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What is the difference between an element, a compound and a mixture, and how are mixtures separated?

Elements, compounds and mixtures, the difference between mixtures and compounds, purity and formulations, and the separation techniques of filtration, crystallisation, simple and fractional distillation and chromatography.

A focused answer to the OCR Gateway GCSE Combined Science A topic C2 on elements, compounds and mixtures, covering the difference between mixtures and compounds, purity and formulations, and the separation techniques of filtration, crystallisation, distillation and chromatography.

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  1. What this topic is asking
  2. Elements, compounds and mixtures
  3. Separating insoluble and soluble solids
  4. Distillation and chromatography

What this topic is asking

OCR wants you to define elements, compounds and mixtures, explain the difference between a mixture and a compound, describe purity and formulations, and describe the separation techniques and when to use each.

Elements, compounds and mixtures

The key difference is the chemical bonding: in a compound the elements are joined by chemical bonds, so the compound has new properties and can only be separated by a chemical reaction, while a mixture's substances are just physically together and can be separated by physical methods. For example, iron and sulfur mixed together can be separated with a magnet, but once heated to form iron sulfide (a compound) they cannot. A pure substance in chemistry is a single element or compound; a formulation is a useful mixture made in carefully measured quantities for a purpose, such as a paint, a fuel, a medicine or an alloy.

Separating insoluble and soluble solids

The choice of technique depends on what you are separating:

  • Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper; the solid stays on the paper (the residue) and the liquid passes through (the filtrate). Use it to separate sand from water.
  • Crystallisation obtains a soluble solid from its solution. The solution is heated so the solvent evaporates, leaving the solid as crystals. Use it to get salt or copper sulfate crystals from solution; gentle heating gives better crystals.

Distillation and chromatography

Simple distillation separates a liquid from a solution (such as pure water from salt water): the solution is heated, the liquid evaporates, then the vapour is cooled in a condenser and collected as a pure liquid, leaving the dissolved solid behind. Fractional distillation separates a mixture of liquids with different boiling points (such as the fractions of crude oil or ethanol from water): the liquid with the lowest boiling point evaporates and is collected first as the mixture is heated. Paper chromatography separates dissolved substances such as the dyes in an ink or food colouring. A spot is placed on a pencil baseline, a solvent rises up the paper and carries the substances different distances depending on their attraction to the paper and solvent, so a mixture splits into several spots. Each spot's position is reported as an RfR_f value:

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

which is always between 00 and 11 and can be used to identify the substance.

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 mixture contains salt dissolved in water and some insoluble sand. Describe how you would obtain pure, dry salt and pure water from this mixture.
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A Chemistry Paper 3 practical question on separation. Reward a clear sequence: first filter the mixture to remove the insoluble sand (the sand stays on the filter paper as the residue, and the salt solution passes through as the filtrate). To get pure water, distil the salt solution: heat it so the water evaporates, then cool the vapour in a condenser so it condenses back to pure liquid water, leaving the salt behind. To get pure dry salt instead, evaporate the water from the salt solution by gentle heating (crystallisation), leaving salt crystals, then dry them. Markers credit filtration to remove sand, distillation to collect pure water, and evaporation/crystallisation to obtain the dry salt.

OCR 20214 marksExplain how paper chromatography can be used to find out whether a food colouring contains a single dye or a mixture of dyes, including how to calculate an Rf value.
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A C2 question on chromatography. Reward: put a spot of the food colouring on a pencil baseline near the bottom of chromatography paper, stand it in a solvent so the solvent rises up the paper (the start line must be above the solvent), and let the solvent carry the dyes up. A single dye produces one spot; a mixture separates into several spots because the different dyes travel different distances. The RfR_f value is calculated as Rf=distance travelled by the spotdistance travelled by the solventR_f = \dfrac{\text{distance travelled by the spot}}{\text{distance travelled by the solvent}}, which is always between 00 and 11 and can be used to identify a dye. Markers credit the pencil baseline, the solvent below the spots, more than one spot meaning a mixture, and the correct RfR_f formula.

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