Choosing How to Separate Mixtures
Choose separation techniques using physical properties: filtration for undissolved solids, distillation using vaporisation and condensation, crystallisation for dissolved solids, and chromatography for components that travel differently
What a learner can do afterwards
- Selects the correct separation technique for a given mixture with justification
- Describes the steps of simple distillation and explains why it works
- Interprets separated chromatography spots, recognises limits of spot counts, and calculates Rf values using distances measured from the same starting line
- Explains why filtration removes sand from water but not salt
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You have used sieving, filtering and evaporation. Filtration traps undissolved grains in suitable filter paper. Water and dissolved salt pass through because their particles are much smaller than its openings. To separate sand from a salt solution, filter out sand first. Ordinary filter paper cannot recover dissolved salt.
Crystallisation recovers a dissolved solid as crystals. Evaporate some water from a salt solution until crystals begin to appear, then allow it to cool. Separate and dry the crystals. Some salt remains in the leftover solution. Use a filter funnel and paper for sand, then an evaporating dish for the solution. Heating needs teacher supervision.
Simple distillation uses vaporisation followed by condensation. Water from a salt solution vaporises, then becomes liquid in a cooled condenser. Salt stays in the flask. With two liquids, both can enter the vapour, which is richer in the more volatile, usually lower-boiling liquid. Simple distillation works better when boiling points are far apart. It does not always give a pure liquid.
This bar chart compares measured Rf values for separate dye spots. Count separate spots to find a minimum number of components detected in a sample. Each separate spot shows at least one component. The sample may contain more: components can overlap at one position, and some may not be visible. The chart compares results; it is not a drawing of the chromatography paper.
The numbers above the bars are Rf values. Separate spots give a minimum component count, not necessarily the total.
In paper chromatography, place a dye spot on a pencil starting line above the solvent level. Rising solvent carries dyes different distances because they interact differently with solvent and paper. A submerged spot may wash off. Rf = spot distance / solvent-front distance. Measure both from the starting line, using the spot's centre. Rf is from 0 to 1 and depends on test conditions. Chromatography can also separate colourless substances.
Filter undissolved solids, crystallise dissolved solids, distil by vaporising and condensing, and interpret chromatography using spots and Rf.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked video: Distillation, Khan Academy.
More to read
Choose what to collect
After filtering sand from a salt solution, inspect both collected parts. Sand is on the paper. Salt is still dissolved in the liquid below.
To collect salt crystals, remove some water until crystals begin to appear, let the solution cool, then separate and dry the crystals. The liquid left over may still contain salt.
To collect water instead, use distillation. Water vapour leaves the flask and is cooled back into liquid. These are different collection goals, so they need different equipment. Heating and laboratory equipment need teacher supervision.
Measure a chromatography result
Draw a pencil starting line and keep the sample spot above the solvent surface. Only the paper's lower edge dips into solvent. This prevents the sample from washing directly into the container.
After the run, measure from the starting line to a spot's centre and to the solvent front. A spot travelling 3 cm while the front travels 10 cm has Rf = 3 / 10 = 0.3.
Use the same solvent and test conditions when comparing results. Similar colours and Rf values can support an identification, but different substances can give similar results.
When spots do not tell the whole story
A sample may begin as one ink spot and finish as several spots. Different dyes interact differently with the solvent and stationary material, so they can separate as the solvent moves.
Count separate spots as detected components. Do not assume this is the sample's total number of substances. Two components may finish in the same place, and some components may not be visible.
Chromatography is not limited to coloured samples. Scientists can use other ways to detect colourless components. For the visible dye examples here, report what the spots show and what they cannot establish.
Why distillation needs cooling
An open dish can lose water by evaporation. Distillation adds a way to cool and collect the vapour, so the water is recovered in another container.
A salt solution works well for explaining this: water vaporises while salt stays behind at the temperatures used. Separating two liquids is less exact. Both may enter the vapour, though it contains a greater proportion of the more volatile liquid.
A fractionating column provides repeated vaporisation and condensation, improving separation when boiling points are close. Laboratory distillation needs teacher supervision. Heated apparatus must not be sealed.
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