Here's the thing nobody tells you in the last fortnight before a chemistry paper: you've already done the hard part. You've learnt the reactions, the equations, the required practicals. The marks you're losing now mostly aren't "I didn't know it" marks. They're "I knew it and threw a mark away anyway" marks, and those are the easiest ones in the entire exam to win back, because fixing them doesn't mean learning more content. It means changing a couple of habits this week.
Mark schemes are surprisingly blunt about where students slip. Read a few published ones and the same patterns show up paper after paper: a number with no unit, an answer rounded to the wrong place, an equation that doesn't balance, a state symbol left off. None of those is a knowledge problem. All of them are checkable in the last thirty seconds of a question. Let's go through the big ones.
Units: the marks you're basically giving away
A number on its own is not an answer in chemistry. "24" is not a mass; "24 g" is. Calculation questions almost always reserve a mark for the correct unit, and it's the single most common easy mark students leave on the table, usually because they're already writing the next question in their head.
The fix is mechanical, not clever: every time you write a final number, your pen doesn't lift until a unit is next to it. Moles in mol, mass in g, volume in cm³ or dm³, concentration in mol/dm³ or g/dm³, energy change in kJ/mol. If you're not sure which unit a quantity takes, that's a revision gap worth ten minutes tonight: far more valuable right now than re-reading a topic you already know.
Significant figures: round at the end, not the middle
Two things go wrong with significant figures. The first is rounding too early: squashing a number down to 2 s.f. halfway through a multi-step calculation, then carrying that error all the way to the final answer. Keep the full number in your calculator until the very last line, then round once.
The second is ignoring the instruction. If the question says "give your answer to 3 significant figures", that's a mark with your name on it, and it's lost the moment you write four figures or two. Underline the s.f. instruction the second you read it, the same way you'd underline a command word. It costs you a pen-stroke and saves you a mark you've otherwise fully earned.
Balancing equations: count every atom, twice
An unbalanced equation is a classic case of knowing the chemistry and still dropping the mark. You know the reactants and products; the slip is in the bookkeeping. The rule examiners apply is simple: the number of each type of atom has to be identical on both sides, and you only ever change the big numbers in front (the coefficients), never the little subscript numbers inside a formula, because that changes the substance entirely.
Build a thirty-second habit: once you've written an equation, run your finger across it and count one element at a time, left side then right side. Do hydrogen and oxygen last. They're the ones that usually need the final tweak. And don't forget the state symbols, which brings us neatly to the next one.
State symbols: the four letters that matter
(s), (l), (g) and (aq). They look like decoration, but in a lot of equation questions (and almost always in ionic equations), they're a required part of the answer. Leave them off and a mark you'd have got vanishes for the sake of four characters.
The one people trip over is (aq) versus (l): "aqueous" means dissolved in water (like sodium chloride solution), while (l) means a pure liquid (like molten lead bromide, or water itself). Water as a product is (l). A salt dissolved in water is (aq). Get that distinction solid and you'll pick up state symbol marks that most of the room is quietly missing.
Command words: "describe" vs "explain" vs "calculate"
Chemistry isn't only a numbers subject, and the longer written questions have their own trap: answering the wrong question well. "Describe" wants what happens. "Explain" wants what happens and why. No "because", no mark. "Compare" wants both things in the same sentence, not two separate paragraphs. "Calculate" wants working shown, because if your final number is wrong you can still pick up method marks from the steps, but only if the steps are on the page.
So show your working even when you're confident, and circle the command word before you write a single line. Half the battle in the six-markers is simply doing the verb the question actually asked for.
Turn your GCSE chemistry common mistakes into one fix
Here's the useful bit: these mistakes are personal. Almost everyone has a signature slip: the one they make again and again. Maybe units are fine but you always round too early. Maybe your equations balance but the state symbols are an afterthought. You don't need to fix all five. You need to find your one or two and drill them until they're automatic.
This is where working from your actual specification helps. GCSE Boss maps every question back to the spec point it's testing, on your exam board, so when the same slip shows up three times it stops being a vague "I keep losing silly marks" and becomes a specific, fixable target ("I drop the unit on moles questions") that you can hammer in one short session. One repeated mistake, found and fixed, is worth more this week than another pass through a topic you already know.
So tonight, don't open a new topic. Grab a past paper you've already done, find the marks you lost, and sort them into knowledge gaps versus silly slips. The slips are free marks waiting to be claimed, and you've already done the hard work to deserve them. You've got this.