Chemistry! The exams are ridiculous especially if you're doing OCR Salters. A2 Chemistry includes everything at AS (concepts are reintroduced) so you're basically learning AS all over again on top of A2. Also, you carry out a 3/4 week investigation and write up 40 pages and if your investigation results are wrong, you're pretty much screwed. It has literally been one of the most stressful things ever. Chemistry is both difficult and demanding at A-level, whereas some subjects are just demanding, or just difficult. Hope that makes sense.