Most candidates attempted all questions successfully; the paper was generally well answered with few blanks.
Planning question (Q4): do not list aims/variables/apparatus at the start. Credit for apparatus only when its use is stated; names alone gain no credit.
(a) Conical flask correctly identified by the majority. Easy
(b) Hot water speeds dissolving / increases solubility; common error: saying temperature increases rate of reaction (this stage is dissolution). Moderate
(c) Process named correctly as filtration; many omitted funnel or filter paper in diagrams. Moderate
(d) Many stated enzymes denature / fermentation does not occur at high T; some incorrect ideas (“reaction becomes too fast”). Moderate
(e) Limewater turns milky/cloudy due to CO2. Easy
(f) Observation = bubbles stop when fermentation complete; “limewater stops changing” and “no more gas made” are not observations. Moderate
(g) Distillation identified to separate ethanol from fermentation mixture. Easy
(a) Reading burettes/adding volumes mostly correct; keep consistent decimal places. Errors: using 21 instead of 21.0; mixing s.f. (e.g. 9.20 with 21.0); adding readings instead of subtracting. Moderate
(b) White tile helps see colour/cloudiness clearly (not just “see colours”); not to protect bench/lift flask. Moderate
(c)(i) Rinse with water to remove residues from burette/conical flask. Easy
(c)(ii) Harder: remove water since it would dilute B; some thought A still present. Difficult
(c)(iii) Rinsing with aqueous ammonia leaves a little NH3 in flask ? more B required; some misread and rinsed burette. Moderate
(d)(i) B more concentrated (smaller volume of B reacts with same volume of NH3); many wrongly claimed A was more concentrated. Moderate
(d)(ii) Correct ratio = larger titre ÷ smaller (?1.5). Errors: divide wrong way (<1 class="tag moderate">Moderate
(e) Say experiment should be repeated and compare titres to judge reliability. Easy
(f) Expected titre based on titre from Exp. 1; errors: using final burette reading; omitting units. Moderate
(g) Volumetric pipette is more accurate than measuring cylinder; fixed volume isn’t a disadvantage here; some compared with dropping/Pasteur pipette. Moderate
(a) Only stronger candidates inferred solid E contained water (hydrated) from steam/condensation.
(b) Gas correctly identified as ammonia. Easy
(c) Negative Test 4 shows which halide ions are absent; “no halogens present” not accepted. Moderate
(d) Sulfate ions identified; Fe(III) often correct but oxidation state sometimes wrong; many said nitrate (not ammonium) in third test; some used formulae not names / wrong charges.
(e) White ppt forms then dissolves in excess to colourless solution (sulfate test). Easy
(f) Positive test for sulfite described correctly by almost all. Easy
(g) “White precipitate” given as often as “no visible change” — nitric acid is used to ensure only sulfate gives a white ppt (sulfite/carbonate also give white ppt without it). Moderate
Qualitative preparation of cobalt(II) sulfate — no measurements needed.
Good plan includes: add excess CoO to dilute H2SO4 in a suitable container (e.g., conical flask); heat & stir; filter to remove excess CoO; evaporate filtrate to crystallisation point; cool and isolate crystals by filtration/drying.
Common omissions: not stating base in excess; no suitable container/poor choice (e.g., measuring cylinder); using cold acid only; skipping stages and starting from CoSO4 solution.
Reminder: don’t list aims/apparatus; state what each item is used for to gain credit.
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