Centres: make up solutions/apparatus exactly as in Confidential Instructions; contact CAIE if substances are hard to obtain.
Readings: record to a consistent resolution (same decimal places) for a given apparatus.
Graphs: plot points as × or ?, not hidden by the line; draw smooth curves or ruler-straight best-fit lines (no wobble).
Qualitative analysis: when told ‘test any gas produced’, perform and record the positive gas test; use the term precipitate for a solid forming and note that a cloudy opaque mixture indicates a precipitate.
Overview: candidates generally completed all questions with very few blanks; results for Q1 were mostly as expected. In the planning task (Q3), do not list apparatus/safety/aims unless asked; credit for apparatus requires stating its use.
(a) Most completed all five experiments correctly. Common error: inconsistent decimal places (e.g., writing 8.0 once implies all water volumes should be to 1 d.p.). Read temperatures to nearest 0.5 °C.
(b) Scales usually appropriate; best-fit line must be a single smooth straight line/curve and need not pass through every point. Expect an even scatter either side when appropriate.
(c) Extrapolation and reading from the line were usually correct, but some did not show working.
(d) Substitution into the given equation to calculate solubility generally correct; a few rounded incorrectly.
(e) Must state the direction of change for both solubility and temperature; some wrote about volume of water instead of temperature.
(f)(i) Burette is more accurate than measuring cylinder. (ii) Volumetric pipette measures one fixed volume (not like Pasteur/dropping pipettes); always use a safety filler.
(g) Adding water in 1.0 cm?3; portions gives fewer points; fewer points make a best-fit line less reliable. Some omitted intermediate volumes (e.g., 8.5 and 9.5 cm?3;).
(h) Many correctly stated ammonium chloride would not fully dissolve in a lower volume. Vague answers like ‘not enough water’ gained no credit.
(a) Flame test: orange-red observed by most; don’t add unrelated observations (effervescence/precipitates) to a flame test.
(b) White precipitate remains with excess aqueous NaOH; use the term ‘precipitate’, not just ‘cloudy’.
(c)(i) When told ‘test any gas produced’ you must record a positive gas test (e.g., damp red litmus blue / pungent smell for NH3). (ii) Gas identified as ammonia.
(d) ‘No visible change’ was expected; some incorrectly wrote a positive halide test (not all tests are positive!).
(e) White precipitate correctly reported by most. (f) Solution C identified as calcium nitrate by the majority.
(g) pH for a strong acid typically 1–3; a few wrote ?7 suggesting the wrong solution was tested.
(h) White precipitate commonly reported correctly.
(i) Better answers recorded a positive CO2 test (limewater milky); some forgot two marks were available and gave only an observation.
(j) Sulfate ion usually identified; fewer spotted the hydrogen ion. A common error was claiming carbonate ions were in solution D (they were added to generate CO2 in part (i)).
Plan an investigation to order three metals by reactivity using either temperature change in an exothermic reaction or gas volume produced.
Fair test: control mass of metal and volume of acid (state concentration and temperature). Avoid vague ‘amount’ wording.
Gas collection: use suitable apparatus (conical flask + bung + delivery tube + gas syringe/upturned cylinder). A beaker is not appropriate for gas collection.
Measurements: Decide on fixed time to measure volume, or fixed volume to measure time. Link how the measurement indicates reactivity (larger ?T/faster/greater gas ? more reactive).
Avoid non-creditworthy phrases such as ‘use the results to determine the order’ without explaining how.
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