Principles of Chemistry
Elements, Compounds, Mixtures and Separation
Pearson Edexcel International GCSE Chemistry
Elements, compounds and mixtures
| Substance | What it is |
|---|---|
| element | a substance that has only one type of atom |
| compound | atoms of two or more different elements chemically bonded together |
| mixture | two or more different substances not chemically joined |
Reading a particle diagram
| What the box shows | Substance |
|---|---|
| one kind of atom only, as single atoms or as molecules | element |
| identical particles, each made of two or more kinds of atom bonded together | compound |
| particles of two or more different substances, not bonded to each other | mixture |
Pure substances
- To test purity, measure the melting point.
- A sharp melting point: the substance is pure. Melts over a range of temperatures: the substance is a mixture.
Choosing a separation method
| Mixture | Method |
|---|---|
| an insoluble solid in a liquid | filtration |
| a soluble solid from its solution | crystallisation |
| a solvent from a solution | simple distillation |
| liquids that mix, with different boiling points | fractional distillation |
- Liquids that mix: write fractional distillation. Never write distillation for liquids that mix or simple distillation for liquids that mix.
- A solvent from a solution: write simple distillation. Never write fractional distillation for a solvent from a solution or distillation alone for a solvent from a solution.
Filtration and crystallisation
- A soluble solid mixed with an insoluble solid: add water, stir and heat to dissolve the soluble solid, then filter.
- The insoluble solid stays in the filter paper as the residue; the solution passes through as the filtrate.
Method: crystals from a solution
- Pour the solution into an evaporating basin and heat until crystals first start to form.
- Leave to cool and crystallise.
- Pour off any excess liquid.
- Leave in a warm place to dry.
Heat only until crystals start to form, not until all the water has gone.
Distillation
- The two changes of state: evaporation in the flask, condensation in the condenser.
- In the condenser the vapour is cooled and condenses.
- Fractional distillation adds a fractionating column between the flask and the condenser; the liquid with the lowest boiling point collects first.
Paper chromatography
Method: separating inks or food colourings
- Draw a start line in pencil near the bottom of the chromatography paper.
- Place a small spot of each sample on the start line.
- Pour some solvent into a beaker.
- Place the paper in the solvent so that the spots are above the level of the solvent.
- Remove the paper when the solvent almost reaches the top of the paper.
- Mark the solvent front.
- Allow to dry.
Use the same solvent and use the same paper.
Draw the start line in pencil: ink would dissolve in the solvent and interfere with the results.
Keep the solvent level below the start line: the spots dissolve in the solvent if it is above.
Reading a chromatogram
- One dye produces only one spot. The number of spots is the number of dyes.
- Two samples contain the same dye when their spots are at the same height.
- A spot left on the start line: the substance is insoluble in the solvent and has not moved. To separate it, repeat using a different solvent.
- The spot that moves furthest is the more soluble and has the greatest Rf value.
Rf values
Rf = distance moved by spot ÷ distance moved by solvent
- Measure both distances from the start line: to the centre of the spot, and to the solvent front.
- An Rf value is always less than 1 and has no unit.
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