How to Show Working in Chemistry Calculations
The method for showing working in IGCSE Chemistry 0620 calculations that earns maximum method marks even when the final answer is wrong.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
A correct final answer with no working earns 1 mark on most 0620 calculation questions. The same question with full working and a wrong answer can earn 3 out of 4 marks. This is not a minor difference; it is the difference between a grade boundary and a comfortable pass. Showing working is a technique, and like any technique, it can be practised until it becomes automatic.
The four-line method
Every chemistry calculation on 0620 can be structured in four lines. Train yourself to write all four, every time, even when the calculation feels simple enough to do in your head.
Line 1: State the formula. Write the relationship you are going to use: moles = mass / Mr, or volume = moles x 24, or concentration = moles / volume.
Line 2: Substitute the values. Replace each variable with the number from the question, including units: moles = 6.5 / 65.
Line 3: Calculate the result. Write the answer to this step with units: = 0.10 mol.
Line 4: Apply the mole ratio or convert to the final answer. If another step is needed (using the equation ratio, or converting units), repeat the pattern: state what you are doing, substitute, calculate.
Example: full working for a reacting mass question
Q: Calculate the mass of carbon dioxide produced when 10 g of calcium carbonate reacts with excess hydrochloric acid. CaCO3 + 2HCl -> CaCl2 + H2O + CO2 (Ar: Ca = 40, C = 12, O = 16, H = 1, Cl = 35.5)
Working:
Mr of CaCO3 = 40 + 12 + (16 x 3) = 100
Moles of CaCO3 = mass / Mr = 10 / 100 = 0.10 mol
From the equation, 1 mol CaCO3 produces 1 mol CO2, so moles of CO2 = 0.10 mol
Mr of CO2 = 12 + (16 x 2) = 44
Mass of CO2 = moles x Mr = 0.10 x 44 = 4.4 g
Five lines, five potential marks. Even if the Mr calculation in line 1 is wrong (say you wrote 110 instead of 100), lines 2 through 5 would still earn error-carried-forward marks because the method is correct.
Why this matters: the mark scheme structure
Cambridge 0620 mark schemes for calculations typically allocate marks as follows:
| Mark | Awarded for |
|---|---|
| M1 | Correct Mr calculation or correct formula stated |
| M2 | Correct moles calculation (or correct substitution) |
| M3 | Correct use of the mole ratio from the balanced equation |
| A1 | Correct final answer with correct unit |
The answer mark (A1) depends on everything above it being correct. The method marks (M1-M3) are independent: you can earn M1 and M3 even if M2 is wrong, provided M3 follows correctly from your M2 answer.
Without working, the examiner sees only a number. If that number is correct, they can award all marks. If it is wrong by even one digit, they award zero because there is no method to credit.
The unit discipline
Write the unit every time you write a number. This habit prevents two types of error:
- Forgetting the unit on the final answer. Some schemes deduct the answer mark for a missing unit.
- Using the wrong unit mid-calculation. If you write “25 cm3” instead of “0.025 dm3” in a concentration calculation, the visible unit makes the error obvious before you finish.
Common unit conversions to show explicitly:
- cm3 to dm3: divide by 1000. Write it: 25.0 cm3 = 25.0 / 1000 = 0.0250 dm3.
- g/dm3 to mol/dm3: divide by Mr. Write it: 4.0 g/dm3 / 40 = 0.10 mol/dm3.
Significant figures and rounding
Give your final answer to 3 significant figures unless the question specifies otherwise or the data suggests fewer. Do not round intermediate steps; round only the final answer. If you round 0.0333… to 0.03 in step 2 and multiply by 44 in step 3, you get 1.32 instead of 1.47, and the error looks like a method mistake rather than a rounding issue.
If the question gives data to 2 significant figures (e.g., 2.0 g), give your answer to 2 or 3 significant figures. Do not give an answer to 6 decimal places from a calculator display.
Error-carried-forward (ECF) marks
ECF is the examiner’s way of rewarding correct method even when earlier numbers are wrong. It only works if:
- The subsequent working is clearly shown.
- The method applied to the wrong number is correct.
- The error is not in the final step (the answer mark cannot be ECF’d from a wrong penultimate step if the question only has 2 marks).
This is the strongest argument for showing every step. A student who makes one arithmetic error but shows perfect method might lose 1 mark out of 4. A student who shows no working and makes the same error loses all 4.
Calculations where students skip steps
Three calculation types where students most often try to do it in their heads:
- Mr calculations. “I can see it’s 100.” But CaCO3 is 40 + 12 + 48 = 100, and if you miscount the oxygens (two instead of three), the visible Mr of 76 tells the examiner what went wrong and preserves your later marks.
- Mole ratio. “It’s obviously 1:1.” But the equation might be 2:1 and the student who did not write the ratio down used 1:1 without noticing.
- Unit conversion. “I just moved the decimal.” But moving it the wrong way (multiplying by 1000 instead of dividing) gives an answer three orders of magnitude wrong, and without the written conversion the examiner cannot award ECF.
Practice drill
Take any past paper calculation question. Solve it twice: once showing full working as described above, and once with only the final answer. Compare the marks you would earn if the final answer were wrong. Do this for five questions and the habit becomes permanent.
The detailed technique for mole calculations specifically is in mole calculations technique. If calculation marks are a persistent weak area despite showing working, a trial lesson can identify whether the issue is arithmetic, formula recall, or mole-ratio application.