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What is the difference between a pure substance and a mixture, and how can mixtures be separated?

Pure substances, mixtures and formulations, and the techniques for separating mixtures: filtration, crystallisation, simple and fractional distillation, and paper chromatography with Rf values.

A focused answer to OCR Gateway GCSE Chemistry A topic C2.3 on pure substances, mixtures and formulations, and the separation techniques of filtration, crystallisation, simple and fractional distillation and paper chromatography including Rf value calculations.

Generated by Claude Opus 4.89 min answer

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  1. What this dot point is asking
  2. Pure substances, mixtures and formulations
  3. Filtration
  4. Crystallisation
  5. Distillation
  6. Paper chromatography

What this dot point is asking

OCR wants you to distinguish a pure substance from a mixture and a formulation, and describe the techniques used to separate mixtures: filtration, crystallisation, simple and fractional distillation, and paper chromatography (including calculating Rf values). These are core practical skills, assessed in the C7 practical activities and the written papers.

Pure substances, mixtures and formulations

A useful test of purity: a pure substance melts and boils at a single, specific temperature, while a mixture melts and boils over a range of temperatures. Impurities lower the melting point and raise the boiling point.

Filtration

Filtration separates an insoluble solid from a liquid (or a solution). The mixture is poured through filter paper in a funnel: the solid is too big to pass through and is trapped as the residue, while the liquid passes through as the filtrate. For example, sand is filtered out of salty water as the residue, leaving the salt solution as the filtrate.

Crystallisation

Distillation

Distillation separates a liquid from a solution or separates liquids with different boiling points:

  • Simple distillation separates a solvent from a solution (for example getting pure water from salty water). The solution is heated; the liquid with the lower boiling point evaporates, then is cooled in a condenser and collected.
  • Fractional distillation separates a mixture of miscible liquids with different boiling points (for example separating ethanol from water, or the fractions of crude oil). A fractionating column lets the liquids separate by boiling point, with the lowest boiling point collected first.

Paper chromatography

The Rf value measures how far a substance travels:

Rf=distance travelled by the substancedistance travelled by the solventR_f = \frac{\text{distance travelled by the substance}}{\text{distance travelled by the solvent}}

The Rf value is always between 00 and 11 and has no units. A pure substance gives one spot; a mixture gives several spots. The same substance gives the same Rf value in the same solvent, so Rf values can be used to identify substances.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20194 marksA student has a mixture of sand and salt. Describe how they could obtain pure, dry samples of both the sand and the salt from the mixture.
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A C2.3 practical question (linked to the separation PAG). Reward a clear method: add water to the mixture and stir, so the salt dissolves but the sand does not. Filter the mixture: the sand is trapped as the residue in the filter paper, and the salt solution passes through as the filtrate. Wash and dry the sand (the residue) to obtain pure dry sand. To get the salt, heat the salt solution (the filtrate) to evaporate the water, leaving salt crystals (crystallisation or evaporation). Markers credit dissolving the salt, filtering to separate the sand as residue, drying the sand, and evaporating or crystallising the filtrate to recover the salt. A common slip is to filter before dissolving, which would not separate the salt.

OCR 20214 marksIn a chromatography experiment, a spot of dye travels 4.5 cm while the solvent front travels 9.0 cm. Calculate the Rf value of the dye, and explain why a substance with a higher Rf value travels further up the paper.
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A Higher tier calculation. Reward: the Rf value is the distance travelled by the substance divided by the distance travelled by the solvent: Rf=4.59.0=0.5R_f = \dfrac{4.5}{9.0} = 0.5. An Rf value has no units because it is a ratio. A substance with a higher Rf value travels further up the paper because it is more strongly attracted to the moving solvent (the mobile phase) and less strongly attracted to the paper (the stationary phase), so it is carried further before the solvent stops. Markers credit the correct formula and answer of 0.5, the point that Rf has no units, and the explanation that a higher Rf substance is more soluble in or attracted to the solvent and less attracted to the paper.

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