Chemical Analysis
Chemical analysis is the GCSE chemistry topic covering how chemists identify what a substance is and how pure it is. It covers the difference between a pure substance and a formulation, paper chromatography as a way to separate the components of a mixture, the standard tests for four common gases, and flame tests and precipitation reactions used to identify ions in an unknown compound.
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Method
- Define a pure substance precisely: a single element or a single compound, not mixed with anything else, which melts and boils at one exact, fixed temperature. This is a stricter meaning than the everyday use of pure (as in pure orange juice).
- Recognise a formulation as a mixture that has been designed as a useful product, with each component present in a carefully measured quantity for a specific purpose, for example a fuel, a cleaning agent, a paint, a medicine, an alloy or a fertiliser.
- Use melting point data to test purity: a pure substance melts sharply at one fixed temperature; an impure substance melts over a range of temperatures and generally below the pure melting point.
- For chromatography, calculate the Rf value of each spot (distance moved by the spot divided by distance moved by the solvent, both measured from the baseline) and use it, or the number of spots produced, to identify substances or test purity.
- Learn the four standard gas tests: a lit splint gives a squeaky pop with hydrogen; a glowing splint relights in oxygen; limewater turns cloudy (milky) with carbon dioxide; damp blue litmus paper is bleached white by chlorine.
- Identify metal cations using a flame test (each metal ion colours a Bunsen flame differently) or by adding sodium hydroxide solution and observing the colour, and solubility in excess, of the precipitate formed.
- Identify anions using precipitation reactions: dilute acid plus limewater for carbonates, dilute nitric acid then silver nitrate solution for halides, and dilute hydrochloric acid then barium chloride solution for sulfates.
Worked example
A colourless solution X is tested. Adding sodium hydroxide solution to X produces a white precipitate that dissolves when excess sodium hydroxide is added. Adding a few drops of dilute nitric acid, then silver nitrate solution, to a fresh sample of X produces a white precipitate. Identify the cation and the anion in X, and name the compound.
- Interpret the sodium hydroxide result: a white precipitate that dissolves in excess sodium hydroxide is the specific test result for aluminium ions, Al3+ (calcium and magnesium also give white precipitates, but theirs do not redissolve).
- Interpret the silver nitrate result: nitric acid is added first to remove any carbonate ions, which would otherwise also give a false precipitate with silver nitrate; a white precipitate with silver nitrate then identifies chloride ions, Cl-.
- Combine the two results: the cation is Al3+ and the anion is Cl-.
- Name the compound from its ions: aluminium chloride.
- Final answer: X is a solution of aluminium chloride, AlCl3.
Practice questions
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Q1State the difference between a pure substance and a formulation.Show answer
Answer: A pure substance is a single element or compound, not mixed with anything else. A formulation is a mixture that has been deliberately designed as a useful product, with each component added in a measured quantity for a specific purpose.
Q2Give one piece of evidence that a solid sample is impure, based on its melting behaviour.Show answer
Answer: It melts over a range of temperatures, and starts to melt below the known melting point of the pure substance, rather than melting sharply at one fixed temperature.
Q3Name the gas that relights a glowing splint.Show answer
Answer: Oxygen.
Q4Describe the result of the test for carbon dioxide.Show answer
Answer: The gas is bubbled through limewater, which turns cloudy (milky).
Q5Which reagent is added, and in what colour change, to test for chlorine gas?Show answer
Answer: Damp blue litmus paper is used; it is bleached and turns white.
Q6A solid gives a lilac flame test. Name the metal ion present.Show answer
Answer: Potassium, K+.
Q7State the reagent added, and the result, that shows a white solid contains sulfate ions.Show answer
Answer: Dilute hydrochloric acid is added, then barium chloride solution; a white precipitate (barium sulfate) forms if sulfate ions are present.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student is given a white solid and told it contains one type of metal cation and one type of halide anion. Describe a series of tests the student could carry out to identify both ions, stating the reagents used and the result expected for each ion that could be present. (Cations to consider: calcium, copper, iron(II), iron(III). Anions to consider: chloride, bromide, iodide.)
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Explain why a pure substance can be identified by its melting point, but this method cannot easily be used to identify one component of a formulation such as a medicine tablet.
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Free printable worksheet
Want more practice on paper? Download the chemical analysis worksheet pack - 15 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 9 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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