Chemical analysis: purity, chromatography and tests — AQA GCSE Chemistry revision

Purity and formulations

• A pure substance contains only one element or compound and has a single, sharp melting or boiling point.
• Impure substances melt or boil over a range of temperatures rather than at one fixed point.
• A formulation is a mixture designed with a precise purpose, such as a medicine, paint or fertiliser, where each component has a specific function.
• Testing melting or boiling points can help identify whether a sample is pure or a mixture.

Chromatography

• Paper chromatography separates substances in a mixture based on how strongly they interact with the paper (stationary phase) compared to the solvent (mobile phase).
• The Rf value is calculated as the distance moved by the substance divided by the distance moved by the solvent front.
• Different substances have different Rf values in the same solvent, allowing identification by comparison with known reference samples.
• A pure substance produces only one spot on a chromatogram, while a mixture produces multiple spots.

Key terms

Pure substance

A single element or compound not mixed with any other substance.

Formulation

A mixture designed to have specific properties, made up of components in carefully measured proportions.

Chromatography

A technique used to separate and identify substances in a mixture based on their movement through a stationary and mobile phase.

Rf value

The ratio of distance travelled by a substance to distance travelled by the solvent front in chromatography.

Stationary phase

The material (e.g. paper) through which the mobile phase moves in chromatography.

Mobile phase

The solvent that moves through the stationary phase carrying dissolved substances.

Common exam traps

• A pure substance in chemistry means a single element or compound, not necessarily something 'unpolluted' or 'natural' as in everyday language.
• Impure substances melt/boil over a range of temperatures, not at a single fixed higher point.
• Rf value is a ratio (no units) and must always be less than or equal to 1.

Practice questions with answers

1. 1. What defines a pure substance in chemistry?

   • • A substance found in nature
   • • A single element or compound with no other substances mixed in
   • • Any liquid that looks clear
   • • A substance with a low boiling point

   Answer: A single element or compound with no other substances mixed in

   Chemical purity refers strictly to composition, not appearance or origin.

2. 2. How does the melting behaviour of an impure substance differ from a pure one?

   Answer: It melts over a range of temperatures rather than at one sharp temperature

   Impurities disrupt the regular structure, causing melting over a range.

3. 3. What is a formulation?

   • • A pure element only
   • • A mixture designed for a specific purpose with measured proportions
   • • Any random combination of chemicals
   • • A type of chemical bond

   Answer: A mixture designed for a specific purpose with measured proportions

   Formulations like medicines and paints are carefully designed mixtures.

4. 4. Name the formula used to calculate an Rf value.

   Answer: Distance moved by substance divided by distance moved by solvent front

   This ratio allows identification and comparison of substances.

5. 5. In paper chromatography, what is the stationary phase?

   • • The solvent
   • • The paper
   • • The sample spot
   • • The container lid

   Answer: The paper

   The paper stays still while the solvent (mobile phase) moves through it.

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