Section 1 of 7Mixtures and choosing a method
What a mixture is
- What a mixture is A mixture contains two or more substances, elements or compounds, that are not chemically joined.
- Each keeps its own chemical properties.
- Because no new bonds are made, the parts can be separated by physical methods.
Using a difference in properties
- Using a difference in properties Every separation method relies on a difference between the substances,
- such as boiling point, solubility or particle size.
- Exam questions often describe a mixture and ask which method is most suitable.
Section 2 of 7Filtration and crystallisation
Solids and solutions
Filtration
- Filtration Separates an insoluble solid from a liquid, such as sand from water.
- Pour the mixture through filter paper in a funnel.
- The solid left on the paper is the residue; the liquid that passes through is the filtrate.
Crystallisation
- Crystallisation Obtains a dissolved solid whose solubility is much greater in hot solvent than cold, for example copper(II) sulfate.
- Heat to evaporate some solvent until a drop on a cold glass rod forms crystals, showing the solution is saturated.
- Leave it to cool slowly, filter the crystals, wash with a little cold distilled water and dry between filter paper.
Evaporation
- Evaporation Heating a solution until all solvent has gone leaves the solid behind.
- This suits a solid that is not damaged by heat when the solvent is not needed.
Section 3 of 7Simple distillation
1
Heat the solution, such as salt water, in a flask fitted with a thermometer and a delivery tube to a condenser.
2
The liquid with the lower boiling point, water at 100 °C, evaporates, and the vapour travels along the side arm.
3
Cold water flows through the outer jacket of the condenser, entering at the bottom and leaving at the top so that the jacket stays full.
4
The vapour condenses into a pure liquid and is collected in a beaker.
5
The solute, for example salt, stays behind in the flask.
Section 4 of 7Chromatography
Setting it up
- Setting it up Draw a start line in pencil, because pencil does not dissolve, and put a spot of each sample on it.
- Stand the paper in a little solvent so that the start line stays above the solvent level, and let the solvent travel up the paper by capillary action.
Interpreting results
- Interpreting results Substances with different solubilities in the solvent move different distances, so they separate into spots.
- A pure substance gives one spot.
- A mixture gives several.
- Matching spots to known reference samples identifies the components.
Rf value
- Rf value Rf = distance moved by the substance ÷ distance moved by the solvent.
- It has no units and is always less than 1.
- If a spot moves 3 cm while the solvent moves 6 cm, then Rf = 3 ÷ 6 = 0.5.
Section 5 of 7Which method to use
| Mixture | Method | Property used |
|---|---|---|
| Sand and water | Filtration | Sand is insoluble |
| Copper(II) sulfate crystals from solution | Crystallisation | Solubility falls on cooling |
| Pure water from salt water | Simple distillation | Different boiling points |
| Colours in an ink | Chromatography | Different solubilities in the solvent |
| Liquids with similar boiling points, such as crude oil fractions | Fractional distillation | Small differences in boiling point |
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Mixture | Two or more substances that are not chemically combined. |
| Residue | The insoluble solid left on the filter paper. |
| Filtrate | The liquid that passes through the filter paper. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
| Condenser | A cooled tube in which a vapour turns back into a liquid. |
| Rf value | The ratio of the distance moved by a substance to the distance moved by the solvent. |
Section 7 of 7Exam technique
1
Drawing the chromatography start line in ink, which dissolves and runs with the solvent.
2
Putting the start line below the solvent level so the samples wash away into the beaker.
3
Connecting the condenser water the wrong way round. It enters at the bottom and leaves at the top.
4
Saying evaporation and distillation are the same. Distillation collects the liquid, but evaporation lets it escape.
5
Calculating Rf upside down. The solvent distance goes on the bottom.