Top 10 Mistakes in GCSE Chemistry Exams
Published 25 April 2026
The most common GCSE Chemistry errors from calculations to bonding and moles. Learn how to avoid them and stay on track for higher marks.
Why Chemistry mistakes are so common
Chemistry combines three things: recall of facts, understanding of concepts, and maths skills. A mistake in any one area can lose marks, and many students make the same errors again and again.
The good news is that once you know what to watch for, you can train yourself to spot and avoid these traps.
- Calculation errors from poor unit conversion
- Missing or wrong chemical formulas
- Bonding misconceptions that affect explanations
1. Wrong mole calculations
Students often forget to convert grams to moles or mix up the formula. For example, number of moles = mass ÷ molar mass, but some students divide the molar mass by the mass instead.
Always write the formula first, then substitute the numbers carefully.
- Moles = mass ÷ molar mass (not the other way round)
- Check your periodic table for exact atomic masses
- Round your final answer to the mark scheme's instruction
2. Forgetting the state symbols
Chemical equations need state symbols (s), (l), (g), (aq) in many exam questions. Missing them can lose a mark even if the equation is otherwise correct.
Solid, liquid, gas, and aqueous are not optional extras—they are part of a complete answer.
- (s) = solid, (l) = liquid, (g) = gas, (aq) = dissolved in water
- Learn which compounds are soluble and which are not
- Check your equation is balanced AND includes state symbols
3. Wrong ionic vs covalent explanations
Students sometimes say an ionic compound has molecules or that a covalent compound has ions. These are opposite mistakes. An ionic compound is made of ions held together by electrostatic forces. A covalent compound is made of molecules where atoms are bonded by shared electrons.
Once you nail this difference, many other chemistry answers become clearer.
- Ionic = ions + electrostatic attraction
- Covalent = atoms + shared electrons
- Metallic = atoms + delocalized electrons
4. Concentration and dilution errors
Many students know that diluting a solution lowers its concentration, but they struggle to calculate by how much. The key equation is concentration = moles ÷ volume in dm³.
A common error is forgetting to convert cm³ to dm³ before using the formula.
- 1 dm³ = 1000 cm³, so divide cm³ by 1000
- Concentration = moles ÷ volume (in dm³)
- In a dilution, the number of moles stays the same but the volume increases
Quick revision tip
These mistakes are predictable, which means they are preventable. Spend 15 minutes a week checking your own answers against a mark scheme and tracking which types of errors you make.
Once you know your personal weak spots, you can focus your revision there and watch your grade improve.
- Keep a list of your repeated mistakes
- Do practice questions under exam conditions
- Compare your answer to the mark scheme line by line
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