Experimental Chemistry is Topic 1 of the Sec 3 Pure Chemistry syllabus. It covers choosing measuring apparatus, collecting and drying gases safely, separation and purification methods, paper chromatography with Rf values, and testing for purity.
The topic is assessed in Paper 1 and Paper 2, in the Paper 3/4 practical assessment, and inside later topics such as preparing a pure, dry salt or collecting gases in mole-concept work. Most marks are ‘suggest and explain’ marks: identify the property difference between the substances, match the method to that difference, and state both in the answer.
What these notes cover
- •Why this topic matters
- •Choosing measuring apparatus
- •Volume of liquids: the precision hierarchy
- •Collecting gases
- •Drying gases
- •Hazard symbols and the fume cupboard
- •Separation and purification methods
- •Choosing a separation method
- •Paper chromatography and Rf values
- •Testing for purity
- •Common mistakes and exam technique
- •Summary
Key definitions
- Residue
- The insoluble solid trapped on the filter paper during filtration.
- Filtrate
- The liquid or solution that passes through the filter paper during filtration.
- Rf value (retardation factor)
- The fraction of the distance travelled by the solvent front that a substance travels, in a given solvent at a given temperature; it acts as a fingerprint to identify the substance.
- Locating agent
- A chemical sprayed on a dried chromatogram that reacts with colourless substances, such as amino acids and sugars, to form visible coloured spots so that their Rf values can be measured.
- Pure substance
- A single element or compound, not mixed with anything else; it has a sharp, fixed melting point and boiling point and gives exactly one spot on a chromatogram.
Key formulas
Volume delivered by a burette
volume delivered = final reading − initial reading
Every burette reading is to 2 decimal places, ending in 0 or 5 (nearest 0.05 cm³). The scale reads downwards from 0.00 cm³, so the initial reading is the smaller number, e.g. 26.55 − 1.20 = 25.35 cm³.
Denser or less dense than air
Mᵣ(gas) > Mᵣ(air) ≈ 29 → gas is denser than air
Used to choose between downward delivery (denser, e.g. CO₂, Mᵣ = 44) and upward delivery (less dense, e.g. NH₃, Mᵣ = 17) for soluble gases.
Rf value
Rf = distance moved by substance ÷ distance moved by solvent front
Measure both distances from the baseline, to the centre of the spot and to the solvent front. Rf lies between 0 and 1, has no units and is usually given to 2 significant figures.
Why concentrated sulfuric acid cannot dry ammonia
2NH₃(g) + H₂SO₄(l) → (NH₄)₂SO₄(s)
Ammonia is a basic gas and is consumed by the acidic drying agent; dry ammonia with quicklime (CaO) instead.
Why quicklime cannot dry acidic gases
CaO(s) + CO₂(g) → CaCO₃(s)
A basic drying agent reacts with acidic gases such as CO₂, SO₂, Cl₂ and HCl. The drying agent must not react with the gas being dried.
Strong heating of hydrated copper(II) sulfate
CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g)
The blue crystals lose their water of crystallisation and turn to white powder, so this salt is obtained by crystallisation, not evaporation to dryness.
Worked example
A stationery company suspects that a rival’s ‘single-dye’ green ink X is really a mixture. A chemist runs a spot of X alongside spots of three pure reference dyes A, B and C on the same paper, using ethanol as the solvent. The solvent front is 8.0 cm above the baseline. Measured from the baseline to the centre of each spot, dye A is at 2.0 cm, dye B at 5.6 cm and dye C at 6.8 cm. Ink X shows two spots, at 2.0 cm and 6.8 cm. (a) Calculate the Rf value of each reference dye to 2 significant figures. (b) Deduce which dyes ink X contains.
- 1Use Rf = distance moved by substance ÷ distance moved by solvent front, with both distances measured from the baseline.
- 2Dye A: Rf = 2.0 ÷ 8.0 = 0.25.
- 3Dye B: Rf = 5.6 ÷ 8.0 = 0.70.
- 4Dye C: Rf = 6.8 ÷ 8.0 = 0.85.
- 5Each value lies between 0 and 1 and has no units, which confirms the two distances were not swapped.
- 6Ink X’s spots at 2.0 cm (Rf = 0.25) and 6.8 cm (Rf = 0.85) are at the same heights as the spots of dyes A and C on the same paper, so X contains dyes A and C.
- 7There is no spot of X at 5.6 cm, so X does not contain dye B; the ‘single-dye’ claim is false because X is a mixture of two dyes.
Watch out: If an Rf value comes out above 1, the solvent-front distance has been divided by the spot distance — swap the two distances and recalculate.
Common mistakes
Drying ammonia with concentrated sulfuric acid
Wrong: Pass the ammonia through concentrated sulfuric acid to dry it.
Right: Ammonia is an alkaline gas, so it reacts with the acid: 2NH₃(g) + H₂SO₄(l) → (NH₄)₂SO₄(s). Dry ammonia with quicklime (CaO), a basic drying agent.
Saying impurities raise the melting point
Wrong: The impure sample melts at a higher temperature.
Right: Impurities lower the melting point and make the sample melt over a range of temperatures instead of sharply; impurities raise the boiling point.
Evaporating hydrated copper(II) sulfate to dryness
Wrong: Heat the copper(II) sulfate solution until all the water has evaporated.
Right: This drives off the water of crystallisation and leaves white anhydrous powder. Use crystallisation: heat gently until saturated, cool to let crystals grow, filter, then dry between sheets of filter paper.
Questions students ask
How do you choose a method to collect a gas in O-Level Chemistry?
If the volume of gas must be measured, use a gas syringe, which works for any gas. Otherwise, collect over water if the gas is insoluble or only slightly soluble in water; if it is soluble, use downward delivery when it is denser than air and upward delivery when it is less dense, comparing its Mᵣ with the average Mᵣ of air (about 29).
Why is the chromatography baseline drawn in pencil and not pen?
Ink is a mixture of dyes, so it would dissolve in the solvent and travel up the paper, adding extra spots that ruin the chromatogram. Pencil graphite is insoluble in the solvent, so the baseline stays put.
When should you use crystallisation instead of evaporation to dryness?
Ask whether the salt survives strong heating. Heat-stable salts such as sodium chloride can be evaporated to dryness, but hydrated copper(II) sulfate loses its water of crystallisation, so it must be crystallised and the crystals dried between sheets of filter paper.
How do you tell if a substance is pure from its melting point?
A pure substance melts sharply at one fixed temperature that matches the data-book value. An impurity lowers the melting point and spreads melting over a range of temperatures, and it raises the boiling point. A pure substance also gives exactly one spot on a chromatogram.
In the full PDF
- •Full apparatus table for time, temperature, mass and volume, with worked examples on burette readings and choosing a gas syringe
- •Gas collection and drying-agent tables, plus worked examples on collecting sulfur dioxide, oxygen and dry ammonia
- •Hazard symbols table and the model fume-cupboard answer
- •Separation decision-flow table and a full scheme for separating iodine, sodium chloride and sand
- •Diagrams and graphs, including the chromatogram figure, vapour pressure against mole fraction, and ammonia yield against pressure
- •Purity data-table worked examples, the remaining three common mistakes, exam-technique tables and the topic summary