OCR GCSE Chemistry A (Gateway)
OCR GCSE Chemistry A C2.1 Purity and Separating Mixtures Exam Questions
Free OCR GCSE Chemistry A exam questions for C2.1 Purity and Separating Mixtures, with a mark scheme for every pack. Covers pure substances and mixtures, formulations, melting points, relative formula mass, empirical formulae, filtration, crystallisation, distillation and chromatography.
C2.1 Purity and Separating Mixtures exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About OCR GCSE Chemistry A C2.1 Purity and Separating Mixtures
C2.1 covers what chemists mean by a pure substance and the techniques used to separate mixtures. On the OCR Gateway Chemistry A course it opens Topic C2 Elements, compounds and mixtures, assessed in Paper 1 (Foundation) or Paper 3 (Higher). It also introduces relative formula mass and empirical formulae, which lead into moles in C3.1 and quantitative chemistry in C5.1. Examiners often ask you to choose and describe a separation technique, use melting point data to judge purity, calculate Rf values from chromatograms (PAG work), and work out empirical formulae from masses or percentages.
What you need to know
- Purity and formulations In chemistry, a pure substance contains only one element or compound. A pure substance melts and boils at a sharp, fixed temperature; impurities lower the melting point and make it melt over a range. Formulations are mixtures designed for a purpose, such as paints, medicines and alloys.
- Relative formula mass Mr is the sum of the relative atomic masses of all atoms in a formula, for example Mr of CaCO3 = 40 + 12 + (3 x 16) = 100.
- Empirical formula The empirical formula is the simplest whole number ratio of atoms in a compound. Divide the mass or percentage of each element by its Ar, then divide by the smallest answer to get the ratio.
- Filtration and crystallisation Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid from a solution by evaporating some solvent and leaving it to cool so crystals form.
- Distillation Simple distillation separates a solvent from a solution, for example pure water from salt water. Fractional distillation separates miscible liquids with different boiling points using a fractionating column.
- Chromatography Paper and thin-layer chromatography separate substances by how they distribute between a stationary phase and a mobile phase. Rf = distance moved by substance / distance moved by solvent; gas chromatography is an instrumental method used to identify substances.
Common mistakes in C2.1 Purity and Separating Mixtures
- Using the everyday meaning of pure, such as 'pure orange juice'. In chemistry, pure means a single element or compound.
- Drawing the start line for chromatography in ink, or below the solvent level. The line should be in pencil and above the solvent so spots do not dissolve into it.
- Measuring the Rf distance to the bottom of a spot. Measure from the start line to the centre of the spot.
- Evaporating all the water when crystallising. Heat until crystals start to form, then leave to cool, so that well-formed crystals are obtained.
- Giving an empirical formula ratio without dividing by the smallest number of moles. The ratio must be simplified to whole numbers.
Try three C2.1 Purity and Separating Mixtures exam-style questions
1. In a chromatogram, the solvent front moved 8.0 cm and a dye moved 3.6 cm. Calculate the Rf value of the dye. (2 marks)
Show the mark scheme answer
Rf = 3.6 / 8.0; = 0.45.
2. A compound contains 2.4 g of carbon and 0.6 g of hydrogen. Calculate its empirical formula. (Ar: C = 12, H = 1) (3 marks)
Show the mark scheme answer
Moles C = 2.4 / 12 = 0.2 and moles H = 0.6 / 1 = 0.6; ratio = 0.2 : 0.6 = 1 : 3; empirical formula CH3.
3. Describe how pure water can be obtained from salty water. (3 marks)
Show the mark scheme answer
Heat the salty water so the water boils and evaporates; the steam passes into a condenser where it cools and condenses; pure water is collected while the salt remains in the flask (simple distillation).
C2.1 Purity and Separating Mixtures FAQs
What does OCR GCSE Chemistry A C2.1 cover?
It covers pure substances and mixtures, formulations, using melting points to test purity, relative formula mass, empirical formulae and separation techniques including distillation and chromatography.
Which paper is C2.1 examined in?
Topic C2 is assessed in Paper 1 (Foundation) or Paper 3 (Higher).
How do I choose the right separation method in C2.1?
Ask whether the substance is soluble, whether you want the solid or the liquid, and whether the liquids have different boiling points; then pick filtration, crystallisation, simple or fractional distillation, or chromatography accordingly.
