Remember this advice isn't official. There is no guarantee it will reflect your experience because university applications can change between years. Check the official Cambridge and Oxford websites for more accurate information on this year's application format and the required tests.
Also, someone else's experience may not reflect your own. Most interviews are more like conversations than tests and like, any conversation, they are quite interactive.
Number of interviews: 3
Skype interview: No
Spread: 1 interview the first day, and 2 the following day.
Length of interviews: 20-30 minutes each
I would definitely suggest that if something unknown to you comes up and you really have no idea of the answer, instead of trying to make something up, just say that you don't know. It is ok not to know the answers, because chemistry is very much a content-based subject, and you won't have learnt it all yet if you are applying during A-Level study (or equivalent). It is also ok to get things wrong. They will nudge you, to see if you can follow the bits of clues to get towards the answers.
Essentially, they want to see if they can teach you in a tutorial-style setting, and tutorials are a learning process, not always a chance to simply show off your knowledge.
I actually didn't really prepare at all for the interviews themselves; many other applicants that I spoke to during the interview process were so concerned with revising content for the interviews that they seemed overly stressed. After all, it doesn't matter if you don't know the content of a question- they will more than happily move on to find something you are familiar with.
I think the best thing to do is keep yourself calm and relaxed during the process, and don't let other applicants get into your head or let them stress you out. The interviewers are looking for potential, not knowledge right now, though obviously you will have to know SOME chemistry!
If you do get in, you will spend a lot of time being taught by, interacting with, and even socialising with the