Chemical analysis: purity and formulations

What you'll learn

This revision guide covers the essential AQA GCSE Chemistry content on purity and formulations. You'll learn how chemists define pure substances differently from everyday language, understand what formulations are and why they're important in industry, and master the analytical techniques used to test purity. These concepts underpin paper chromatography and other separation methods you'll encounter in exam questions.

Key terms and definitions

Pure substance — a single element or compound that is not mixed with any other substance; has a sharp, specific melting and boiling point

Formulation — a mixture that has been designed as a useful product, where each component is present in a measured quantity and contributes to the properties of the formulation

Melting point — the temperature at which a solid changes to a liquid; pure substances melt at a specific temperature, whilst impure substances melt over a range of temperatures

Boiling point — the temperature at which a liquid changes to a gas; pure substances have a sharp boiling point, whilst impurities raise or lower this value

Rf value — in chromatography, the ratio of the distance moved by a substance to the distance moved by the solvent (retention factor)

Mobile phase — in chromatography, the phase that moves through or over the stationary phase, carrying the components of the mixture

Stationary phase — in chromatography, the phase that does not move and over which the mobile phase passes

Chromatogram — the pattern of spots or bands produced when a mixture is separated by chromatography

Core concepts

Purity in chemistry vs everyday use

In everyday language, "pure" often means natural, clean, or of high quality. Orange juice labelled "pure" might contain pulp, water, sugars, and vitamins. However, in chemistry, pure has a precise, scientific definition.

A pure substance in chemistry contains only one element or one compound with nothing else mixed with it. Examples include:

• Pure water (H₂O only)
• Pure copper metal (Cu only)
• Pure sodium chloride (NaCl only)

Most substances you encounter daily are mixtures, not pure substances. Tap water contains dissolved minerals and gases. "Pure orange juice" is a mixture of water, sugars, acids, vitamins, and other compounds.

Why purity matters:

• Chemical reactions require known quantities of reactants
• Impurities can affect product quality in manufacturing
• Pharmaceutical companies must ensure drugs are pure and safe
• Research chemists need pure samples for accurate analysis

Testing purity using melting and boiling points

Pure substances have characteristic fixed points at which they change state. These values are published in data books and can be used to test purity.

Melting point testing:

• Pure substances melt at a specific, sharp temperature
• If a substance melts over a range of temperatures, it contains impurities
• Impurities generally lower the melting point and broaden the melting range
• Example: Pure ice melts at exactly 0°C; ice containing salt melts below 0°C over a temperature range

Boiling point testing:

• Pure substances boil at a specific, sharp temperature
• Impurities typically raise the boiling point
• A substance that boils over a range indicates impurity
• Example: Pure water boils at 100°C (at standard atmospheric pressure); seawater boils at approximately 102°C

Practical application:

To test if a sample is pure aspirin (melting point 136°C):

1. Measure the melting point of your sample using apparatus like a melting point tube
2. If it melts sharply at 136°C, the sample is pure
3. If it melts at 130-135°C (a range), the sample contains impurities

What are formulations?

A formulation is a mixture made to a specific recipe where each component is in a carefully measured quantity. Formulations are designed products where every ingredient has a specific purpose.

Key characteristics of formulations:

• Each component is present in a measured, precise amount
• Components are mixed together in a particular way
• Each ingredient contributes specific properties to the product
• The proportions are optimised through research and testing
• Changing the recipe would affect the product's performance

Examples of formulations:

Formulations in everyday life

The development of formulations involves chemists, pharmacologists, and engineers working together. Understanding formulations helps explain why products perform differently even when they seem similar.

Paint formulations:

Modern paints are complex formulations containing:

• Pigment — provides colour and opacity
• Binder (polymer resin) — forms a film that holds the pigment and adheres to surfaces
• Solvent — makes the paint spreadable; evaporates as paint dries
• Additives — modify properties like drying time, texture, or resistance to mould

The ratio of these components determines whether paint is gloss or matt, quick-drying or slow-drying, for interior or exterior use.

Pharmaceutical formulations:

A paracetamol tablet contains:

• Active ingredient (paracetamol) — provides pain relief
• Binder — holds ingredients together
• Filler (starch) — adds bulk to make tablet large enough to handle
• Lubricant (magnesium stearate) — helps tablet pass through machinery and be swallowed
• Coating — makes swallowing easier, controls release rate, masks taste

The precise formulation ensures consistent, safe dosing and optimal delivery of the drug.

Alloys as formulations:

Alloys are formulations made from metals mixed in specific proportions:

• Steel — iron with small amounts of carbon and sometimes other metals; stronger than pure iron
• Brass — copper and zinc; harder than copper, corrosion-resistant
• Bronze — copper and tin; harder than copper, used for sculptures and ship fittings

Paper chromatography and purity analysis

Chromatography is an important analytical technique used to separate mixtures and test purity. Paper chromatography is specifically tested at GCSE level.

How paper chromatography works:

1. A pencil baseline is drawn near the bottom of chromatography paper (the stationary phase)
2. Small spots of the mixture to be tested are placed on the baseline
3. The paper is placed in a beaker containing a shallow layer of solvent (the mobile phase)
4. The solvent must be below the baseline
5. The solvent rises up the paper, carrying different substances different distances
6. Substances separate based on their solubility in the solvent and attraction to the paper
7. The resulting pattern is called a chromatogram

Why substances separate:

• More soluble substances travel further up the paper with the solvent
• Substances more attracted to the paper travel less far
• Different substances have different balances of these properties
• Each substance produces a spot at a characteristic height

Testing purity using chromatography:

• A pure substance produces a single spot on the chromatogram
• An impure substance (mixture) produces multiple spots
• The number of spots indicates the minimum number of different substances present

Identifying substances using Rf values:

The Rf value (retention factor) is calculated using:

Rf = distance moved by substance / distance moved by solvent

Key points about Rf values:

• Always between 0 and 1 (substance cannot travel further than solvent)
• No units (it's a ratio)
• Characteristic for each substance in a particular solvent system
• Can be compared with reference data to identify unknown substances
• Same substance in same conditions always gives same Rf value

Practical considerations:

• Use pencil for the baseline (ink would dissolve and interfere)
• Keep the solvent level below the baseline spots
• Use a lid on the beaker to prevent evaporation affecting results
• Allow chromatogram to dry before measuring distances
• Some colourless substances need locating agents (e.g., UV light, iodine vapour)

Worked examples

Example 1: Melting point determination

Question: A student heats a sample of a white solid. The solid starts to melt at 78°C and completely melts by 84°C. Pure stearic acid has a melting point of 69°C. Pure benzoic acid has a melting point of 122°C. What can you conclude about the sample? [3 marks]

Answer:

The sample is not pure [1 mark] because it melts over a range of temperatures (78-84°C) rather than at a single sharp temperature [1 mark]. Pure substances melt at specific temperatures. The sample does not contain pure stearic acid or pure benzoic acid as the melting range does not match either value [1 mark].

Examiner note: Award marks for stating the sample is impure, explaining the evidence (melting range), and comparing to the given data.

Example 2: Chromatography Rf calculation

Question: A student uses paper chromatography to analyse food colourings. The solvent travels 8.0 cm up the paper. One of the dyes produces a spot with its centre 6.4 cm from the baseline. Calculate the Rf value for this dye. [2 marks]

Answer:

Rf = distance moved by substance / distance moved by solvent [1 mark]

Rf = 6.4 / 8.0 = 0.8 [1 mark]

Examiner note: Must show the formula or working. Rf values have no units. Do not write 0.80 cm/cm.

Example 3: Analysing formulations

Question: Suncream is a formulation containing several components. Explain what is meant by the term 'formulation' and suggest why suncream needs to contain different components. [4 marks]

Answer:

A formulation is a mixture made to a specific recipe [1 mark] where each component is present in a measured quantity [1 mark]. Suncream needs different components because each has a specific function [1 mark]. Examples: UV-blocking chemicals protect skin, moisturisers prevent drying, emulsifiers keep ingredients mixed, waterproofing agents make it water-resistant [1 mark for any appropriate example with function].

Examiner note: Definition must include both the idea of a designed mixture and measured quantities. Functions should be specific, not vague statements.

Common mistakes and how to avoid them

• Confusing everyday and scientific meanings of "pure" — Remember that in chemistry, pure means only one substance with nothing else added, not "natural" or "high quality"

• Writing that impurities always lower melting points — While impurities generally lower melting points and broaden the range, the key indicator of impurity is melting over a range rather than at a sharp temperature

• Forgetting Rf values have no units — Rf is a ratio (distance ÷ distance), so units cancel. Never write "0.8 cm" or "0.8 cm/cm"

• Measuring to the wrong point in chromatography — Always measure to the centre of the spot, not the edge. Measure the solvent front (furthest point reached), not where the solvent currently sits

• Thinking more spots means less pure — Each spot represents a different component. Two spots means at least two substances (still impure), but that doesn't mean it's "more impure" than something with three spots

• Using pen instead of pencil for the baseline — Ink dissolves in the solvent and interferes with results. Always use pencil, which doesn't dissolve

Exam technique for "Chemical analysis: purity and formulations"

• "Explain" and "suggest" command words — These require you to give reasons and link ideas. For formulations, state what each component does and why this contributes to the product's overall function (typically 2-4 marks)

• Calculation questions for Rf values — Always write the formula first (Rf = distance moved by substance / distance moved by solvent), substitute values, and give answer to 2 significant figures without units (usually 2 marks: 1 for working, 1 for answer)

• Drawing conclusions from chromatograms — Count spots to determine number of components. Compare Rf values or spot positions with reference samples to identify substances. State "pure" only if single spot appears (typically 1-2 marks per conclusion)

• Describing practical procedures — Use correct scientific terminology (baseline, stationary phase, mobile phase, solvent front). Explain why each step is important, such as "use pencil because ink would dissolve in the solvent and contaminate the chromatogram" (typically 3-6 marks for full method)

Quick revision summary

Pure substances in chemistry contain only one element or compound and have sharp, specific melting and boiling points. Impurities cause melting over a range and alter boiling points. Formulations are designed mixtures with each component in measured quantities serving a specific purpose, such as paints, medicines, and alloys. Paper chromatography separates mixtures: substances dissolve in the mobile phase and move up the stationary phase at different rates. Pure substances produce one spot; mixtures produce multiple spots. Rf values (distance moved by substance ÷ distance moved by solvent) help identify substances.