Eduqas GCSE Chemistry
Eduqas GCSE Chemistry 1 Pure Substances and Mixtures Exam Questions
Free Eduqas GCSE Chemistry revision for 1 Pure Substances and Mixtures: what you need to know, common mistakes and exam-style questions with answers. Covers pure substances, elements, compounds and mixtures, formulations, filtration, crystallisation, distillation, paper chromatography and Rf values.
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About Eduqas GCSE Chemistry 1 Pure Substances and Mixtures
Pure substances and mixtures opens the Eduqas GCSE Chemistry specification. You learn what chemists mean by a pure substance, how to tell elements, compounds and mixtures apart, what a formulation is, and how to separate mixtures by filtration, crystallisation, simple and fractional distillation and paper chromatography. Melting and boiling points are used to test purity. The ideas link to particles and states of matter in topic 2, salt preparation in topic 7 and crude oil in topic 10. Examiners often ask you to choose and describe a separation method, calculate Rf values and interpret chromatograms. Two specified practicals belong here: SP1A on melting point and SP1B on distillation and chromatography.
What you need to know
- Pure substances In chemistry a pure substance is a single element or compound with nothing else mixed in. A pure substance melts and boils at a fixed temperature; impurities lower the melting point and make it melt over a range. SP1A: determine the melting point of a substance such as naphthalene or candle wax.
- Elements, compounds, mixtures An element contains only one type of atom. A compound contains two or more elements chemically bonded in fixed proportions. A mixture contains substances that are not chemically joined and can be separated by physical methods.
- Formulations A formulation is a mixture designed as a useful product, with each component in a measured quantity, for example paints, medicines, fuels, alloys and fertilisers.
- Filtration and crystallisation Filtration separates an insoluble solid from a liquid. Crystallisation obtains a soluble solid from a solution: heat to evaporate some solvent, then leave to cool so crystals form, and dry them.
- Distillation Simple distillation separates a solvent from a solution, such as water from salt solution. Fractional distillation separates miscible liquids with different boiling points, such as ethanol and water, using a fractionating column. SP1B includes distilling ethanol from water.
- Chromatography Paper chromatography separates dyes because they have different attractions to the paper (stationary phase) and the solvent (mobile phase). Rf = distance moved by the substance / distance moved by the solvent. A pure substance gives one spot.
Common mistakes in 1 Pure Substances and Mixtures
- Saying a pure substance is a natural one with nothing added. In chemistry, pure means a single element or compound only.
- Drawing the start line for chromatography in ink. Use pencil, because ink would dissolve and separate too.
- Placing the start line below the solvent level. The spots must be above the solvent, otherwise they dissolve into it.
- Measuring Rf from the bottom of the paper. Measure both distances from the pencil start line.
- Heating a solution to dryness when making crystals. Evaporate some of the water and leave to cool so that crystals form.
Try three 1 Pure Substances and Mixtures exam-style questions
1. In a chromatogram, a dye spot moved 3.6 cm and the solvent front moved 6.0 cm from the start line. Calculate the Rf value. (2 marks)
Show the mark scheme answer
Rf = 3.6 / 6.0; = 0.60.
2. A student has a mixture of ethanol (boiling point 78 °C) and water. Describe how fractional distillation separates them. (4 marks)
Show the mark scheme answer
Heat the mixture; ethanol has the lower boiling point so it evaporates first; vapour rises up the fractionating column and passes into the condenser; it is cooled and condenses, and liquid ethanol is collected when the thermometer reads about 78 °C.
3. Explain how a melting point test could show that a sample of naphthalene is impure. (2 marks)
Show the mark scheme answer
An impure sample melts below the melting point of pure naphthalene; and melts over a range of temperatures rather than at a sharp, fixed temperature.
1 Pure Substances and Mixtures FAQs
What does Eduqas GCSE Chemistry 1 Pure Substances and Mixtures cover?
It covers pure substances and how to test purity, elements, compounds, mixtures and formulations, and separation by filtration, crystallisation, distillation and paper chromatography with Rf values.
Which paper is 1 Pure Substances and Mixtures examined in?
It is examined in Component 1: Concepts in Chemistry (2 hours 15 minutes, 120 marks). Component 2: Applications in Chemistry can also test it through practical contexts and an unseen article.
How do I choose a separation method in Eduqas GCSE Chemistry 1 Pure Substances and Mixtures?
Insoluble solid from liquid: filtration. Soluble solid from solution: crystallisation. Solvent from solution: simple distillation. Liquids with different boiling points: fractional distillation. Coloured substances such as dyes: paper chromatography.
