I got A*A* at GCSE Science and found the jump up quite big. I've heard that it's less of a jump though if you do GCSE Chemistry rather than GCSE Science...either way, there's a fair bit of work involved and there's a bit more maths than there was at GCSE
I thought it was a really big jump! I often found my teacher saying well we told you this at GCSE but actually this isn't the truth. I'm so glad i never have to do chemistry again wahoooo
All the Sciences are a big jump from GCSE to AS, because as ppl have already said GCSE Science is so simplified a lot of it isn't really true. I got an A in GCSE Chemistry and just got an A at AS with a brilliant teacher and then dropped it. It's really tough and apparently gets an equally big jump between AS and A2 (for CCEA anyways)
I can't personally comment, it might just have been that all the chemists thought they were being martyrs carrying on with such a tough subject. Plus my school makes you do Mod 4 in Jan and they hadnt finished the module by that point so everyone had to resit. I reckon if they didn't have to do that they would have found it more manageable
There are few questions which you can get wrong if you know the material. Just learn the defintions (which I foolishly didn't because none were on past papers...but guess what the first question on the June paper was?...).