Chemistry: Chemical Analysis

Pure substances and formulations

In chemistry, a pure substance is a single element or compound rather than a mixture. A pure substance melts and boils at specific temperatures. An impurity usually changes the measured melting or boiling behaviour, so comparing experimental data with accepted values can provide evidence about purity.

A formulation is a mixture designed for a particular purpose. Fuels, paints, medicines, cleaning products and fertilisers can all be formulations. Each component has a role, and the quantities are controlled so the product has consistent properties. That makes a formulation useful and reproducible, but it does not make it a pure substance.

Using paper chromatography

Paper chromatography separates soluble substances because they distribute differently between a stationary phase and a mobile phase. The solvent carries the sample up the paper. A mixture may form several spots, while a pure compound should form one spot under the same experimental conditions.

An Rf value is calculated by dividing the distance travelled by the centre of a substance spot by the distance travelled by the solvent front. Rf values can help identify compounds only when the solvent, stationary phase and other conditions are comparable. A spot colour or Rf value on its own is not enough if the conditions differ.

• Draw the start line in pencil so it does not dissolve in the solvent.
• Keep the sample spots above the solvent level.
• Mark the solvent front before it evaporates.
• Measure from the same origin line when calculating Rf values.

Required tests for common gases

Gas tests use a specified procedure and a distinctive positive result. Hydrogen gives a squeaky pop with a lit splint. Oxygen relights a glowing splint. Carbon dioxide turns limewater milky or cloudy. Chlorine bleaches damp litmus paper after it may first turn red.

The observation matters as much as the name of the test. A conclusion should match the evidence: for example, limewater turning cloudy supports the presence of carbon dioxide. It does not prove that every other substance in a mixture has been identified.

• Hydrogen: lit splint; squeaky pop.
• Oxygen: glowing splint; the splint relights.
• Carbon dioxide: bubble through limewater; it turns milky or cloudy.
• Chlorine: damp litmus paper; the colour is bleached.

Keep related chemistry topics separate

This Year 10 practice collection also includes linked activities on acids, alkalis, neutralisation and cracking. Those are useful GCSE Chemistry topics, but they are not all methods of chemical analysis. Keep the definitions separate when revising so an answer about identifying an unknown substance does not drift into an unrelated reaction topic.

A reliable revision answer names the analytical method, states the observation or measurement and then gives a conclusion that the evidence supports. Use the related activities to strengthen broader chemistry knowledge without treating every chemical reaction as an analysis test.