How do we classify matter, and how do we separate mixtures?
Elements, compounds and mixtures, the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, distillation, fractional distillation and chromatography.
A CCEA GCSE Chemistry answer on elements, compounds and mixtures, covering how each is defined, the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, simple and fractional distillation and chromatography with their uses.
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What this dot point is asking
CCEA wants you to classify matter as an element, a compound or a mixture, tell a physical change from a chemical change, and choose and describe the right technique to separate a mixture: filtration, crystallisation, simple distillation, fractional distillation or chromatography.
Elements, compounds and mixtures
The key difference is the chemical bond. In a compound the elements are joined by chemical bonds, so the compound has new properties and a fixed formula, and it can only be split by a chemical reaction. In a mixture the substances keep their own properties and can be separated by physical methods, because no bonds hold them together.
Choosing a separation technique
The method you choose depends on what kind of mixture you have.
Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel; the solid stays as the residue and the liquid passes through as the filtrate. Use it to separate sand from water.
Crystallisation gets a soluble solid back from its solution. Warm the solution to evaporate some of the water until it is saturated, then cool it slowly so that crystals grow. Use it to obtain copper(II) sulfate crystals. Do not boil to dryness, or you get a powder instead of pure crystals.
Simple distillation separates a solvent from a solution, for example pure water from salty water. The solution is heated, the water boils off and the vapour is cooled in a condenser back to a liquid (the distillate). The dissolved solid stays in the flask.
Fractional distillation and chromatography
Paper chromatography separates dissolved substances such as the dyes in an ink. A spot of the mixture is placed on chromatography paper, the bottom of which dips into a solvent. The solvent rises up the paper and carries the substances with it; those more soluble in the solvent travel further. The result is a chromatogram of separated spots.
Worked example
Examples in context
Example 1. Getting drinking water at sea. Desalination plants use distillation to separate pure water from sea water: the water is boiled and the vapour condensed, leaving the dissolved salts behind. This is a direct industrial use of simple distillation built on the idea that water and dissolved salt have very different volatilities.
Example 2. Forensic ink analysis. Investigators use chromatography to compare the ink on a document with inks from suspect pens. Because each ink separates into a characteristic pattern of spots with fixed Rf values, a match or mismatch can be identified, showing how a school separation technique becomes a real analytical tool.
Try this
Q1. State the difference between a compound and a mixture. [2 marks]
- Cue. A compound is chemically bonded in a fixed ratio; a mixture is not bonded and has a variable composition.
Q2. Name the technique used to separate two miscible liquids with different boiling points. [1 mark]
- Cue. Fractional distillation.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksDescribe how you would obtain pure, dry copper(II) sulfate crystals from a mixture of copper(II) sulfate solution and insoluble sand.Show worked answer →
Markers want two separation steps in the right order, each with a reason.
First filter the mixture. The sand is insoluble, so it stays on the filter paper as the residue, while the copper(II) sulfate solution passes through as the filtrate. This separates the solid from the solution.
Then obtain crystals from the filtrate by crystallisation: warm the solution to evaporate some water until it is saturated (a hot concentrated solution), then leave it to cool slowly so that crystals form. Filter off the crystals and dry them between filter papers or in a warm oven.
Markers reward naming filtration to remove the insoluble sand, then crystallisation (evaporate to the point of crystallisation and cool) rather than boiling to dryness, which would give a powder, not crystals.
CCEA 20213 marksExplain how fractional distillation separates a mixture of ethanol (boiling point 78 degrees C) and water.Show worked answer →
The marks are for the link between boiling point and the order of collection.
Heat the mixture. Both liquids vapourise, but the vapour is richest in the substance with the lower boiling point, ethanol. As the vapour rises up the fractionating column it cools, and the higher-boiling water condenses and runs back down, so the vapour reaching the top is almost pure ethanol.
The ethanol vapour passes into the condenser, where it cools and condenses, and is collected first at about 78 degrees C. The water is left behind (or distils later at 100 degrees C).
Markers reward the idea that fractional distillation separates liquids by their different boiling points, that the lower-boiling liquid (ethanol) comes off first, and that the column improves the separation.
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Sources & how we know this
- CCEA GCSE Chemistry specification (1110) — CCEA (2017)