Most candidates attempted all questions and finished in time; full mark range seen; very few blanks.
Planning Q4: no list of aims/apparatus/safety needed. Credit for apparatus only when its use is stated.
(a)(i) Apparatus purpose correctly stated as distillation by the majority. Easy
(a)(ii) B identified as a beaker by most; fewer identified A as a condenser. Moderate
(a)(iii) Heat should be applied under the liquid in the flask; a few heated the beaker or the condenser. Moderate
(b)(i) Careful reading required: answer was residue, not “filtration”. Moderate
(b)(ii) Some suggested filtration again or heating (would not remove NaCl). Best responses: wash the calcium carbonate to remove aqueous sodium chloride. Difficult
(b)(iii) Correct methods to obtain solid sodium chloride from solution: evaporation / crystallisation / distillation. Easy
(a) Nearly all recorded masses and temperatures; keep thermometer readings to the nearest half-division (e.g. 11.5 °C) and include negative signs; use one decimal place consistently. Moderate
(b) Choose a proper scale (continue below 0 °C; avoid non-linear scales) and plot six points; draw a single thin best-fit line (needn’t pass through points). Moderate
(c) Reaction was endothermic; explanation should refer to temperature decrease during the reaction (not generic energy in/out of surroundings). Moderate
(d) Identify which experiment had the greatest temperature change; a small minority chose Experiment 2 incorrectly. Easy
(e) Show working on the graph: vertical from 3.5 g to the line, then horizontal to the y-axis reading. Many omitted this. Moderate
(f) Stirring mixes the solids so they can react; a few added that it keeps temperature uniform. Easy
(g) Polystyrene beaker acts as an insulator, reducing heat gain from surroundings (prevents temperature rise). Moderate
(a) Better candidates inferred condensation + pink CoCl2 paper ? water given off ? solid G was hydrated.
(b) Test 4 (for sulfite) was negative ? solid G is not that ion; negative tests tell us what a substance is not.
(c) Test 3 showed the cation was either Al3+ or Zn2+.
(d) Using test 1 many deduced K+; some wrongly said “sulfite ions” rather than the correct sulfate from test 5.
(e) Adding dilute HCl to CuCO3: observe effervescence and blue/green solution; test gas by bubbling through limewater ? milky.
(f) The solution contains Cu2+; adding NaOH dropwise then excess ? blue precipitate (does not redissolve). Two-colour descriptions not credited.
(g) White precipitate expected (appropriate confirming test).
Quantitative task about how much oxalic acid dissolves (not the rate).
All good methods include: known/stated volume of solvent; known/stated mass of oxalic acid; suitable container (e.g. beaker); stir solvent + acid to aid dissolving; optionally repeat with ethanol and water.
Two common workable methods: (1) Add known mass of oxalic acid to fixed volume of solvent, remove undissolved solid and determine its mass. (2) Add small portions of known mass to a set volume until no more dissolves.
Notes: Use the word volume for liquids and mass for solids. Do not list aims/apparatus/variables; state the use of any apparatus to earn credit.
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