What you'll learn
Chemical analysis is the unit of AQA GCSE Combined Science: Trilogy concerned with identifying substances and judging how pure they are. It is a short unit but a precise one, because much of it turns on using words in their exact scientific sense rather than their everyday sense — pure, formulation and clear all mean something specific here. The unit covers what a pure substance is in chemistry, how melting and boiling point data reveal purity, what a formulation is and why so many everyday products are formulations, how paper chromatography separates a mixture and how the Rf value identifies its components, and the four tests for common gases. By the end you should be able to distinguish a pure substance from a mixture using melting point data, explain how chromatography works in terms of two phases, calculate an Rf value, and state the test and positive result for hydrogen, oxygen, carbon dioxide and chlorine. This unit is assessed on Chemistry Paper 2 and includes the required practical on chromatography.
Key terms and definitions
Pure substance — in chemistry, a single element or compound not mixed with any other substance
Formulation — a mixture designed as a useful product, in which each component is present in a measured quantity for a particular purpose
Chromatography — a technique that separates the components of a mixture according to how they distribute between a stationary phase and a mobile phase
Stationary phase — the phase that does not move, which in paper chromatography is the paper
Mobile phase — the phase that moves through the stationary phase, which in paper chromatography is the solvent
Solvent front — the furthest point reached by the solvent as it moves up the paper
Rf value — the distance moved by a substance divided by the distance moved by the solvent, always a number between 0 and 1
Baseline — the pencil line on which the samples are spotted at the start of a chromatography run
Core concepts
Purity in chemistry and in everyday use
In everyday speech, pure often means natural or unadulterated: pure orange juice, pure spring water. In chemistry the word has a much narrower meaning. A pure substance is a single element or compound, not mixed with anything else at all. Pure orange juice is, chemically speaking, a complex mixture.
Questions frequently set up this contrast deliberately, giving a product labelled pure in the everyday sense and asking whether it is pure in the chemical sense. The answer turns on whether it consists of one substance only.
Using melting and boiling points to judge purity
Pure substances melt and boil at specific, sharp temperatures. Pure water melts at exactly 0 degrees Celsius and boils at exactly 100 degrees Celsius at atmospheric pressure.
Mixtures behave differently. They melt and boil over a range of temperatures rather than at a single point, and the presence of an impurity lowers the melting point and raises the boiling point relative to the pure substance.
This gives a practical test. If a sample melts sharply at the accepted value for a substance, it is pure. If it melts gradually over several degrees, or melts at a lower temperature than expected, it is impure. Questions typically present a table of melting ranges and ask which sample is purest; the answer is the one with the narrowest range at the expected value.
Formulations
A formulation is a mixture that has been designed as a useful product. What distinguishes a formulation from any old mixture is that each component is present in a carefully measured quantity and is there to contribute a particular property to the product as a whole.
Formulations are everywhere: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all formulated. A paint, for example, contains a pigment for colour, a binder to hold the pigment to the surface, a solvent to make it spreadable, and additives to alter how it dries.
When a question asks you to explain why something is a formulation, the answer must contain both halves: it is a mixture, and the components are present in measured amounts each having a specific purpose. Saying only that it is a mixture is not enough.
Paper chromatography
Chromatography separates substances by exploiting how differently they distribute themselves between two phases. The stationary phase is the chromatography paper; the mobile phase is the solvent that travels through it.
A substance that is more strongly attracted to the solvent, and less strongly attracted to the paper, will be carried further up the paper. A substance more strongly attracted to the paper will move only a short distance. Since different substances have different balances of attraction, they separate into distinct spots.
The practical detail matters and is regularly examined. The baseline must be drawn in pencil, because pencil is insoluble in the solvent and so will not run and contaminate the result, whereas an ink line would separate into its own components. The baseline must sit above the level of the solvent in the container, because if the spots were below the solvent level they would simply dissolve into it and be washed away rather than travelling up the paper. A lid on the container reduces evaporation of the solvent.
Interpreting the result is straightforward once the principle is clear. A pure substance produces a single spot in all solvents. A mixture separates into two or more spots, and the number of spots gives the minimum number of components present.
Rf values
The Rf value is a measure of how far a substance travelled relative to the solvent. It is calculated as the distance moved by the substance divided by the distance moved by the solvent.
Both distances are measured from the baseline. The distance to the substance is measured to the centre of its spot, and the distance to the solvent is measured to the solvent front. Because the substance can never travel further than the solvent, the Rf value is always between 0 and 1, which gives a quick check on any answer.
The same substance in the same solvent always gives the same Rf value, so Rf values can be compared with reference data to identify an unknown. Change the solvent and the Rf value changes, which is why identification requires the solvent to be stated.
Tests for gases
Four gas tests are required, and each has a specific procedure and a specific positive result.
Hydrogen is tested with a burning splint held at the open end of a test tube. A positive result is a squeaky pop, produced as the hydrogen burns rapidly.
Oxygen is tested with a glowing splint inserted into a test tube of the gas. A positive result is that the splint relights.
Carbon dioxide is tested by bubbling the gas through an aqueous solution of calcium hydroxide, known as limewater. A positive result is that the limewater turns milky, or cloudy white.
Chlorine is tested with damp litmus paper placed in the gas. A positive result is that the litmus paper is bleached white. Note that the paper must be damp, and that the result is bleaching rather than a colour change to red; chlorine solution is acidic, so the paper may briefly turn red before being bleached, but the bleaching is the identifying result.
Worked examples
Example 1: Judging purity from melting point (3 marks)
Pure aspirin melts at 135 degrees Celsius. A student's sample melts between 128 and 132 degrees Celsius. What does this tell you about the sample?
The sample melts over a range of temperatures rather than at a single sharp temperature, and that range is below the accepted melting point of pure aspirin. Both observations indicate that the sample is impure, because impurities lower the melting point and cause melting to occur over a range rather than at one value.
Example 2: Calculating an Rf value (3 marks)
On a chromatogram, the solvent front is 8.0 centimetres from the baseline and the centre of a spot is 3.2 centimetres from the baseline. Calculate the Rf value.
The Rf value is the distance moved by the substance divided by the distance moved by the solvent, which is 3.2 divided by 8.0. This gives 0.40. Rf values have no units, and as a check the answer lies between 0 and 1 as it must.
Example 3: Interpreting a chromatogram (4 marks)
A chromatogram of a food colouring shows three spots. A known dye run alongside it produces a single spot at the same height as one of the three. What can be concluded?
The food colouring is a mixture, because it has separated into more than one spot, and it contains at least three different substances. Because one of its spots has travelled the same distance as the known dye in the same solvent, that spot has the same Rf value as the known dye, so the food colouring probably contains that dye. The known dye is itself pure, since it produced only one spot.
Common mistakes and how to avoid them
The most frequently penalised error in this unit is saying that limewater turns clear or that bromine water turns clear in the related organic test. Limewater turns milky or cloudy white; describing a change as turning clear almost never earns a mark, because most of these solutions are transparent throughout.
Students regularly draw the chromatography baseline in ink. Pencil is required because it is insoluble in the solvent, and this is worth a mark on its own.
Another routine error is starting the paper with the spots below the solvent level. The spots would dissolve into the solvent instead of travelling, and no separation would occur.
In Rf calculations, many students measure to the top or bottom of a spot rather than to its centre, or divide the wrong way round and obtain a value greater than 1. Any Rf above 1 is impossible and signals that the division has been inverted.
Finally, in formulation questions, answers often stop at calling the product a mixture. The definition requires the measured quantities and the specific purpose of each component.
Exam technique for "Chemistry: Chemical Analysis"
Gas tests are pure recall and are guaranteed marks, so learn them as three-part statements: the gas, the test, the positive result. Write all three in the answer even if the question seems to ask for fewer.
When given melting point data, quote the numbers from the table in your answer. Saying the sample is impure earns less than saying it melts from 128 to 132 degrees rather than sharply at 135, so it is impure.
For chromatography, be ready to explain the method in terms of the two phases and the relative attractions. Answers that describe only what is seen, without the mechanism, cap out quickly.
Show the division in Rf calculations even though it is a single step, and give the answer to two significant figures unless told otherwise. Include no unit, because the two distances cancel.
Quick revision summary
A pure substance in chemistry is a single element or compound, and it melts and boils at sharp temperatures; impurities lower the melting point and spread it over a range. A formulation is a mixture designed as a product, with each component in a measured quantity for a specific purpose. Paper chromatography separates substances between a stationary phase, the paper, and a mobile phase, the solvent, according to their relative attractions; the baseline is drawn in pencil above the solvent level. A pure substance gives one spot; a mixture gives several. The Rf value is the distance moved by the substance divided by the distance moved by the solvent, measured from the baseline to the spot centre and to the solvent front, always between 0 and 1. Hydrogen gives a squeaky pop with a burning splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, and chlorine bleaches damp litmus paper.