Key messages: use correct subscripts/upper–lower case; draw all bonds with correct valencies in displayed formulae; always show working in calculations.
New syllabus effects: overall standard high, but some unfamiliarity with new content led to rubric slips (e.g., giving multiple answers when only one requested) and overlong prose in extended parts (e.g., 3(b)(v)).
Strengths: CO? as greenhouse gas; % N? = 78%; catalytic converters; diffusion idea linked to lower Mr; haemoglobin references sometimes too biological; bonding/electrons in CO? usually correct.
Weak spots: 1(c) flue-gas desulfurisation detail; 1(e)(i) many picked CH? (OK) but some chose CO; 1(e)(ii) wrong phrasing like “lightest gas/lowest atomic mass”; 1(f) answers beyond syllabus scope.
Bonding & tests: metallic bonding known but “ionic” seen; H? test = lighted splint/pop (avoid vague “splint test”).
pH & indicator: strong alkali often given as 14; universal indicator required (not litmus/methyl orange).
Equation in (b)(iv): common errors: missing (aq), H?O(aq), unbalanced, Na(OH)?/Na?O.
Isotopes: definition secure; isotopic mass × abundance calc usually right to 1 d.p.
Ions & shells: many omitted the 4th shell of K or gave wrong charges.
New content reliance: sources of H?/N? for Haber; definition/sign of ?H (exothermic negative) poorly cited; conditions known except pressure value (200 atm ? 20 000 kPa) often wrong.
Kinetics explanation: many wrote “more collisions” without linking to frequency/activation energy; fewer mentioned “greater fraction above Ea”.
Acids: name H?SO? not just “sulfuric acid”; ammonium sulfate as fertiliser widely known; RFM often left as unsimplified sums.
Alloys & properties: brass = Cu + Zn; “alloy” term known; ductility vs malleability muddled; conduction in metals sometimes wrongly attributed to ions.
Crystallisation & oxides: water of crystallisation and “class of oxide” (acidic/basic, metallic/non-metallic) poorly known; meaning of Roman numerals (Cu(II) ? Cu?2;?) often missed; “valency” not an IGCSE term.
Calcs & equations: RFM sometimes not completed; failed mole conversions for gases/solids; unbalanced equations (e.g., using Al?).
Homologous series: similar properties due to functional group/general formula; 5(b) almost universally correct.
General formulae & isomers: common errors with carboxylic acid general formula and subscripts; displayed structures often missing O–H bond; naming unbranched isomer known but structures inaccurate.
Reactions: adding equations fine for stronger candidates; catalytic cracking widely known.
Organic challenges: 5(f) hardest—many drew 9-carbon chains or repeated C? fragments; (g) sodium propanoate OK, anion formula harder; (h) acid catalyst identified (avoid Fe/Ni); ester naming mostly correct though “propanoate” misspelt; displayed formula of ethyl propanoate generally accurate.
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