Chemical Analysis — AQA Combined Science: Trilogy
Chemical analysis is about identifying substances and checking purity. In Combined Science this covers purity and formulations, chromatography, and tests for common gases.
Pure substances
In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
A pure substance:
- melts and boils at a specific, sharp temperature.
- An impure substance melts/boils over a range of temperatures, and impurities usually lower the melting point and raise the boiling point.
Measuring melting point is therefore a way to test purity — a sharp melting point at the expected value indicates a pure substance.
Formulations
A formulation is a useful mixture with a precise purpose, made by mixing components in carefully measured quantities so the product has the required properties.
Examples of formulations:
- medicines (active drug plus fillers and coatings)
- fuels, cleaning products, paints, fertilisers, alloys and foods.
Each component is present in a measured amount to give the desired effect.
Chromatography
Paper chromatography separates mixtures of soluble substances (e.g. coloured dyes) and helps identify them.
How it works:
- The stationary phase is the chromatography paper.
- The mobile phase is the solvent that moves through the paper.
- Substances that are more attracted to the solvent (more soluble) and less attracted to the paper travel further.
A pure substance produces a single spot; a mixture produces several spots.
Rf values
Each substance has an Rf value under the same conditions:
$$R_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}$$
The Rf value is always between 0 and 1. Comparing Rf values (or running known substances alongside) helps identify the components.
Required practical: using paper chromatography to separate a mixture of food colourings or inks, measuring distances and calculating Rf values. Remember to draw the start line in pencil (so it doesn't dissolve) and keep it above the solvent level.
Tests for gases
Learn these four standard gas tests:
| Gas | Test | Positive result |
|---|---|---|
| Hydrogen | Lighted splint near the mouth of a tube | Squeaky pop |
| Oxygen | Glowing splint inserted into the tube | Relights the glowing splint |
| Carbon dioxide | Bubble the gas through limewater | Limewater turns cloudy/milky |
| Chlorine | Hold damp litmus paper in the gas | Paper is bleached white (turns red first if litmus is blue) |
Why analysis matters
Identifying substances and checking purity is essential in:
- the food and drink industry (checking for contaminants),
- forensics (identifying substances at a crime scene),
- medicine (ensuring drugs are pure and correctly formulated),
- and quality control in manufacturing.
Instrumental methods (used at higher levels) are faster, more accurate and more sensitive than manual chemical tests, and can detect tiny amounts.
Exam tips
- A pure substance has a sharp melting/boiling point; impurities broaden and shift it.
- Define a formulation as a mixture made in measured quantities for a purpose — give an example.
- Learn the Rf equation and that a pure substance gives one spot.
- Memorise the four gas tests and their exact positive results.
- In chromatography practicals, the pencil start line and keeping it above the solvent are common mark points.