CARA'S TIPSPractical 6: Question 3 activities and exam-day coaching
WAECNECO
What the syllabus asks
WAEC: separation techniques, chromatography, melting/boiling points, heats of neutralisation, rates of reaction, organic and food tests, preparation of salts and gases, solubility and pH; exam-day technique.
NECO: separation techniques, chromatography, melting/boiling points, heats of neutralisation, rates of reaction, organic and food tests, preparation of salts and gases, solubility and pH; exam-day technique.
What you will learn
- Describe and justify separation techniques.
- Carry out and interpret paper chromatography.
- Use melting point as a test of purity.
- Measure and calculate heats of neutralisation.
- Investigate rates of reaction and interpret graphs.
- Carry out tests for organic compounds and food substances.
- Describe preparation of salts and gases.
- Apply exam-day strategies and precautions.
Key points
- Q3: short, specific answers.
- Separation = physical difference.
- Rf has no unit.
- Sharp m.p. = pure.
- q = mcΔT; ΔH negative.
- Rate ∝ 1/t.
- Precautions must fit the experiment.
Start here: Question 3: what it tests
The third question “tests candidates’ familiarity with the practical activities suggested in their teaching syllabuses” (WAEC). It is worth around 10–11 marks and usually has three or four short parts: name apparatus; state a precaution; explain a technique; describe a test; do a short calculation (dilution, Rf value, solubility, heat change). It rewards breadth more than depth.
The WAEC syllabus lists the general skills it expects: measurement of mass and volume; preparation and dilution of standard solutions; filtration, recrystallisation and melting point determination; measurement of heats of neutralisation and solution; determination of pH values of solutions by colorimetry (indicator papers); and determination of rates of reaction from concentration-against-time curves. Add to these the tests for organic functional groups, preparation and collection of gases, and the separation techniques from SS1.
| Recent kinds of item | What a full answer contains |
|---|---|
| Separation: property used for distillation, chromatography | the physical difference exploited (boiling point, adsorption) |
| Why a white tile / two decimal places | see cp1 |
| Volume of stock acid for a dilution | C₁V₁ = C₂V₂ with units |
| Two gases that cannot be collected over water | NH₃, HCl (very soluble); also SO₂ |
| Two items of apparatus that measure volume accurately | burette, pipette (measuring cylinder only approximate) |
| Three items of protective wear | lab coat, goggles, gloves (face mask accepted) |
| Purpose of a fume cupboard | to remove poisonous gases |
Short, specific answers win here. One accurate sentence per mark is ideal.
How Question 3 differs between school and private candidates
In the 2-hour practical paper, Question 3 may ask you to do something small at the bench (test a gas, find the pH of a solution, observe a reaction), then answer questions. In the 1 h 30 min alternative to practical, everything is described or drawn, and you may be shown a diagram of apparatus with a deliberate fault to identify, such as a thermometer bulb dipping into the liquid during distillation, water entering the condenser at the top, a delivery tube for ammonia leading into water, or an unsealed apparatus. Learn to spot these faults.
| Faulty diagram | What is wrong |
|---|---|
| Thermometer bulb in the boiling liquid | should be at the side arm to read vapour temperature |
| Condenser water in at the top | should enter at the bottom |
| NH₃ collected over water | NH₃ is very soluble; collect by upward delivery |
| Thistle funnel end above the liquid | gas escapes through the funnel; it must dip below the liquid |
| Closed flask being heated | pressure builds up; risk of explosion |
| Cl₂ prepared on an open bench | must be in a fume cupboard |
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Practice questions
- 0.40
- 2.5
- 0.30
- 4.5
Show answer
A: 0.40
3.0 ÷ 7.5.
- pencil is cheaper
- ink is too dark
- it can be erased
- graphite does not dissolve in the solvent
Show answer
D: graphite does not dissolve in the solvent
Ink would itself separate.
- immiscible liquids
- dyes in ink
- miscible liquids with different boiling points
- an insoluble solid from a liquid
Show answer
C: miscible liquids with different boiling points
E.g. ethanol and water.
- a burette
- a separating funnel
- a filter paper
- a condenser
Show answer
B: a separating funnel
They form two layers.
- sharply at a fixed temperature
- over a wide range
- below room temperature always
- only under pressure
Show answer
A: sharply at a fixed temperature
Impurities lower and widen the melting range.
Key words
- Rf value
- Ratio of distance moved by a spot to distance moved by the solvent front.
- Heat of neutralisation
- Heat evolved when one mole of water is formed from acid and alkali.
- Specific heat capacity
- Heat needed to raise 1 g by 1 K (water 4.2 J g⁻¹ K⁻¹).
- Solubility
- Maximum mass dissolving in 100 g of solvent at a given temperature.
- Suck-back
- Cold water drawn into hot apparatus when heating stops.
- Fair test
- Experiment in which only one variable is changed.
- Reducing sugar
- Sugar that reduces Benedict’s/Fehling’s solution (e.g. glucose).
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