Common Exam Mistakes and How to Avoid Them

Most marks lost in GCSE Chemistry are not lost because students don't know the chemistry — they are lost to a small set of avoidable, repeating mistakes. Examiners' reports describe the same errors every year: unbalanced equations, missing state symbols, cm³/dm³ slips, ignoring mole ratios, vague "it goes faster" answers, confusing command words, and mis-reading flame or precipitate colours. The good news is that because these errors are predictable, they are fixable. This lesson names the highest-frequency mistakes and shows you exactly how to avoid each one.

By the end of this lesson you should be able to recognise the mistakes that cost the most marks and apply a specific habit to prevent each one.

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Mistake 1: Not Balancing Equations

An unbalanced symbol equation usually scores zero even if the formulae are right, because conservation of mass demands equal atoms on both sides.

• $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$H2​+O2​→H2​O is wrong — the oxygen does not balance.
• $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$2H2​+O2​→2H2​O is correct — 4 H and 2 O on each side.

How to avoid it: count every element on both sides, and balance using big numbers in front of formulae — never change a formula (e.g. you cannot write $\text{H}_2\text{O}_2$H2​O2​ just to balance oxygen; that is a different substance). Tick each element off once it balances.

Exam Tip: The only things you may change to balance an equation are the multipliers in front of formulae. Altering a subscript inside a formula changes the chemical, which is always wrong. Always re-count after balancing.

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Mistake 2: Forgetting State Symbols

When state symbols are asked for, leaving them out — or getting them wrong — loses the mark. The four symbols are (s) solid, (l) liquid, (g) gas and (aq) aqueous (dissolved in water).

• $\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)$HCl(aq)+NaOH(aq)→NaCl(aq)+H2​O(l) — note water is (l), the salt is (aq).

How to avoid it: if a question says "include state symbols", add one to every species. Think about whether each is dissolved (aq), a pure liquid (l), a solid precipitate (s) or a gas (g).

Exam Tip: A salt that stays dissolved is (aq); an insoluble product (a precipitate) is (s); water made in neutralisation is (l), not (aq). Mixing these up is a frequent, easily avoided slip.

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Mistake 3: Confusing cm³ and dm³

This is the single biggest source of lost calculation marks in chemistry. Concentrations are in mol/dm³ or g/dm³, but volumes are usually measured in cm³.

• A titre of $25 \text{ cm}^3$25 cm3 is $0.025 \text{ dm}^3$0.025 dm3 — you must divide by 1000 before using it in a concentration formula.
• Forgetting the conversion makes the answer 1000 times too big or too small.

How to avoid it: before any concentration or moles-in-solution calculation, convert every volume to dm³ (÷1000). Write the converted value down so you can see you have done it.

Exam Tip: There are 1000 cm³ in 1 dm³ (a dm³ is the same as a litre). If a concentration answer comes out absurdly large or tiny, the first thing to check is whether you converted cm³ to dm³.

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Mistake 4: Ignoring the Mole Ratio

In a reacting-mass calculation, you must use the balanced equation to get from moles of one substance to moles of another. Skipping the ratio gives the wrong answer whenever it is not 1 : 1.

• For $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$2Mg+O2​→2MgO the Mg : MgO ratio is 1 : 1, but for $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$N2​+3H2​→2NH3​ the H₂ : NH₃ ratio is 3 : 2.

How to avoid it: always lay out the route mass → moles → (×mole ratio) → moles → mass, and read the ratio straight from the big numbers in the balanced equation.

Exam Tip: Underline the mole ratio in the balanced equation before you convert. The most common reacting-mass error is going straight from grams of reactant to grams of product as if the ratio were always 1 : 1.

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Mistake 5: Vague "It Goes Faster" Rate Answers

Rate questions are marked on collision theory, and a vague answer earns nothing. "It speeds up because it's hotter" misses every marking point.

• "It goes faster" → "the particles have more kinetic energy, move faster and collide more frequently, and more collisions have energy above the activation energy, so there are more successful collisions per second."

How to avoid it: for any rate-of-reaction explanation, mention frequency of collisions and, for temperature, energy of collisions / activation energy. Name the factor (concentration, temperature, surface area, catalyst) and link it to collisions.

Exam Tip: Build every rate explanation around the phrase "frequency of successful collisions". Concentration, pressure and surface area increase the frequency; temperature increases both the frequency and the proportion of collisions above the activation energy; a catalyst lowers the activation energy.

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Mistake 6: Mis-reading Flame and Precipitate Colours

Identifying-ions questions depend on exact colours, and a swapped colour loses the mark.

Often confused	The correct test result
Lithium / sodium / potassium flame	red / yellow / lilac
Calcium / copper flame	orange-red / green
Copper(II) / iron(II) / iron(III) hydroxide precipitate	blue / green / brown
Carbonate vs sulfate vs halide	fizz + limewater (carbonate) / white ppt with barium chloride (sulfate) / coloured ppt with silver nitrate (halide)

How to avoid it: make a flashcard for every flame colour, precipitate colour and gas test, and drill them. These are pure recall (AO1) marks you can bank if you learn them precisely.

Exam Tip: Learn the gas tests too — hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper. These short, factual marks appear on most papers.

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Mistake 7: Unit and Conversion Slips

Numerically correct answers lose the final mark when the unit is missing, wrong, or unconverted.

• Forgetting the unit entirely (a concentration of "20" instead of "20 g/dm³").
• Putting a unit on $M_r$Mr​, atom economy or percentage yield, which have none.
• Mixing cm³ with dm³, or g with mol, in one calculation without converting.

How to avoid it: in every calculation, write the formula, substitute, and finish with the answer and its unit, having converted to consistent units first. Then sanity-check the size — does the magnitude look reasonable? An answer a thousand times too big or small usually signals a conversion error.

Exam Tip: Treat the unit as an inseparable part of the number. The quantities with no unit are $M_r$Mr​, percentage yield, percentage composition, atom economy and $R_f$Rf​ — everything else (mass, volume, concentration, rate) needs one.

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Mistake 8: Not Answering the Command Word

Knowing the chemistry is not enough if you answer the wrong instruction. The classic is describing when asked to explain — giving what happens but never why.

• Explain needs reasons ("...because there are more frequent collisions").
• Compare needs similarities and differences, not a list of one thing.
• Evaluate needs two sides and a conclusion.
• Calculate needs working and units.

How to avoid it: underline the command word before you write, and check your answer matches it. Revisit Lesson 3 until each command word's demand is automatic.

Exam Tip: Confusing the AO/command word is a top mark-drainer. A "describe" answer to an "explain" question forfeits the reasoning marks, which are usually most of the marks on offer.

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What the Examiner Wanted vs What Students Wrote