What is the difference between elements, compounds and mixtures, and how do we separate them?
Elements, compounds and mixtures, the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, simple and fractional distillation, and chromatography.
A focused CCEA GCSE Double Award Science (Chemistry Unit C1) answer on elements, compounds and mixtures, covering the difference between physical and chemical change, and the separation techniques of filtration, crystallisation, distillation and chromatography.
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What this dot point is asking
CCEA Double Award wants the definitions of element, compound and mixture, the difference between physical and chemical change, and the main separation techniques. The separation methods are practical, so you must know which method suits which mixture and why.
Elements, compounds and mixtures
Because a compound is chemically bonded, it can only be split by a chemical reaction. A mixture is not bonded, so it can be separated using physical methods like the ones below.
Physical and chemical change
A physical change (melting, boiling, dissolving) does not make a new substance and is usually easily reversed. A chemical change (such as burning or rusting) makes a new substance and is usually hard to reverse. Dissolving salt is physical; burning magnesium is chemical. Because the separation techniques below are all physical, they can only separate mixtures, not split compounds - a compound can only be broken apart by a chemical reaction such as electrolysis.
Separation techniques
In chromatography, a spot of the mixture is placed on paper and a solvent moves up, carrying the substances different distances. A pure substance gives a single spot; a mixture separates into several. In fractional distillation, a fractionating column lets liquids separate by boiling point - the basis of separating crude oil and ethanol from water.
Examples in context
Example 1. Getting drinking water from seawater. Seawater is a mixture of salt dissolved in water. Simple distillation boils off the water and condenses it, leaving the salt behind, giving pure water. This is desalination, used where fresh water is scarce.
Example 2. Testing food colourings. Chromatography of a green sweet coating can reveal it is a mixture of blue and yellow dyes, shown by two separate spots. This is how scientists check which dyes are present in a product.
Try this
Q1. What is a compound? [1 mark]
- Cue. Two or more elements chemically combined.
Q2. Which technique separates an insoluble solid from a liquid? [1 mark]
- Cue. Filtration.
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-style4 marksDescribe how you would obtain pure water from a salt solution, naming the technique.Show worked answer →
Describe simple distillation for four marks.
Heat the salt solution so the water boils and evaporates, leaving the salt behind.
Pass the water vapour through a condenser, where it cools.
The vapour condenses back to liquid water, which is collected as the distillate.
The technique is simple distillation. Markers reward evaporation, condensing in a condenser, and collecting pure water.
CCEA-style3 marksExplain how chromatography can show whether an ink is a pure substance or a mixture.Show worked answer →
Describe the result for three marks.
A spot of ink is placed on chromatography paper and a solvent is allowed to move up.
A pure substance gives a single spot.
A mixture separates into several spots because the different dyes travel different distances. Markers want one spot for pure and several spots for a mixture.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)