Key Stage 4 science · Chemistry – Chemical analysis
GCSE chemistry: purity, separation and reacting masses
This GCSE chemistry topic is about finding out what a sample contains and how much of it there is. Students learn the scientific meaning of pure, choose the right method to separate a mixture, and then move into quantitative work: using a balanced equation to predict masses, and calculating the concentration of a solution in grams per cubic decimetre. It joins practical skills to maths, which is why many students find it demanding.
Purity, separation and reacting masses, explained
In everyday life, “pure” often means nothing added, as in pure orange juice. In chemistry, a pure substance is a single element or a single compound, with nothing else mixed in. Pure substances melt and boil at fixed temperatures, so a sharp melting point is evidence of purity, while a mixture melts over a range of temperatures and at a different temperature from the pure substance. Air, seawater and milk are all mixtures, even though each may look like one substance.
Separating a mixture depends on a physical difference between its parts. Filtration separates an insoluble solid from a liquid, such as sand from water. Crystallisation recovers a dissolved solid: heat the solution gently to evaporate some water, leave it to cool so crystals form, then filter and dry them. Simple distillation recovers the solvent from a solution, for example pure water from salty water, by evaporating and condensing it. Fractional distillation separates liquids with different boiling points that are mixed together, such as ethanol and water, using a column in which the lower-boiling liquid reaches the top first.
Chromatography separates substances by how far they travel across paper in a solvent. The start line is drawn in pencil, because pen ink would dissolve and run, and the line must sit above the level of the solvent. Each substance spends a different proportion of time in the solvent and sticking to the paper, so they spread out. Students calculate an Rf value by dividing the distance moved by the substance by the distance moved by the solvent. This value is always less than 1 and can be compared with known values to identify substances.
The quantitative half starts with a balanced equation, which tells you the ratio in which reactants combine and products form. Students use relative atomic masses to work out reacting masses, finding what mass of product to expect or how much reactant is needed. Many schools introduce moles for this, although the statutory wording only asks for quantitative interpretation of balanced equations. They also calculate concentration as mass of solute divided by volume of solution, usually in g/dm³, converting from cm³ by dividing by 1,000.
Words your child will hear
- Pure substance
- A single element or compound with nothing else mixed in, which melts and boils at fixed temperatures.
- Mixture
- Two or more substances together that are not chemically joined and can be separated physically.
- Solute
- The substance that dissolves, such as salt in salty water.
- Solvent
- The liquid that does the dissolving, such as water.
- Residue
- The solid left on the filter paper after filtration.
- Rf value
- Distance moved by a substance divided by distance moved by the solvent in chromatography.
- Concentration
- The mass of solute in a given volume of solution, commonly in g/dm³.
What the National Curriculum says
The programme of study for Key Stage 4 says:
- Students should be taught about distinguishing between pure and impure substances
- Students should be taught about separation techniques for mixtures of substances: filtration, crystallisation, chromatography, simple and fractional distillation
- Students should be taught about quantitative interpretation of balanced equations
- Students should be taught about concentrations of solutions in relation to mass of solute and volume of solvent
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example
Magnesium burns in oxygen to make magnesium oxide: 2Mg + O₂ → 2MgO. What mass of magnesium oxide forms when 6.0 g of magnesium burns completely? (Relative atomic masses: Mg = 24, O = 16.)
- Check the equation is balanced: two magnesium and two oxygen atoms on each side. The ratio is 2 magnesium to 2 magnesium oxide.
- Work out the relative formula mass of each substance, and multiply by the number in front in the equation.
2 Mg = 2 × 24 = 48 2 MgO = 2 × (24 + 16) = 80
- Read this as a mass ratio: 48 g of magnesium makes 80 g of magnesium oxide. Notice that the masses are not in the ratio 2 : 2 even though the numbers in the equation are.
- Scale down to the mass you have. 6.0 g is one eighth of 48 g, so the product is one eighth of 80 g.
6.0 ÷ 48 = 0.125 0.125 × 80 = 10.0 g
- Sense check: the oxide is heavier than the metal because oxygen from the air has joined it, so 10.0 g is more than 6.0 g. The extra 4.0 g is the oxygen.
10.0 g of magnesium oxide forms, which includes 4.0 g of oxygen that combined with the magnesium.
Common mistakes and how to help
Thinking pure means natural, untreated or without additives
Labels on food and bottled water use “pure” as a marketing word, and mineral water is described as pure when it contains dissolved minerals.
What helps: In chemistry, if it has more than one substance in it, it is a mixture. Ask “could I separate it by a physical method?” A sharp, fixed melting point or boiling point is the evidence chemists use for a pure substance.
Using the numbers in front of a formula as a ratio of masses
The equation says 2Mg and 2MgO, so students assume 2 g of magnesium makes 2 g of magnesium oxide.
What helps: The numbers count particles, not grams. Convert to masses using the relative atomic masses first, then compare. Writing a “mass under each formula” line below the equation helps.
Mixing up cm³ and dm³ when working out concentration
Both are volumes, and the numbers are easy to confuse. 1 dm³ is 1,000 cm³, so a 250 cm³ sample is only 0.25 dm³.
What helps: Always write the unit with every number. If 12 g is dissolved in 500 cm³, convert first: 500 ÷ 1,000 = 0.5 dm³. Then the concentration is 12 ÷ 0.5 = 24 g/dm³.
Setting up chromatography incorrectly or misreading the result
Pen is easier than pencil, the line can end up in the solvent, and Rf values larger than 1 are sometimes reported when the wrong distances are measured.
What helps: Pencil does not run, and the start line must be above the solvent. Measure both distances from the start line, not from the bottom of the paper. For a spot 3.0 cm from the line and a solvent front 5.0 cm, Rf = 3.0 ÷ 5.0 = 0.60.
How to help at home
- Try kitchen chromatography. Draw a thick spot or line with a water-based felt-tip pen about 2 cm from the bottom of a strip of coffee filter paper. Stand the strip in a little water, with the line above the water, and watch the colours separate as the water rises. Compare different pens and brands of black, and measure the distances to find an Rf.
- Grow crystals slowly. Stir salt into a cup of tap-warm water until no more will dissolve, pour the clear liquid into two saucers and leave them somewhere safe for several days, one in a cool room and one on a sunny windowsill. Crystals form as the water evaporates. Do not taste them. This shows crystallisation, and comparing the saucers usually shows that slower evaporation gives bigger crystals.
- Compare drink concentrations. Read the sugar content per 100 ml on two different drinks and work out how many grams are in a whole glass. Then convert to grams per litre. A litre is the same as 1 dm³ and a millilitre is 1 cm³, so this is exactly the g/dm³ unit used in class, in a familiar setting.
- Practise scaling with recipes. Reacting-mass questions are scaling problems. If a recipe needs 200 g of flour for 12 buns, how much for 30? (500 g.) Finding the scale factor, or the amount for one bun first, is the same method as scaling 48 g of magnesium down to 6.0 g.
Related topics
Builds on
- Key Stage 3: Chemical reactions and equations
- Key Stage 3: Acids, alkalis and neutralisation
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