Pearson Edexcel International GCSE Science Single Award Practical Skills 3: Chemistry measurements and reactions

This note covers the explicitly labelled Chemistry practical work in Pearson Science Single Award 4SS0: paper chromatography, estimating the percentage of oxygen in air, measuring temperature changes, and investigating the rate of reaction between marble chips and hydrochloric acid. These experiments are assessed through written questions, so methods must be understood as evidence-producing systems rather than memorised recipes.

Paper chromatography

Paper chromatography separates soluble substances because they divide differently between the mobile solvent and the stationary paper. Draw a pencil baseline near the bottom of the paper, place small concentrated spots of ink or food colouring on the line, and stand the paper in a shallow solvent. The solvent level must begin below the spots. Cover the container where appropriate and let the solvent rise. Remove the paper before the solvent reaches the top and mark the solvent front immediately in pencil.

Pencil is used because graphite does not dissolve and travel with the solvent. A pen line could separate into pigments and contaminate the chromatogram. Small spots give sharper components; a broad wet spot can merge neighbouring substances. Reapply a spot only after the previous drop has dried.

If a component and solvent front are measured from the same baseline, calculate:

$$R_f = \frac{\text{distance travelled by component}}{\text{distance travelled by solvent front}}$$