IGCSE Chemistry topics that lose students marks aren’t always the hardest ones… That’s the thing that surprises people. After years of marking Edexcel papers, the topics that cost students marks most consistently aren’t the obscure corners of the specification — they’re the familiar ones. The ones students think they know.
That’s what makes them dangerous.
Here are the five topics I’ve seen drain marks from otherwise well-prepared students, and exactly what to do about each one.
1. Moles and quantitative chemistry
This is the one that causes the most visible distress. Students either love moles or they’ve decided they can’t do them — and that decision usually happened the first time the formula didn’t make sense and nobody explained the reasoning behind it.
The mistake most students make is treating moles as a memory task. They memorise the triangle, plug in numbers, and hope the question matches what they practiced. The moment a question changes the format slightly — asking for concentration instead of mass, or introducing a limiting reagent — everything falls apart.
What actually works: understand that moles is just a counting system. One mole of anything always contains the same number of particles. Once that clicks, the calculations follow logically rather than feeling like guesswork.
Practice the calculation types in order — mass to moles, moles to volume, concentration problems — and don’t move to the next type until the previous one feels automatic.
2. Electrolysis
Electrolysis questions are on almost every Edexcel paper. They follow a very consistent pattern. And yet students consistently drop marks on them — usually because they’ve learned the rules without understanding why they work.
The two most common errors I see in mark schemes:
Getting cathode and anode mixed up. Students write the correct product but assign it to the wrong electrode, losing the mark entirely.
Forgetting the half-equations. Electrolysis questions at the higher grades almost always ask for half-equations. Students who haven’t practised writing them under timed conditions freeze.
The fix: drill the two rules until they’re automatic. At the cathode, positive ions gain electrons and are reduced. At the anode, negative ions lose electrons and are oxidised. Then practise writing half-equations for the common electrolytes — copper sulfate solution, sodium chloride solution, molten lead bromide — until the format is instinctive.
3. Rates of reaction
Rates of reaction is one of the most reliably examined topics on the Edexcel specification. And it’s also one of the topics where students most consistently write too vaguely.
The typical question asks students to explain why increasing temperature increases the rate of reaction. A common student answer: “the particles move faster and collide more.”
That answer will get one mark out of three. Here’s what gets full marks: increasing the temperature increases the kinetic energy of the particles. They move faster, so collisions happen more frequently. Crucially — and this is the part students miss — a greater proportion of those collisions have energy equal to or greater than the activation energy, so more successful collisions occur per second.
That last part is what most students leave out. It’s also what the mark scheme is looking for every single time.
4. Organic chemistry
Organic chemistry feels overwhelming because students try to learn every reaction in isolation — memorising each transformation as a separate fact. By the time they’ve covered alcohols, carboxylic acids, esters, and addition reactions, it all becomes a blur.
The students who do well in organic chemistry questions have a reaction map in their head. They know how the functional groups connect to each other: alkenes can be hydrated to alcohols, alcohols can be oxidised to carboxylic acids, carboxylic acids and alcohols react together to form esters. Once you see the relationships between the groups, the reactions stop feeling like a list and start feeling like a system.
Exam questions on organic chemistry are also very predictable in their structure. They almost always give you a starting material and ask you to identify a product or name a reagent. Practising past paper questions on this topic gives you an accurate picture of exactly what’s expected.
5. Command words — across every topic
This isn’t a topic in the traditional sense, but it might be the single biggest source of avoidable mark loss across the entire paper.
I’ve marked answers that showed genuine understanding of the chemistry but received zero marks because the student described when the question asked them to explain, or explained when the question asked them to state. The examiner cannot award marks for correct information given in the wrong format.
The command words that trip students up most often: describe means say what you observe or what happens. Explain means give the scientific reason why. Suggest means apply your knowledge to an unfamiliar situation — there may not be one right answer. State means give a concise factual answer with no elaboration needed.
Read the command word before you read the rest of the question. It tells you exactly what kind of answer will earn the marks.
What to do now
If you recognised your own revision habits in any of the five points above, that’s useful information — not a reason to panic.
Every one of these issues is fixable with the right practice. The common thread is that they all reward students who understand what the examiner is looking for, not just students who know the content.
A free topic diagnosis quiz on mintnotescience.pro will show you which of these areas need your attention most. It takes five minutes and gives you a clear starting point.
There’s also a full IGCSE Chemistry study pack with tiered practice questions and examiner tips built into every topic — so you can practise the right things in the right way.
👉 Take the free topic diagnosis quiz →
👉 Explore the IGCSE Chemistry study pack →
Written by a qualified teacher and Edexcel examiner with over 10 years of IGCSE Chemistry experience. All MintNote Science resources are checked against the current Edexcel specification.