Mixtures and separation
Mixtures
A contains two or more elements or compounds that are not chemically combined. Air (mostly nitrogen and oxygen) and sea water (water with dissolved salts) are both mixtures.
Each substance in a mixture keeps its own chemical properties, so mixtures can be separated by physical processes. No chemical reaction is needed, and no new substances are made.
This is the key difference from a compound, where the elements are chemically bonded and can only be separated by a chemical reaction.
Key terms
Separation techniques
| Technique | Separates | Example |
|---|---|---|
| An insoluble solid from a liquid | Sand from water | |
| A soluble solid from its solution, as crystals | Copper sulfate crystals | |
| Evaporation | A soluble solid from its solution, quickly | Salt from salt water |
| A liquid (the solvent) from a solution | Pure water from sea water | |
| Liquids with different boiling points | Fractions from crude oil | |
| Different soluble substances | Dyes in an ink |
Evaporating to dryness is quick, but some salts break down when heated strongly. Crystallisation is slower: heat until crystals start to form, then leave the solution to cool.
Key terms
Distillation
In , the solution is heated so the solvent boils. The vapour passes into a , where it cools and turns back into a liquid that is collected. The dissolved solid is left behind.
separates liquids that mix together but have different . A is fitted above the flask. It is hottest at the bottom and coolest at the top. Each liquid condenses and is collected when the temperature at the top reaches its boiling point, lowest boiling point first. It is used to separate ethanol from water and crude oil into fractions.
Purifying rock salt
Rock salt is a mixture of salt (sodium chloride, which dissolves in water) and sand and rock (which do not). To get pure salt:
- Grind the rock salt into a powder.
- Add water and stir, so the salt dissolves.
- the mixture. The sand stays on the filter paper as the ; the salt solution passes through as the .
- Evaporate the water from the filtrate to leave salt crystals.