The hardest idea in IB Biology, Chemistry and Economics, explained

We read the examiner subject reports and counted the questions in the past papers to find the idea in each subject that students understand least. Each one has a 35-second video, a full explanation and a free live session.

Biology HL Chemiosmosis A 7-mark Paper 2 part in May 2025 and again in May 2026, and examiners flag the direction of the protons. Chemistry HL Rate equations and the slow step About 1 in 4 got this right on one May 2025 paper, and two-step mechanisms came up in 6 of the last 8 sittings. Economics SL and HL Which exchange-rate curve moves The diagram is in 24 of the 30 Paper 2 questions since 2022, and one 2024 report called a part on it the worst attempted.

How we chose them

• What the examiners say. We looked for ideas the subject reports flag as misunderstood, with a figure where they give one.
• How often they are set. We counted every appearance across the HL Biology and Chemistry papers since May 2025 and the Economics Paper 2 questions since May 2022.
• When you meet them. We picked what May-session students in Europe and the Middle East are studying or revising before mocks.

The examiner evidence is from the May 2025 subject reports for Biology and Chemistry and the 2022 to 2025 reports for Economics, the most recent we could find. This page was developed independently from and is not endorsed by the International Baccalaureate Organization. Examples and wording are ours.