Which IAL Chemistry Unit Is the Hardest? A Unit-by-Unit Guide (2026 Data)

TL;DR: On paper, Unit 4 (WCH14) and Unit 5 (WCH15) are the hardest IAL Chemistry units, because they are the longest papers at 90 marks each and Unit 4 carries the most maths. In my own classes, though, students score lowest on Unit 3, the AS practical paper. The paper you sit matters too: in June 2026, two versions of Unit 2 needed 65 and 49 out of 80 for an A, a 16-mark gap in the same session. The table below compares all six units side by side: marks, timing, maths load and 2026 A-grade boundaries.

In this guide: The hardest unit · All six units compared · Why boundaries change · Unit 4 vs Unit 5 · Units 3 and 6 · Unit 1 vs Unit 2 · Planning your revision · FAQs For all the students starting A2 this year, one question keeps coming up. Which is the hardest IAL Chemistry unit, and where am I going to lose my A? Forum threads give you opinions. I'm going to give you Pearson's own figures, unit by unit, and then show you what they mean for your revision.

What is the hardest IAL Chemistry unit?

On paper, Unit 4 (WCH14) and Unit 5 (WCH15) are the hardest IAL Chemistry units, because they are the longest papers at 90 marks and 1 hour 45 minutes each. In my own classes, though, students score lowest on Unit 3 (WCH13), the AS practical paper. Why these two? Because they sit at the top of the course and build on everything before them. According to Pearson's IAL Chemistry Getting Started Guide, Unit 4 includes at least 24 marks of maths. Unit 5 can ask what we call synoptic questions, which are questions that pull in content from Units 1, 2 and 4. Both papers also include at least one six-mark extended-response question, where the examiner marks how logically you build your answer, not just the facts you write. So which of the two is harder for you? That depends on your weak point. I'm going to show you how to find it below. And Unit 3? It is only 50 marks, yet it is where my students score lowest. That's why it gets its own section further down.

How do the six IAL Chemistry units compare?

The six units differ in length, maths load and weighting. Units 1, 2, 4 and 5 are each worth 120 UMS, while the two practical papers, Units 3 and 6, are worth 60 UMS each. Here is everything in one table. The structure comes from the Getting Started Guide, and the boundaries come from Pearson's January 2026 and June 2026 grade boundary documents.

How do you read the last two columns? Pearson lists two versions of each paper, such as WCH12 and WCH12A, and each version gets its own boundary. The first figure is the main paper and the second is the A version. Pearson adjusts raw boundaries for the difficulty of each paper, so a lower boundary means students found that paper harder.

Why do grade boundaries change for the same unit?

Pearson sets raw grade boundaries after each exam to account for how difficult that particular paper was. That's why, in June 2026, the two versions of Unit 2 needed 65 and 49 out of 80 for an A, a 16-mark gap in the same unit and the same session. Think about what that means. On one version you needed about 81% for an A. On the other you needed about 61%. Same unit. Same content. Same month. And it is not only Unit 2. In June 2026 the two Unit 3 papers needed 40 and 31 out of 50. The main Unit 4 paper needed 72 out of 90 in January 2026, then only 66 in June. So how is that fair? Pearson converts your raw mark into a fixed scale, and a harder paper with a lower boundary still gives you the same grade. This is what we call the uniform mark scale, or UMS. That's why, when a student on a forum says "Unit 4 is the hardest", they often mean the Unit 4 paper they sat was the hardest. What should you do with this? Practise papers from several exam series, not just last year's. One paper tells you very little. Five papers tell you where you lose marks. You can find them in the solved IAL past papers.

Unit 4 vs Unit 5: which is harder?

Unit 4 is harder if calculations are your weak point, because it carries at least 24 marks of maths. Unit 5 is harder if you struggle to connect topics, because it can test Units 1, 2 and 4 as well as its own content. Let's start with Unit 4. It covers kinetics, entropy and energetics, chemical equilibria, acid–base equilibria, and carbonyls, carboxylic acids and chirality. Section C is a data or calculation question. The Getting Started Guide itself notes that Unit 4 has many more opportunities to test maths than Unit 5. On the other hand, Unit 5 covers redox equilibria, transition metals, arenes, organic nitrogen compounds and organic synthesis. It has fewer maths marks, a minimum of 14. But its Section C is a contemporary-context question, and organic synthesis can ask you to plan a reaction scheme of up to four steps. So which is harder for you? Look at your own marks. If you drop marks in pH and equilibrium constant calculations, Unit 4 is your hardest unit. If you lose marks linking reactions together, Unit 5 is. In my classes, Unit 4 costs students more marks than Unit 5. So if you are planning your A2 revision, start with Unit 4.

Are Units 3 and 6 easier because they are practical papers?

Units 3 and 6 are shorter, at 50 marks and 1 hour 20 minutes each, but they are not automatically easier. They are written papers set in unfamiliar practical situations, so you have to apply lab skills rather than recall a method. There is no coursework and no practical exam. Instead, Pearson's guide explains that these papers test your understanding of techniques, your planning, and how you evaluate data and methods. You may be asked to do calculations, draw graphs, and explain what you would observe. Unit 3 is the unit where my students score lowest. The most common mistake I see on Units 3 and 6 is revising only the practicals. Revising the theory part is just as important as the practicals themselves. Why? Because you cannot explain an observation or evaluate a method without the chemistry behind it. Do not judge these papers by their length. In June 2026, the two Unit 3 versions needed 40 and 31 out of 50 for an A. A 9-mark swing on a 50-mark paper is substantial. If Unit 3 is next for you, work through my practical guide to Unit 3.

Is Unit 1 or Unit 2 harder at AS?

Unit 2 (WCH12) asks more of you than Unit 1, because it adds a six-mark extended answer, a contemporary-context section and questions that can draw on Unit 1. But Unit 1 sets the foundations, so a weak Unit 1 makes Unit 2 harder still. What does each unit cover? Unit 1 is formulae, equations and amount of substance, atomic structure, bonding, and your first organic chemistry: alkanes and alkenes. Unit 2 moves on to energetics, intermolecular forces, redox with Groups 1, 2 and 7, kinetics and equilibria, and alcohols and halogenoalkanes. The 2026 boundaries did not pick a winner. In January, the main Unit 2 paper needed 60 for an A against 63 for Unit 1. In June it flipped: Unit 1 needed 59 and Unit 2 needed 65. In my experience, the move from Unit 1 to Unit 2 is usually frictionless if Unit 1 was strong. That's why my advice is simple. Secure Unit 1 first. If moles and amount of substance are shaky, see how to pass Unit 1 in one month.

How should you plan revision around your hardest unit?

Find your weakest unit from timed past-paper scores, not from forum opinion, and give it the most revision time. Because IAL Chemistry is modular, each unit is graded on its own, and your total UMS across all six units decides your final grade. Here is the plan, in five steps:

1. Sit two recent papers for each unit under timed conditions.
2. Mark them with the mark scheme and convert each score to a grade using that series' boundaries.
3. Rank your units from weakest to strongest.
4. Use the IAL UMS calculator to see what you need in your remaining units. An A overall needs 480 out of 600 UMS.
5. Read the command word every time. Pearson warns that answering an "explain" question with a description scores few marks.

Before you start, check every topic in each unit against the IAL Chemistry specification checklist. Make sure nothing is missing from your notes.

What else do students ask about IAL Chemistry units?

Here are four common questions about IAL Chemistry units, each with a short, direct answer.

Is IAL Chemistry hard?

IAL Chemistry is demanding, because about 20% of the marks across the papers test maths and many questions are set in unfamiliar contexts. It becomes manageable when you practise past papers unit by unit and learn exactly what each mark scheme rewards. I explain this further in my video, Is IAL Chemistry Hard?

Why are IAL Chemistry grade boundaries so high?

Pearson sets boundaries after each exam to reflect how difficult that paper turned out to be. A high boundary means students found the paper more accessible, so you needed more marks for the same grade.

How many UMS marks do I need for an A in IAL Chemistry?

You need 480 out of 600 UMS across all six units for an A. Units 1, 2, 4 and 5 are worth 120 UMS each, and Units 3 and 6 are worth 60 each.

Can I resit just one IAL Chemistry unit?

Yes. IAL Chemistry is modular, so you can sit a unit in any exam series in which it is offered and cash in once you have completed the units you need. To work through every unit with me, join the IAL Chemistry live classes, or start with the IAL AS Chemistry course and my AS Chemistry notes. That's all for today. I hope that this was helpful and I wish you all best of luck in your exam.

Hosni Showike is the author of eight published science textbooks with more than 20,000 copies sold worldwide. He has taught IGCSE and International A Level Chemistry and Biology for over 20 years, including 16 years teaching to Cambridge and Edexcel mark schemes at A-level, and writes at chem-bio.info where he publishes grade-threshold analysis, solved past papers and live exam-preparation classes.