Cambridge Examiners Revealed Why You're Losing Marks on IGCSE Combined Science (0653/0654) — And It's Not What You Think

What you're about to read was taken directly from Cambridge IGCSE Combined Science examiners — the people physically marking your paper.

Not a teacher's guess. Not a YouTube tutor's theory. The actual patterns your examiners see, year after year, that cost students marks they already earned.

Here's what nobody tells you: you probably know enough science to pass. Maybe even to do well. Many students who underperform aren't failing because their knowledge is weak. They're failing because there's a gap — between what they know and what they wrote down — and they don't even know the gap exists.

Read every pillar below. Take it seriously. Because this isn't about studying harder or learning more content. It's about finally understanding the game — and realising you were one step away from winning it the whole time.

Pillar 1: General Exam Mastery — The Mental Game

Before we even get into the science content, I want to talk about something that examiners bring up time and time again: marks lost not because students didn't know the answer, but because they weren't thinking carefully when they read the question. This is what I call the mental side of the exam, and getting it right costs you nothing except a bit of deliberate habit-building.

The Context Trap — Your Past Paper Knowledge Can Work Against You

This one is so common and so preventable. You've done twenty past papers. You see a question about osmosis, or electromagnetic induction, or enzyme activity, and your brain goes "I know this" — and you're already halfway through writing before you've actually read what's being asked. And then you answer a slightly different question to the one on the page.

Examiners are wise to this. They deliberately vary the angle of familiar topics precisely because they know candidates will go on autopilot. So even when something looks familiar, treat it like you've never seen it before. Read every word. The context might be the same, but the question might not be.

Time Management — One Question Is Never Worth the Whole Paper

I'll keep this simple because the logic is straightforward, even if following it under pressure is harder. You have a fixed amount of time and a fixed number of questions. No single question — no matter how many marks it's worth — deserves to eat up so much time that you're rushing or skipping things at the end.

If a calculation isn't coming together, mark it and move on. Come back with fresh eyes. You'd be amazed how often a question that felt impossible suddenly clicks when you return to it after answering a few others. What you cannot recover is time spent on a question you've already lost while other questions sit unanswered.

Negative Constraints — The Sneakiest Word in Any Exam

"Not." One syllable. Three letters. Responsible for a genuinely shocking number of lost marks every single year.

"Which of the following ions does not have a noble gas configuration?" Miss that word and you've answered the exact opposite question. Same with "least," "except," "unlike," and similar words — they completely flip what the examiner is asking for.

My advice is simple and it works: as you read through each question, actively hunt for these words. When you find one, underline it. Make it impossible to miss. It takes two seconds and it keeps your brain locked onto what's actually being asked rather than what you assumed was being asked.

Pillar 2: The Mathematical Toolkit — Where Easy Marks Disappear

Here's something that genuinely frustrates me as a teacher, because it comes up in almost every set of papers I mark: a student who clearly understands the physics or chemistry behind a question, sets it up correctly, and then loses the mark because of a unit conversion or a rounding slip. The science was right. The execution let them down. Let's make sure that's not you.

Unit Conversions — Do This Before Anything Else

This is non-negotiable in my classroom and it should be non-negotiable for you in that exam hall. Before you touch a single number in a calculation, check what units your data is in and what units the formula needs.

15 milliamperes is not 15 amperes. 3 minutes is not 3 seconds. 250 grams is not 250 kilograms. These feel like obvious things, but under exam pressure, with a formula you recognise and numbers ready to plug in, it is incredibly easy to skip the conversion step entirely. And then you get a wrong answer on a question you actually knew how to do.

My rule: write your conversions out explicitly before you start. "15 mA = 0.015 A." One line. It takes five seconds and it keeps you honest.

Rearranging Formulas — Letters Before Numbers

A lot of algebraic errors happen at the moment you're trying to rearrange and substitute at the same time. You're thinking about the numbers, and your algebra gets sloppy. The fix is simple: rearrange the formula in its symbolic form first, before a single number appears.

So if you need to find resistance, write V = IR, rearrange to R = V/I, and then substitute your values. Don't try to do both at once. Formula triangles are also a completely valid tool — there is no shame in using them, and they work. Whatever helps you get the algebra right is the right method for you.

And of course — show every step of your working. Even if your final answer is wrong, examiners can award marks for correct method. A page of clear working that leads to a wrong answer is worth far more than a wrong answer on its own.

Significant Figures — Read the Question, Then Round Once

When an examiner tells you to give your answer to two significant figures, that instruction is part of the question. Ignoring it costs you the mark even if your number is otherwise correct. I've seen it happen more times than I can count.

The golden rule here is: don't round early. Carry full values through your calculation and only round at the very last step, to exactly the precision the question specifies. Rounding halfway through introduces errors that compound, and you can end up with an answer that's both rounded incorrectly and imprecise. Do the maths in full, then round once, deliberately, at the end.

Pillar 3: Discipline-Specific Deep-Dives — The Mistakes I See Every Year

These are the ones that make me sigh when I'm marking. Not because the students are careless, but because these are such fixable misconceptions — the kind that, once you understand them properly, you'll never get wrong again. Let's go through them subject by subject.

Biology — Say What You Mean, Precisely

Language in biology really matters, and there are two areas where I see candidates lose marks constantly.

First: enzymes do not die. I know it's tempting to say it that way, and I understand the instinct — but enzymes are proteins, not living things, and they cannot die. What happens at high temperatures is that the enzyme denatures — the shape of the active site changes, meaning the substrate can no longer fit. That's the explanation the mark scheme wants, and "the enzyme died" gets you nothing. Learn the word, understand what it means, use it correctly.

Second: excretion and egestion are not the same thing and are not interchangeable. Excretion is the removal of metabolic waste — things your body has produced through chemical reactions, like urea or carbon dioxide. Egestion is the removal of undigested food through the bowel. One is waste your body made. The other is material that was never really part of your body at all. Mix these up and you've demonstrated a fundamental misunderstanding, and the examiner will mark accordingly.

Chemistry — Precision in Definitions Is Everything

Two things here that come up year after year.

The word "only" in hydrocarbon definitions. A hydrocarbon contains hydrogen and carbon — but writing just that is not enough. Plenty of other compounds contain hydrogen and carbon alongside other elements. A hydrocarbon contains hydrogen and carbon only, and that word does real definitional work. Leave it out and the mark is gone.

In electrolysis, I want you to be very clear on this: electrons do not flow through the electrolyte. Electrons flow through wires and external circuits. What carries charge through the electrolyte — the liquid or dissolved solution — is the movement of ions. Mobile ions conduct electricity through the electrolyte. If you write "electrons flow through the solution," that is incorrect, and examiners will not credit it.

Physics — Logic Over Instinct

Physics marks are often lost because candidates apply a rule they know without checking whether the conditions of the question actually allow it.

Energy questions are a perfect example. In a vacuum, with no air resistance, gravitational potential energy lost equals kinetic energy gained — clean and simple. But the moment air is involved, some of that energy is doing work against air resistance, which means the kinetic energy gained is less than the GPE lost. Always check: does this question involve air? If so, your energy equation needs to account for that.

In circuits, parallel versus series behaviour trips up so many candidates. Here's how I want you to remember it: in a parallel circuit, every branch gets the same potential difference — voltage doesn't split, it stays the same across each branch. What splits is the current. In a series circuit it's the other way around — the current is the same throughout, and the voltage divides. Get this the wrong way round and every subsequent part of the question goes with it.

Pillar 4: The Practical Scientist's Protocol — Getting Papers 5 and 6 Right

Practical papers are where well-prepared students can really pull ahead — but they're also where some very specific, very avoidable mistakes consistently cost marks. The good news is that once you know the rules, following them is mostly just discipline.

Graphing — There Is a Right Way and an Almost-Right Way

Plotting a graph feels straightforward, and that's exactly why candidates get complacent with it. Let me give you the non-negotiables.

Your scale must be linear — equal intervals, consistently spaced, every time. And it must be generous enough that your plotted points spread across more than half the grid. A graph where all your data is squashed into one corner is not a good graph, regardless of how accurate the points are. Choose a scale that uses the space you've been given.

Label both axes with the quantity and the unit. "Temperature / °C" not just "Temperature." That slash matters and examiners look for it.

And please — plot your points as small, neat crosses. Not dots, not blobs, not circles. A cross shows the examiner exactly where you intend the point to be. A blob is ambiguous and can cost you accuracy marks. This is a tiny thing that takes zero extra effort once it's a habit.

Planning Investigations — Specificity is Everything

When a question asks you to plan an investigation, structure your answer around the bullet points provided. They're not decorative — they're telling you exactly what the mark scheme is looking for.

The biggest language trap in planning questions is the word "amount." Amount of what? The examiner needs to know. Say "volume of acid in cm³" or "mass of magnesium in grams" — something precise and measurable. "Amount" tells a scientist nothing, and in an exam context it tells the examiner you haven't quite thought it through. The same applies to your variables: name them specifically, state how you'll measure them, and state what you'll keep constant and why.

Biological Drawing — Less Is More

This one has a very clear set of rules, and following them is mostly about slowing down and being deliberate.

Use a sharp pencil. A single, continuous, unbroken line for each outline — no sketching, no feathering, no going over the same line twice. And absolutely no shading. I know shading feels like it adds detail, but in biological drawing it does the opposite — it obscures the structures the examiner needs to see clearly. If you want to show a feature, use a label line, not shading.

Your drawing should be large. It should take up at least half the space you've been given, and ideally it should be bigger than the actual specimen. Tiny drawings crammed into a corner cannot show the level of detail that earns marks. Use the space. Be bold with your pencil lines.

Papers 5 and 6 reward candidates who treat practical skills as seriously as content knowledge — because they are just as learnable, just as mark-worthy, and honestly, once you know the protocol, just as straightforward.