Filtration, crystallisation, simple and fractional distillation and chromatography: how each one works, and which to choose for a given mixture.
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1.What is filtration used for?
Separating an insoluble solid from a liquid.
2.Describe how filtration works.
The mixture is poured through filter paper in a funnel. The liquid passes through as the filtrate, while the solid stays behind as the residue.
3.What is the residue?
The insoluble solid left in the filter paper.
4.What is the filtrate?
The liquid that passes through the filter paper.
5.What is crystallisation used for?
Separating a soluble solid from the solution it is dissolved in.
6.Describe how crystallisation works.
The solution is warmed so some of the solvent evaporates, leaving a saturated solution. It is then left to cool, and crystals of the solid form.
7.Why is a solution not evaporated to complete dryness?
Some solids decompose when heated strongly, and slow cooling produces larger, better formed crystals.
8.What is simple distillation used for?
Separating a liquid from a solution, when you want to keep the liquid.
9.Describe how simple distillation works.
The solution is heated until the solvent boils. The vapour passes into a condenser where it cools back into a liquid and is collected. The dissolved solid stays in the flask.
10.What is the limitation of simple distillation?
It can only separate substances with very different boiling points, so it cannot separate a mixture of similar liquids.
11.What is fractional distillation used for?
Separating a mixture of two or more liquids that have different boiling points.
12.Describe how fractional distillation works.
The mixture is heated in a flask with a fractionating column. The liquid with the lowest boiling point evaporates first and reaches the top of the column, where it is condensed and collected.
13.Why does a fractionating column improve separation?
Vapours repeatedly condense and evaporate as they move up the column, so by the top only the substance with the lowest boiling point remains as a vapour.
14.What is chromatography used for?
Separating substances in a mixture, often coloured substances such as inks or dyes.
15.Name the two phases in paper chromatography.
The stationary phase, which is the paper, and the mobile phase, which is the solvent.
16.Why do different substances travel different distances?
They have different attractions to the paper and different solubilities in the solvent, so more soluble substances travel further.
17.Why is the starting line drawn in pencil?
Pencil is insoluble, so it will not dissolve and move up the paper with the solvent as ink would.
18.Why must the solvent level start below the spots?
If the solvent covered the spots, the substances would dissolve straight into the solvent instead of travelling up the paper.
19.How can you tell a substance is pure from a chromatogram?
A pure substance produces only one spot in any solvent.
20.How do you calculate an Rf value?
Rf = distance moved by the substance divided by the distance moved by the solvent.
21.A spot moves 3 cm and the solvent moves 12 cm. Calculate the Rf value.
3 divided by 12 = 0.25.
22.Why does an Rf value never exceed 1?
The substance is carried by the solvent, so it can never travel further than the solvent front.
23.What does a pure substance mean in chemistry?
A single element or compound, not mixed with anything else. This is different from the everyday use of the word.
24.How can melting point be used to test purity?
A pure substance melts at one specific temperature. A mixture melts over a range of temperatures and usually at a lower value.
25.Which technique would separate sand from water?
Filtration, because sand is insoluble.
26.Which technique would separate salt from salty water, keeping the salt?
Crystallisation.
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