This module is different from the topic modules. It does not add new content. It trains the habits that turn a half-right Chemistry answer into a fully right one.
The habits are using the correct ratio, explaining at particle level and checking an equation on its own. You also learn to link what you see to what you conclude, and to describe your exact sticking point.
Check the current Cambridge IGCSE Chemistry 0620 syllabus for what is examined in your exam year. The habits here apply to whichever content points you are studying. Our Chemistry learning guide shows how this module sits with the topic modules.
What should you already know?
You should have met moles and balanced equations, the particle model, and the idea of rate of reaction at least once. You do not need to be confident in them yet. The lessons revisit each one through the errors students actually make.
An orienting example
Magnesium burns in oxygen to form magnesium oxide. All data in this example are invented for practice. A student burns 0.24 g of magnesium and is asked for the mass of magnesium oxide formed. Use Ar: Mg = 24, O = 16.
Step 1, equation: 2Mg + O₂ → 2MgO. Check the atoms: Mg 2 and 2, O 2 and 2.
Step 2, amount of Mg: 0.24 g ÷ 24 g/mol = 0.010 mol.
Step 3, ratio: Mg : MgO is 2 : 2, which is 1 : 1. So 0.010 mol of MgO forms.
Step 4, mass: Mr of MgO is 24 + 16 = 40. So 0.010 × 40 = 0.40 g.
Independent check: oxygen used is 0.0050 mol (ratio 2 : 1), which is 0.0050 × 32 = 0.16 g. Mass in: 0.24 + 0.16 = 0.40 g. Mass out: 0.40 g. The two agree.
Particle explanation: each magnesium atom gives up two electrons to oxygen, so the solid is made of Mg²⁺ and O²⁻ ions. The product is heavier than the magnesium because oxygen atoms have joined it.
That one example used every lesson in this module: the ratio, the explanation, the equation check, the link from change to conclusion, and a check that shows where an error would sit.
In which order should you study it?
- Explain why correct arithmetic can use the wrong chemical ratio: the most common way to lose marks in a calculation you understood.
- Repair a particle-level explanation that names only a trend: turns “it goes faster” into a reason.
- Check a balanced equation independently: a two-minute test that catches a wrong coefficient before it spreads.
- Connect observation, model and conclusion: stops you from writing a conclusion the evidence cannot support.
- Prepare an exact sticking point for individual help: makes any future help, from a teacher or from yourself, faster and more exact.
Then work through the mixed practice set. Two tools are built for this module: the mole and equation-ratio tutor and the equation balance reasoning trainer.
Which traps catch most students here?
- Using 1 : 1 by habit when the equation says 2 : 1 or 4 : 2.
- Writing the trend and stopping, for example “a higher temperature makes the reaction faster” with no particles in the sentence.
- Balancing by changing a formula, such as turning NaCl into NaCl₂, instead of changing a coefficient.
- Writing a conclusion as if it were an observation, or the other way round.
- Saying “I don’t get moles” when the real gap is one specific step.
How should you use the practice set?
Answer on paper first, in full sentences where a question asks you to explain. Then open the worked answer and compare your reasoning, not only your final value.
When an answer is wrong, the routing table at the end sends you to the matching lesson. Record the error in the mistake log and retest queue, and retry a fresh question a few days later. If the same ratio or explanation slip keeps coming back, a teacher in online one-to-one Chemistry tuition can work from your written answers and find the step that fails.