How to Bridge the Gap Between IGCSE and A-Level Chemistry

Hosni Showike • 11 July 2026

How much IGCSE Chemistry you really need, why the mole decides your AS grade, and the 40-day rule — from a Cambridge and Edexcel examiner.

A hand-drawn AS Chemistry bonding mind map branching into ionic, covalent and metallic bonding, with sub-branches for polarising power, sigma and pi bonds, dative bonding, molecular shapes and giant structures.

TL;DR: AS Chemistry is an expansion of IGCSE Chemistry, not a different subject — bonding, atomic structure and organic chemistry all return, with more explanation, more detail and far more exam application. What changes is where the marks live: recall drops from half the qualification to about a third, while application rises to nearly half. The mole is the real bridge, because it moves from an optional IGCSE sub-topic to Topic 1 of Unit 1, feeding every unit that follows. Bridge the gap by learning conceptually, compiling each topic onto one summary sheet, and finishing the content 40 days before the exam so the final month is pure application.

Every July I get the same message from students about to start AS. They want to know whether they should spend the summer re-revising IGCSE Chemistry, and whether the jump is as brutal as everyone says. After more than fifteen years teaching this transition and marking papers for both boards, my answer has not changed: you need the IGCSE knowledge, but you almost certainly do not need a separate IGCSE revision programme to get it back. What you need is a different way of learning.

How much of IGCSE Chemistry do you need to carry into A-Level Chemistry?

You need the knowledge, but not a separate revision course — a good AS teacher rebuilds the prerequisites inside the A-Level lesson itself. Pearson designed the International A-Level Chemistry specification for students who have already achieved a Chemistry qualification at GCSE or IGCSE level, so the prior knowledge is assumed rather than retaught from zero.

Here is what I see in practice. Most students retain more than 40% of their IGCSE content — which is more than they fear, but less than their new teacher may assume. The reason for the loss is not stupidity and it is not the summer holiday. It is that most students crammed the IGCSE syllabus in the weeks before the exam, and crammed content evaporates. Retention was never built in the first place.

So I do the opposite of assuming. I start every AS course as though my students know nothing, and I revise the prerequisite ideas at the beginning of each lesson — but only the ones that lesson requires. Everything from the IGCSE syllabus that does not feed the A-Level topic in front of us, I cut. That filtering is the whole trick. I have taught students who arrived with very weak IGCSE Chemistry backgrounds and who still built a strong foundation, because those short, targeted revisions at the start of each session did the work that a summer of unfocused re-reading never would.

If your teacher does this, do not spend August re-reading your old notes. Spend it getting comfortable with the mole.

What really changes between IGCSE and AS Chemistry?

The marks move. At IGCSE, half your grade comes from recall; at AS, recall is worth about a third, and application is worth nearly half. The Cambridge IGCSE Chemistry 0620 syllabus gives 50% of the qualification to knowledge with understanding and 30% to handling information and problem-solving. Pearson's assessment objective weightings for IAS Chemistry invert that balance: 34–36% for knowledge and understanding, and 43–47% for applying it in familiar and unfamiliar contexts.

That single shift explains almost everything students find hard about the first term.

Dimension	Cambridge IGCSE Chemistry (0620)	Edexcel IAL AS Chemistry (WCH11-WCH13)
Recall marks (AO1)	50%	34-36%
Application and analysis marks (AO2)	30%	43-47%
Where the mole sits	Topic 3.3, Supplement content only; Core calculations exclude it	Topic 1 of Unit 1, applied to all other units
Gas calculations	Molar gas volume (24 dm3) only	Molar gas volume and the ideal gas equation, pV = nRT
Guaranteed maths marks	None stated	At least 20% of marks across the papers; at least 18 in Unit 1 alone
Multiple-choice pace	40 questions in 45 minutes	20 questions in roughly 20 minutes
Guided learning hours	About 130	360 for the full IAL
Synoptic questions	None	Units 2, 4 and 5

Read the last two rows again. Nothing at IGCSE asks you to carry Unit 1 into Unit 4. At IAL, that is the design.

Why the mole concept is the real bridge

The mole is the single biggest structural difference between the two courses, and it is where I would spend your first weeks. At IGCSE the mole is Supplement content — the Cambridge syllabus states plainly that Core candidates' calculations will not involve the mole concept at all. At IAL it is Topic 1 of Unit 1, Formulae, Equations and Amount of Substance, and Pearson writes in the specification that the ideas from this topic will be applied to all other units.

That is not a warning from a tutor. That is the exam board telling you where your grade comes from.

Look at what it means on a real paper. I audited the Edexcel WCH11 Unit 1 paper from May 2023 mark by mark. Of its 80 marks, 23 — roughly 29% — were amount-of-substance calculations: parts per million, the Avogadro constant, a limiting reactant, an empirical formula from percentage composition, atom economy, identifying an unknown Group 1 element from a precipitate mass, the volume of carbon dioxide produced when a kilogram of aluminium is extracted, and a five-mark ideal gas question on a pharmaceutical inhaler. Nearly three marks in every ten, on the very first paper of the course.

And the clock is the problem. Section A gives you 20 multiple-choice marks and tells you to spend no more than 20 minutes on them. One minute per question — and some of those questions are multi-step mole calculations. This is exactly why I rebuild the mole from scratch at the start of my AS Chemistry course rather than assuming it carried over.

The one-minute mole method

Speed on these questions is a technique, not a talent. Four habits get my students there:

Find the target first. Read the last line before the options and identify exactly what the question wants. Not the chemistry — the quantity.
Eliminate on units. If an option's units cannot be right for the target, it is a distractor. You can often kill two options before calculating anything.
Go straight to the calculator. Apply the formula on the keypad without writing intermediate steps. Writing is what costs you the minute.
Master the mole ratio. The faster you read the ratio out of the balanced equation, the faster everything downstream falls out.

One more thing from marking these papers: percentage yield is where students haemorrhage multiple-choice marks — not the multi-step ideal gas questions, which tend to appear as written calculations where partial credit rewards your working. When a percentage yield question appears in Section A, I tell my students to skip it, mark it, and come back at the end of the paper. Do not let one question eat five minutes of a twenty-minute section.

Which IGCSE topics should you revise before you start?

Bonding, atomic structure and organic chemistry all return — but the topic that damages students most in the first term is organic chemistry, because it demands sheer memorisation and most students left IGCSE without building any. I revise it extensively before I start the A-Level organic content, and I would rather you arrive knowing your functional groups than knowing nothing else.

Bonding is the clearest example of what "expansion" really means. At IGCSE you learned that ionic bonding is electrostatic attraction between oppositely charged ions. At AS you keep that sentence — and then you add evidence for the existence of ions from electron density maps, polarising power and covalent character, sigma and pi overlap, dative bonds, electronegativity and molecular polarity, and electron-pair repulsion theory for shapes. Same foundation. Four floors built on top.

Learn it conceptually, not factually

Students who learn Chemistry as a set of facts cap out around a B or C. In fifteen years I have never seen a student reach A* by memorising content and solving a couple of past papers at the end. The A and A* students learn why and how things happen, not what happens — and they decide to do that in the first month, not the last.

The exam board says the same thing in colder language. Pearson's guidance warns that a student who answers an "Explain" question with a series of factual statements, without reasoning or justification, is likely to score few marks — and it names the failure mode precisely: the "scatter gun" approach, where a student latches onto a key word in the question and writes everything they know about that topic.

Watch your command words. In Chemistry the ones that decide your grade are calculate, determine, state and explain — and they are not interchangeable. State wants a fact. Determine wants an answer established from the information given. Explain wants the reason, and a beautiful description will score zero against it.

Compile everything onto one summary sheet

After each topic, merge your own notes and your teacher's lesson into a single sheet — a mind map, a data sheet, a summary page, whatever suits you. The point is not to collect the information. The point is that the act of drawing the map forces you to see which connections you cannot make, and those blanks are your gaps.

![A hand-drawn AS Chemistry bonding mind map branching into ionic, covalent and metallic bonding, with sub-branches for polarising power, sigma and pi bonds, dative bonding, molecular shapes and giant structures]

That is one of my own bonding maps. Notice that the IGCSE material sits in it as the trunk — electrostatic attraction, giant structures, diamond and graphite — and every AS addition hangs off it as a branch. That is the correct mental model for this whole transition, and it is the model behind the way my IAL Chemistry notes are written: mark-scheme language, structured so the links between topics are visible rather than buried.

Then take the map to a past paper question. Compiling trains understanding; applying trains the brain to retrieve it under pressure. You need both, in that order.

Finish the content 40 days before the exam

Learning the content is essential, and on its own it may not even get you a pass. The goal is to finish the main syllabus about 40 days before your exam so the final stretch belongs entirely to application. Those 40 days are not padding — they have a shape:

Revise individual topics first, one at a time, using your summary sheets.
Make the connections between them, because synoptic questions in Units 2, 4 and 5 will demand exactly that.
Then solve full papers at exam pace, counting them — a target number, timed, start to finish. Working through classified solved past papers topic by topic first, then whole papers, is the sequence that works.
Keep the last week for mistakes only. Go back through everything you got wrong and understand why.

That final week matters more than students believe. Examiner reports on these papers return again and again to careless losses — answers copied down incorrectly from one line of working to the next, units ignored or omitted. Those are not knowledge failures. They are habits, and habits are fixed by review, not by revision.

The students who score Cs and Ds and the students who score A*s often know a similar amount of chemistry in March. What separates them is that one group spent April learning content and the other spent it applying it.

Do you need A-Level Maths for A-Level Chemistry?

No — but you do need confident algebra, ratios, standard form and unit conversion. Pearson guarantees that a minimum of 20% of the marks across the IAL Chemistry papers target Level 2 mathematics or above, with at least 18 of Unit 1's 80 marks in that category. Maths is rarely the thing that limits a Chemistry student. Fluency with rearranging a formula under time pressure is.

If you want to see how the first weeks of the mole are taught before you commit to anything, sit in on a free AS Chemistry class and judge it yourself. If you would rather ask a question directly, message me on WhatsApp at +965 5137 5709 or email admin@chem-bio.info.

Frequently asked questions

How big is the jump from IGCSE to A-Level Chemistry? The content jump is smaller than students expect, because bonding, atomic structure and organic chemistry all reappear in expanded form. The real jump is in assessment: application marks rise from 30% to nearly half of the qualification.

Do I need to revise IGCSE Chemistry before starting A-Level? You need the knowledge, but a separate summer revision programme is usually wasted effort if your teacher rebuilds prerequisites inside each lesson. If you want to prepare for something specific, prepare for the mole.

What is the hardest topic in AS Chemistry? Amount of substance causes the most damage, because it appears in roughly 29% of the marks on a real Unit 1 paper and reappears throughout the course. In multiple-choice sections, percentage yield questions are where students lose the most marks fastest.

Is A-Level Chemistry harder than IGCSE Chemistry? It is more demanding in depth and pace, but it is built on the same foundation, so it is not a new subject. Students struggle when they carry IGCSE study habits — cramming and memorising — into a course that rewards explanation.