Paper 6 (Alternative to Practical): The Complete Guide

Paper 6 is 20% of your IGCSE Chemistry grade, and most students give it less than 5% of their revision. It feels easy (no Supplement content, simple chemistry), so it never makes the revision plan. Then it returns 24/40 and takes the A* with it.

Why students under-prepare this paper

The trap is built into how the paper reads. A Paper 6 question looks friendly: a diagram, a half-finished table, a question that starts “describe how you would…”. Nothing in it looks like the hard chemistry of Paper 4. So students assume reading is preparing.

But Paper 6 marks a different skill set: recording data correctly, reading apparatus, evaluating methods, and writing experimental procedures in mark-worthy steps. School lessons rarely train these explicitly, because labs are used for teaching content, not for exam write-ups. The result is a paper where the marks bleed silently: one for a missing unit, one for a conclusion written where an observation was asked, one for a graph line drawn point-to-point. Ten such marks is a full grade.

The standard experiments that repeat

Paper 6 draws from a short list of experiments, recombined each series. Know these and you have seen most of the paper before opening it.

Rates of reaction. Marble chips with hydrochloric acid (gas volume against time), sodium thiosulfate with acid (disappearing cross). Expect: reading a gas syringe, plotting volume-time curves, explaining why the curve flattens (a reactant is used up), and comparing curves for different concentrations or temperatures.

Electrolysis. Labelled diagrams of cells: electrodes, electrolyte, power supply. Expect: naming the electrodes (anode positive, cathode negative), identifying the products and the gas tests that confirm them.

Salt preparation. Excess insoluble base or carbonate added to warm dilute acid, filter, evaporate to crystallisation point, dry the crystals. Expect: ordering method steps and explaining each one (excess, to make sure all the acid reacts; filter, to remove unreacted solid). The chemistry behind it lives in Acids, Bases and Salts.

Titration. Burette, pipette, conical flask, indicator, colour change at the end-point. Expect: reading burette diagrams to 0.1 cm³, completing titre tables, identifying concordant results (within 0.2 cm³) and averaging only those.

Chromatography. Baseline in pencil, solvent below the baseline, Rf = distance moved by spot ÷ distance moved by solvent. Expect: calculating Rf to 2 significant figures and explaining why the baseline must be pencil (ink would dissolve and run).

Energetics. Temperature changes when solids dissolve or acids neutralise alkalis, in a polystyrene cup. Expect: reading thermometer diagrams, identifying exothermic (temperature rises) versus endothermic, and suggesting why heat loss makes the measured change too small.

Qualitative analysis. Flame tests, NaOH and ammonia tests on unknowns, gas tests. Every result must match the official notes word for word. The full set is in our ion and gas tests guide.

The apparatus knowledge underneath all of this (what measures what, and to which precision) is the Experimental Techniques topic.

How to read apparatus diagrams

Three reading rules earn quiet marks across the paper.

Read the scale direction first. Burettes read downward from 0 at the top; measuring cylinders read upward. A burette diagram showing the liquid level at the 7.0 line means 7.0 cm³ has been delivered only if the start was 0.0. Check what the question actually asks.

Read to the instrument’s precision. Burette readings to 0.1 cm³ (and the half-division estimate makes 7.35 acceptable where schemes allow), thermometers to 0.5 °C on a 1 °C scale, balances to whatever the diagram shows. Writing “7” where the instrument reads “7.0” can drop the mark.

Name apparatus exactly. “Conical flask”, not “flask”. “Measuring cylinder”, not “beaker with lines”. “Evaporating basin/dish” for crystallisation. The mark scheme is literal about names because the skill being tested is recognising real equipment.

Answering “describe how you would…” questions

These planning questions carry 3-6 marks and follow a fixed grammar. Each mark is one decision: what you measure, what you change, what you keep the same, and how you make it fair or reliable.

The template: name the apparatus and quantities → state the measurement and when you take it → state what you change between runs → state what you keep constant → say how you handle reliability (repeat and average) or safety where relevant. Write it as numbered steps. An examiner can only tick a step they can find.

Example: “Describe how you would investigate the effect of concentration on the rate of reaction between magnesium and hydrochloric acid. [4]” earns its marks as: measure 25 cm³ of acid with a measuring cylinder and add a fixed mass/length of magnesium [1]; measure the volume of gas in a gas syringe every 10 seconds / time how long the magnesium takes to disappear [1]; repeat with different concentrations of acid [1]; keep the temperature, the mass and form of magnesium, and the acid volume the same [1].

Table and graph marks

Tables: include the quantity AND unit in the header (Time / s, Volume of gas / cm³), never units in the body of the table, all readings to consistent precision. A table mark is the cheapest mark on the paper and is dropped constantly.

Graphs: the standard four marks are scales (sensible, plotted points covering more than half the grid in both directions), plotting (within half a small square), line (single smooth best-fit line or curve: never dot-to-dot, never forced through the origin unless it belongs there), and any value read off the graph (show construction lines). Label both axes with quantity and unit.

Sig-fig and unit traps

• Rf values have no units and are quoted to 2 significant figures (e.g. 0.67), and must be less than 1. A “calculated” Rf of 1.3 means the two distances were inverted.
• Averages of burette titres keep burette precision: concordant titres of 24.30 and 24.50 cm³ average to 24.40 cm³, written to two decimal places like the readings.
• Temperature change needs a sign or a direction: “decrease of 4.5 °C”, not “4.5”.
• Volume answers without cm³ or dm³ drop the mark on most schemes. Write the unit every time; it is never penalised for being there.

Worked exam question

A student investigates the temperature change when ammonium chloride dissolves in water. She measures 25 cm³ of water into a polystyrene cup and records the temperature as 28.0 °C. She adds the ammonium chloride, stirs, and records the lowest temperature reached as 21.5 °C. (a) Calculate the temperature change. [1] (b) State whether the process is exothermic or endothermic. Explain your answer. [2] (c) Suggest why a polystyrene cup is used instead of a copper can. [1]

Model answer with mark breakdown:

• (a) 28.0 − 21.5 = 6.5 °C decrease [1: the value with its direction or sign]
• (b) Endothermic [1]; the temperature falls because energy is taken in from the surroundings/the water [1]
• (c) Polystyrene is an insulator, so less heat is transferred in from the surroundings (making the temperature change more accurate) [1]

The most common dropped mark is (a): “6.5” with no direction, or the subtraction reversed. The second most common is (b) answered with one word. The question pays one mark for the label and one for the reason.

The mistakes that cost marks

• Writing conclusions where observations are asked. “Hydrogen is produced” scores zero under “state what you observe”. Bubbles, fizzing, the solid disappearing: those score.
• Dot-to-dot graph lines. One smooth best-fit line or curve. Every series, this single habit costs a mark.
• Units missing from table headers and answers. The cheapest marks on the paper, lost by the strongest students.
• Averaging all the titres. Only concordant ones (within 0.2 cm³). Including the rough or an outlier loses the processing mark.
• Vague planning answers. “Measure the gas” earns nothing; “measure the volume of gas in a gas syringe every 10 s” earns the mark. Each step must name apparatus, quantity and timing.

How to phrase it for full marks

Student language: “The reaction slows down and stops.” Mark-scheme language: “The gradient of the graph decreases because the acid is being used up; the line becomes horizontal when the reaction is complete.”

Student language: “Repeat the experiment to make it accurate.” Mark-scheme language: “Repeat and calculate an average, ignoring anomalous results, to make the results more reliable.”

Student language: “Use the same things each time.” Mark-scheme language: “Keep the volume and concentration of acid and the temperature constant; change only the surface area of the solid.”

The pattern: name the variable, name the instrument, name the number. Paper 6 examiners credit specifics and ignore intentions.

The Malaysia note

Nearly every Malaysian international school enters its candidates for Paper 6 rather than Paper 5, so this paper is effectively compulsory here, and it is the paper Malaysian students are least prepared for, because school lab time is teaching time, not write-up training. In our experience the gap shows most in observation language and graph discipline, both fixable in 2-3 focused sessions with marked past papers. We run a Paper 6 diagnostic as part of the free 1-hour trial lesson: one real past-paper question, marked live against the official scheme, so you see exactly which of the 40 marks your child is currently giving away. No forms. WhatsApp us. We reply the same day.