How do chemists test for and identify the substances in a sample?
Pure substances and formulations, chromatography and the Rf value, and the tests for common gases (hydrogen, oxygen, carbon dioxide and chlorine).
A focused answer to the AQA GCSE Combined Science: Trilogy Chemical analysis topic, covering pure substances and formulations, chromatography and the Rf value, and the laboratory tests for hydrogen, oxygen, carbon dioxide and chlorine.
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What this topic is asking
AQA wants you to define pure substances and formulations, describe paper chromatography and calculate Rf values, and recall the laboratory tests for hydrogen, oxygen, carbon dioxide and chlorine.
Pure substances and formulations
A formulation is a mixture that has been designed as a useful product, with each component present in a carefully measured quantity so the product has the right properties. Examples include fuels, cleaning products, paints, cosmetics, fertilisers, metal alloys and medicines (where the dose of active drug is precisely controlled). The key idea AQA tests is that a formulation is deliberately mixed for a purpose, not an accidental or impure mixture. The purity of a substance can be checked by its melting point: a pure substance melts sharply at a single temperature, whereas impurities lower the melting point and make it melt over a range, so a measured melting-point range is evidence of impurity.
Chromatography
In the required practical, the start line is drawn in pencil (pencil does not dissolve in the solvent and so does not move), the solvent level starts below the spots, and the paper is removed before the solvent reaches the top so the solvent front can be marked. A pure substance produces a single spot in any solvent; a mixture separates into several spots. Because a substance has a characteristic Rf value in a given solvent, comparing Rf values (or running known reference substances alongside) allows the components of a mixture to be identified.
Tests for gases
These tests are quick, qualitative and use simple apparatus, which is why they are favourite exam questions: you must give both the test and the exact positive observation to gain full marks.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20193 marksIn a chromatography experiment a spot of dye travels 3.6 cm and the solvent front travels 8.0 cm. Calculate the Rf value of the dye, and explain why a pure substance produces only one spot.Show worked answer →
A Chemistry Paper 2 chromatography calculation. Method: Rf value . The value has no units and must be between 0 and 1. For the explanation: a pure substance is a single compound, so all of it has the same attraction to the paper and the solvent and travels at the same speed, producing one spot; a mixture would separate into several spots. Markers credit the correct ratio (a common slip is to invert it), an answer with no units between 0 and 1, and the reasoning that one spot indicates a single substance.
AQA 20214 marksDescribe the chemical tests you would use to identify samples of hydrogen, oxygen and carbon dioxide gas, giving the positive result for each.Show worked answer →
A Chemistry Paper 2 recall question on the required gas tests. Reward each test paired with its result: for hydrogen, hold a lit (burning) splint at the mouth of the tube and a squeaky pop is heard; for oxygen, place a glowing splint into the gas and it relights; for carbon dioxide, bubble the gas through limewater (calcium hydroxide solution) and it turns milky (cloudy white). Markers award a mark for each correct test-and-result pair. The most common confusion is between the oxygen test (glowing splint relights) and the hydrogen test (lit splint, squeaky pop), so keep them distinct.
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