Pearson International A Level Chemistry Core Practical 8: Analyse inorganic and organic unknowns

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Identify ions or functional groups through a planned sequence of discriminating tests with controls and confirmatory evidence.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use small test tubes, spotting tile, droppers, approved test reagents, water bath, pH materials and waste containers. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Record initial appearance and divide the unknown into fresh small portions for independent tests.
2. Begin with tests that separate broad possibilities without contaminating later samples.
3. Add reagents dropwise, then in excess when solubility is diagnostic, and record observations rather than inferred identities.
4. Use at least one confirmatory test and reconcile all positive and negative evidence.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: test reagent or condition applied to separate aliquots.
• Dependent variable: colour, precipitate, gas, pH or other analytical observation.