The Most Common IGCSE Chemistry Exam Mistakes

Read three years of 0620 examiner reports and a strange fact emerges: the reports barely change. The same eight errors appear series after series, costing the same marks, flagged in nearly identical sentences. That is good news. These are not random slips. They are predictable, named, and trainable. Fix the eight below and you recover marks without learning a single new fact.

1. Observations without colours and states

The mistake: “A precipitate forms.” “The solution changes colour.” “Gas is given off.”

Observation marks pay for specifics: which colour, what state, what change. “Aqueous copper(II) sulfate reacts with sodium hydroxide to give a precipitate” earns nothing; “a light blue precipitate forms” earns the mark. The same trap kills gas answers: “a gas is produced” scores zero where “effervescence” or “bubbles of colourless gas” scores.

The fix: every observation answer must contain at least one of: a colour (including colour change, from X to Y), a state or texture (precipitate, bubbles, dissolves), or a temperature change. And learn the one-word rule: colourless, never “clear”. Coloured solutions are clear; only colourless is creditable.

2. “Amount” instead of moles

The mistake: “The amount of hydrochloric acid is increased.” Amount of what: moles, volume, concentration? In chemistry, amount officially means amount of substance in moles, so a loose “amount” either says something you did not mean or says nothing the examiner can credit.

The fix: ban the word from your answers unless you mean moles, and then write “moles”. Name the actual quantity every time: concentration of acid, volume of solution, mass of magnesium, number of moles of gas. This single substitution recovers marks across rate, equilibrium and stoichiometry questions.

3. Wrong electrode products

The mistake: electrolysis questions have the highest wrong-answer rate on Paper 4. Students send the metal to the anode, produce oxygen from concentrated chloride solutions, or forget that aqueous solutions can discharge hydrogen instead of the metal.

The fix: drill the rules until they are reflex. Cathode (negative electrode): metal or hydrogen. Hydrogen is produced if the metal is above hydrogen in the reactivity series. Anode (positive electrode) with inert electrodes: halogen if a concentrated halide is present, otherwise oxygen from hydroxide ions. Then check your answer against charge: positive ions travel to the cathode, negative ions to the anode. The full decision routine is on our electrochemistry pages.

4. Describing instead of explaining

The mistake: the question says explain; the student writes what happens, beautifully, and earns zero. “The powdered marble reacts faster” is a description. The examiner wanted the why: larger surface area, more frequent collisions.

The fix: underline the command word before reading anything else. Every explain answer needs a because that reaches particles, electrons, bonds or energy. Test your own answer: delete the word “because” and everything after it. If the answer still makes sense, you described. The full word-by-word breakdown is in our command words guide.

5. Ignoring “use data from the table”

The mistake: the question supplies a table or graph and says “use the data”. The student answers from memory (correct general chemistry, zero marks) because the question was testing whether you can read evidence, not recall facts.

The fix: when a question references data, your answer must visibly touch it: quote a number, name a trend between two table rows, cite the graph’s gradient. A good habit is to tick each data source on the page once your answer has used it. If your answer would work without the table existing, rewrite it.

6. Unit and significant-figure slips

The mistake: a perfect calculation ending in “25” with no grams; a concentration left in g/dm³ when mol/dm³ was asked; an answer copied off the calculator as 2.4583333.

The fix: three habits. Write the unit in the same pen-stroke as the number. Check what unit the question demands before starting. Converting cm³ to dm³ first prevents the factor-of-1000 error that wrecks concentration answers. Round to 3 significant figures unless told otherwise, and only round at the final step, never mid-calculation.

7. Balancing slips

The mistake: equations balanced by changing a formula instead of a coefficient (turning O2 into O to make the oxygens fit), or diatomic elements written as lone atoms (H, Cl, O instead of H2, Cl2, O2).

The fix: formulae are fixed by chemistry; only the large numbers in front may change. Memorise the diatomic seven: H2, N2, O2, F2, Cl2, Br2, I2. After balancing, audit every element with a quick atom count on each side, and in ionic equations check that total charge balances too. Equal atoms with unequal charge is still wrong.

8. Mixing up oxidation and reduction

The mistake: redox definitions recited backwards under pressure (“oxidation is gaining electrons”), or correct definitions applied to the wrong species in the equation.

The fix: OIL RIG (Oxidation Is Loss of electrons, Reduction Is Gain), plus the oxygen version: oxidation is gain of oxygen, reduction is loss of oxygen. Then anchor it to electrolysis: at the cathode, positive ions gain electrons, so reduction happens at the cathode; oxidation at the anode. When a question asks “which substance is oxidised”, track one element’s electrons (or oxidation number, on Extended) across the equation and state the change explicitly: “iron loses electrons / its oxidation number rises from 0 to +3, so it is oxidised”.

Worked exam question

One question, four of the eight mistakes available.

Q (Paper 4 style): Concentrated aqueous sodium chloride is electrolysed using inert electrodes. (a) Name the product at each electrode. (2) (b) Describe what you would observe at the anode. (1) (c) Explain why sodium metal is not produced. (2)

Model answer: (a) Cathode: hydrogen (1). Anode: chlorine (1). (b) Bubbles of pale yellow-green gas (1). (c) Sodium is more reactive than hydrogen / above hydrogen in the reactivity series (1), so hydrogen ions are discharged at the cathode instead of sodium ions (1).

Mark-scheme logic: (a) punishes mistake 3: students who write sodium at the cathode or oxygen at the anode score zero there. (b) punishes mistake 1: “gas is given off” without colour earns nothing; the colour is the observation. (c) punishes mistake 4: restating “hydrogen is made instead” is a description; the reactivity comparison is the explanation. And throughout, chlorine must be Cl2 in any equation, or mistake 7 collects its toll.

How to phrase it for full marks

Student language: “More acid means a faster reaction.” Mark-scheme language: “Increasing the concentration of the acid means more acid particles per unit volume, so collisions with the magnesium occur more frequently.”

Student language: “The copper goes to the negative electrode.” Mark-scheme language: “Copper(II) ions are positively charged, so they move to the cathode, where each ion gains two electrons and is reduced to copper metal.”

Student language: “The answer is 25.” Mark-scheme language: “n = m/Mr = 2.5/100 = 0.025 mol, so mass of product = 0.025 × 1000 = 25 g.”

The pattern in all three: name the quantity, name the particle, show the unit. Long answers built the same way are covered in the 6-mark extended response technique.

The Malaysia note

Marking mocks from Malaysian international-school students, the top three of these eight are always the same: describe-instead-of-explain, missing colours in observations, and the cm³-to-dm³ slip in concentration work. None is a chemistry gap. All three are habits, which is why they survive months of content revision untouched. Habits need an outside eye: a specialist watching you answer can catch the error mid-sentence, which is exactly what the first free trial lesson is built to do. Bring a marked past paper and we will tell you in an hour which of the eight are costing you a grade.