It's not so much about the textbook you choose, but I think it's more on your understanding of the syllabus.
Download a copy of the Stage 6 Chemistry syllabus from the Board of Studies website, and really thoroughly know each of the dot points and do not go beyond them.
Another key thing about Chemistry is that people, including myself sometimes, really fail to answer the question. For example, the simple question of "Define what an element is" for one mark, here are two types of responses, one incorrect and one correct:
1. An element is the purest form of matter. An example is magnesium.
2. An element is the simplest form of a substance.
1 is incorrect, and 2 is correct. 1 is incorrect because "pure" usually means a substance that is uniform in composition, and so it is not the best word to describe an element. It also provides an un-necessary example, when the question only asks for 'define'. IF it were two marks, then you'd provide an example. Response 2 is naturally correct because it does exactly what the question asks.
If you get used to writing exactly what the examiner WANTS to hear, you'll do well, in my opinion, in Chemistry.
