, the solvent boils off, the vapour cools in a condenser and drips into a beaker as pure liquid. The thermometer at the top shows the boiling point of what is distilling over. Fractional distillation adds a fractionating column, so liquids with closer boiling points can be separated: the one with the lowest boiling point comes over first.
Which method would you use to get pure water from salty water?
Show the answer
Simple distillation.
Part 2 of 3: See it worked
Worked examples
Example 1
Describe how to get pure salt from rock salt (salt mixed with sand and grit).
- Grind the rock salt and stir it into warm water so the salt dissolves
- Filter: the sand and grit stay on the filter paper and the salt solution passes through
- Heat the filtrate to crystallise the salt, then filter off and dry the crystals
Answer: Dissolve, filter, then crystallise.
Example 2
Ethanol boils at 78 °C and water at 100 °C. Which method separates them, and which liquid is collected first?
- The two liquids mix completely, so filtration will not work
- They have different boiling points, so use fractional distillation
- The liquid with the lower boiling point distils over first
Answer: Fractional distillation; ethanol is collected first.
Common mistakes
- Choosing filtration to separate a dissolved solid. Dissolved particles pass through filter paper.
- Saying distillation separates by melting point instead of boiling point.
- Evaporating all the water when the question asks for crystals.
- Calling a mixture a compound. In a mixture the substances are not chemically bonded.
Why does filtration not separate salt from salt water?
Show the answer
The salt is dissolved, and dissolved particles pass through the filter paper.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Which method would you use to get pure water from salty water?
Simple distillation.
Why does filtration not separate salt from salt water?
The salt is dissolved, and dissolved particles pass through the filter paper.
In fractional distillation, which liquid comes over first?
The one with the lowest boiling point.
Jobs that use this
- Forensic scientist (opens a new tab)
- Chemical engineer (opens a new tab)
- Laboratory technician (opens a new tab)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses