I do Chemistry and Maths, so I can tell you that it isn't going to be a 'walk in the park' if you are predicted less than As in any of your sciencey-subjects. Chemistry is similar to what you already know from GCSE, but the huge leap from GCSE Maths to A level Maths is big.
Maths - 7
Chemistry - 5
Maths, Chemistry and Physics are some of the hardest A levels out there, by the way.
Here is what I heard from my friends about your other choices.
Physics - absolutely hard, quite boring, only choose if you're good at math.
Geography - also hard, quite hard to keep interested, good school trip though.
xx