Pure substances and mixtures
A pure substance is a single element or compound with nothing else in it; a mixture contains more than one substance not chemically joined. The practical test for purity is the melting point: a pure substance melts sharply at one fixed temperature, while a mixture melts over a range and at a lower temperature. Chemistry is full of separating mixtures into pure substances — which method you use depends on what you are separating.
The separation methods
Each method exploits a different physical difference:
| Method | Separates | Uses the difference in |
|---|---|---|
| Filtration | an insoluble solid from a liquid | particle size (solid caught by the paper) |
| Crystallisation / evaporation | a soluble solid from its solution | the solid stays as the solvent evaporates |
| Simple distillation | a solvent from a solution | boiling point (vapour condensed back) |
| Fractional distillation | two or more miscible liquids | their different boiling points |
| Chromatography | a mixture of soluble/coloured substances | how strongly each is carried by the solvent |
Chromatography and Rf
In paper chromatography, a spot of mixture is carried up the paper by a solvent; substances that dissolve better and stick to the paper less travel further, so a mixture separates into a line of spots. It shows how many substances are present and can identify them.
Each substance has an Rf value — the distance it travelled divided by the distance the solvent travelled — which is constant for a given substance and solvent, so it acts like a fingerprint.
Testing for gases
Once you have made something, a few standard tests identify the common gases:
| Gas | Test and result |
|---|---|
| Hydrogen | a lit splint gives a squeaky pop |
| Oxygen | relights a glowing splint |
| Carbon dioxide | turns limewater milky |
| Chlorine | bleaches damp litmus paper |
| Ammonia | turns damp red litmus paper blue |
A worked example
You have salty water (salt dissolved in water) and want BOTH the pure water and the pure salt. What do you do?
- To get the pure water: simple distillation.The water boils off, is condensed and collected, leaving the salt behind.
- To get the pure salt: crystallisation (or evaporation).The salt is soluble, so it cannot be filtered; evaporating the water leaves the salt as crystals.
- Filtration would NOT work here.Filtration only removes an insoluble solid; dissolved salt passes straight through the paper.
Distil to collect the pure water; crystallise the residue to get the pure salt. Filtration fails because the salt is dissolved.
Testing for positive ions (cations)
A cation can often be identified twice over — by a flame test and by adding sodium hydroxide solution. In a flame, different metal ions give characteristic colours. With sodium hydroxide, many metal ions form coloured insoluble hydroxides whose colour identifies them; ammonium ions instead release ammonia gas on warming, which turns damp red litmus blue.
| Ion | Flame colour | With sodium hydroxide |
|---|---|---|
| Lithium Li⁺ | red | — |
| Sodium Na⁺ | yellow | — |
| Potassium K⁺ | lilac | — |
| Calcium Ca²⁺ | orange-red | white precipitate |
| Copper(II) Cu²⁺ | blue-green | blue precipitate |
| Iron(II) Fe²⁺ / Iron(III) Fe³⁺ | — | green ppt / red-brown ppt |
Testing for negative ions (anions)
Anions have their own set of confirmatory tests. A carbonate fizzes with dilute acid, releasing carbon dioxide that turns limewater milky. A sulfate gives a white precipitate with barium chloride (and dilute acid). The halides give coloured precipitates with silver nitrate: chloride white, bromide cream, iodide yellow. A nitrate releases ammonia when warmed with sodium hydroxide and aluminium foil.
| Ion | Test | Positive result |
|---|---|---|
| Carbonate CO₃²⁻ | add dilute acid | fizzes; CO₂ turns limewater milky |
| Sulfate SO₄²⁻ | acid + barium chloride | white precipitate |
| Chloride / bromide / iodide | acid + silver nitrate | white / cream / yellow precipitate |
| Nitrate NO₃⁻ | warm with NaOH + aluminium | ammonia (damp red litmus turns blue) |