Recording & graphs: Use the same resolution for all readings from the same apparatus. Plot ×/? points, draw smooth best-fit lines (straight or curve), never jagged.
Names vs formulae: If asked for a name, a correct formula is accepted; an incorrect formula loses credit.
Gas tests: When the paper says “Any gas produced is tested”, give the positive test observations for that gas.
Notes for Qualitative Analysis: Provided in this session—performance on Q3 improved markedly.
General: Most candidates attempted all questions with few blanks. For numerical ranges (e.g. Q3(e)), both limits must lie within the allowed range; otherwise give a single value. In planning (Q4), list apparatus only with its use; aim/safety lists are not required unless asked.
(a) Most supplied an appropriate name for apparatus A; vague terms (e.g., “container”, “evaporating basin”) or naming the gas-collection tube gained no credit.
(b) Majority explained the faster reaction using SA:V — small pieces give larger surface area; some simply repeated “small pieces are faster”.
(c)(i) Correct heating: Bunsen burner heats long-chain alkane and catalyst; water bath/match were inappropriate.
(c)(ii) Many drew arrows; careless arrows not pointing to correct parts lost marks.
(d) Gas collection often weak: first bubbles are air; heated alkane on mineral wool condenses back after the trough; claiming the collected gas was the heated alkane was common; a gaseous alkane would also form alkenes that decolourise bromine water.
(e) ‘Suck-back’ understood by some: cooling gas contracts, water is pulled into apparatus; others predicted unrelated reactions/“inaccurate results”.
(a) Tables mostly correct using the diagram; common errors: inconsistent decimal places or half-scale divisions.
(b) Scales chosen well; many drew a straight line through points instead of a single smooth straight-line/curve best-fit with even scatter either side.
(c) Extrapolation and reading from the line generally good; some failed to show how the value was obtained.
(d) Substitution from Experiment 1 into the given equation usually fine; a few rounded incorrectly.
(e) Must state the direction for both variables: e.g. “as temperature increases, solubility increases”. Some described volume of water instead of temperature.
(f)(i) Burette identified as more accurate than measuring cylinder.
(f)(ii) Volumetric pipette = one fixed volume; many confused with burette/Pasteur pipettes; always use a safety filler.
(g) Adding water in 1.0 cm?3; portions gives fewer points ? less reliable best-fit; missing 8.5 and 9.5 cm?3; points was common.
(h) Many correctly stated ammonium chloride will not fully dissolve in the lower volume; vague statements like “not enough water” earned no credit.
(a) Flame test result usually correct; a minority did not consult the Notes pages at the end of the paper.
(b) Addition of aqueous NaOH to solution C: result generally correct; a small minority did not use the Notes.
(c) Ammonia gas: some wrote the gas name without the positive test observation.
(d) Many stated “no reaction/no change”—correct; a few assumed every test must give a positive outcome.
(e) Strongly acidic pH accepted if within scheme range; giving a wide range where part lies outside the permitted limits lost credit.
(f) Gas correctly identified as carbon dioxide by almost all.
(g) Sulfate ion correctly identified by most; far fewer gave hydrogen ion. A frequent error was to say carbonate ions were present in solution D (they were added to produce CO? in part (i)).
Plan to place three metals in order of reactivity using either temperature change of an exothermic reaction or gas produced.
Better answers controlled mass of metal and volume of acid (avoid “amount”); many omitted acid concentration and temperature control.
Most chose to measure volume of gas. A beaker is not suitable; use a conical flask with bung + delivery tube and a gas-collection device.
Link measurement to reactivity: measure gas after a fixed time or time to reach a fixed volume; avoid vague claims like “use the results to decide the order”.
Write a public review