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What pure means in chemistry, how melting points show purity, what a formulation is, and how to run and read a paper chromatogram, including Rf values.
Specification references this topic covers: AQA 5.3.1, 5.8.1; Edexcel C0.1, C2.5, C2.6, C2.9, C2.10, C2.11, C7.5.
In everyday life "pure" means nothing has been added. In chemistry it means something stricter. This lesson is about telling pure substances from mixtures, and about a method called chromatography that separates coloured mixtures.
It matters because people need to know what is in their medicine, their food and their water. A chemist checks purity before a product is sold, and chromatography is one of the standard checks.
By the end you will be able to use melting point data, explain what a formulation is, run and read a chromatogram, and calculate an Rf value.
In chemistry a pure substance is a single element or a single compound, not mixed with anything else. Orange juice from a carton might be called pure in a shop, but a chemist sees a mixture of water, sugars, acids and other substances.
Pure substances melt and boil at specific temperatures. Pure ice melts at 0 °C and pure water boils at 100 °C (at normal pressure). A mixture melts over a range of temperatures, and it usually starts to melt at a lower temperature than the pure substance.
So melting point data can show purity. A sharp melting point means the sample is pure. A melting range means there is something else in it.
A formulation is a mixture that has been designed as a useful product. Each chemical in it has a job, and the amounts are measured carefully so that the product has the properties it needs.
Fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods are all formulations. A paint, for example, has a pigment for the colour, a binder to make a hard film and a solvent so that it can be spread.
A formulation is not a pure substance. If you are given the label of a product that lists several ingredients in set amounts, you can say it is a formulation. You do not need to know the names of the chemicals in products sold under a brand name.
Chromatography separates a mixture of soluble substances. In paper chromatography, a spot of the mixture is placed on a line near the bottom of a piece of paper. The paper stands in a solvent such as water. The solvent moves up the paper and carries the substances with it.
The paper is the stationary phase because it stays still. The solvent is the mobile phase because it moves. Each substance in the mixture spends its time between the two phases differently. A substance that dissolves well in the solvent travels far. A substance that sticks to the paper travels a short way.
So the substances spread out into separate spots. A pure substance gives one spot. A mixture can give several spots.
Draw the start line (the origin) in pencil about 1 cm from the bottom. Pencil is used because it does not dissolve in the solvent. An ink line would run up the paper with the solvent and spoil the results.
Put small spots of each sample on the line, with space between them, and let them dry. Stand the paper in a beaker with a shallow layer of solvent. The solvent level must be below the pencil line, or the spots would just dissolve into the beaker. Put a lid on the beaker to stop the solvent evaporating.
When the solvent is near the top, take the paper out and draw a line at the solvent front straight away, because it will evaporate. Let the paper dry. In the inks practical you can also use simple distillation to separate the water from the ink, so that you can see the solvent and the dyes separately.
Count the spots in each lane. One spot means a pure substance. Two or more spots mean a mixture. A mixture with three spots contains at least three substances.
To identify a substance, compare it with known substances run on the same paper. If a spot in your sample is at the same height and is the same colour as a known substance, they may be the same substance.
Under the same conditions a compound always travels the same fraction of the solvent distance. This fraction is the Rf value, so Rf values can be compared with reference tables.
The Rf value is a ratio: Rf = distance moved by the substance ÷ distance moved by the solvent. Measure the substance distance from the start line to the centre of the spot. Measure the solvent distance from the start line to the solvent front.
Rf has no unit because the units cancel. It is always between 0 and 1. A substance that does not move has an Rf of 0. One that moves with the solvent has an Rf of 1. Quote it to 2 significant figures unless told otherwise.
Different compounds usually have different Rf values in the same solvent, so an Rf value helps to identify a compound. A different solvent gives different Rf values, so compare only like with like.
Pure means a single element or compound. Pure substances have sharp melting points. Mixtures melt over a range.
A formulation is a designed mixture with measured amounts, such as a paint or a medicine.
Chromatography separates soluble substances. Rf = distance moved by substance ÷ distance moved by solvent.
A spot moves 3.0 cm up the paper. The solvent front moves 6.0 cm. Calculate the Rf value.
Answer: 0.50
A sample gives two spots, at 2.4 cm and 5.6 cm from the start line. The solvent front is 8.0 cm from the start line. Calculate the Rf value of the spot that moved further, and say whether the sample is pure.
Answer: Rf = 0.70. The sample is a mixture
A student wants to find out whether a food colouring X contains dye A or dye B. Pure A has Rf 0.30 and pure B has Rf 0.55. In the same solvent, X gives spots with Rf 0.30 and 0.80. The solvent front moved 10.0 cm. Say what X contains, and how far the spot with Rf 0.80 moved.
Answer: X contains dye A and another substance. The 0.80 spot moved 8.0 cm
Purity, formulations and chromatography practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
In paper chromatography, what name is given to the paper?
Answer: stationary phase
In paper chromatography, what name is given to the solvent?
Answer: mobile phase
Should the start line on chromatography paper be drawn in pencil or pen?
Answer: pencil
How many spots does a pure substance give on a chromatogram?
Answer: one
Rf equals the distance moved by the substance divided by the distance moved by what?
Answer: the solvent
A spot moves 4.5 cm. The solvent front moves 9.0 cm. Calculate the Rf value.
Answer: 0.50
A spot moves 3.0 cm. The solvent front moves 7.5 cm. Calculate the Rf value.
Answer: 0.40
Does a pure substance melt at one sharp temperature or over a range of temperatures?
Answer: one sharp temperature
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
(a) A bottle of orange juice is labelled "pure orange juice". Explain why a chemist would say it is not pure. [2 marks] (b) Describe how melting point data can show that a solid is pure. [2 marks]
Answer: (a) orange juice is a mixture of several substances, but a pure substance is a single element or compound (b) a pure solid melts at one sharp temperature, but an impure solid melts over a range of temperatures
A student runs a chromatogram. The solvent front moves 10.0 cm. Dye A moves 3.0 cm, dye B moves 5.0 cm and dye C moves 7.5 cm. Sample X gives two spots, at 3.0 cm and 7.5 cm. (a) Calculate the Rf value of dye A. [2 marks] (b) Which dyes are in sample X? [1 mark] (c) Explain why the start line is drawn in pencil. [1 mark] (d) Explain how the chromatogram shows that X is a mixture. [1 mark]
Answer: (a) 0.30 (b) A and C (c) pencil does not dissolve in the solvent, but ink would run (d) there are two spots, so more than one substance
Using an ink pen for the start line
Use pencil. Ink dissolves and runs up the paper with the solvent.
Putting the start line under the solvent
The solvent level must be below the pencil line, or the spots dissolve into the beaker.
Working out Rf the wrong way up
Rf = distance moved by substance ÷ distance moved by solvent. It is never more than 1.
Measuring to the edge of the spot
Measure from the start line to the centre of the spot.
Saying a mixture has a sharp melting point
A pure substance has a sharp melting point. A mixture melts over a range.
Purity, formulations and chromatography is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 4.
Using an ink pen for the start line. Use pencil. Ink dissolves and runs up the paper with the solvent.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
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