well, many science courses require an UAI of >90, so the hard work is necessary.
i don't want to offend you, but it does sound like you're struggling a bit in chem. i know what it's like because i had the exact same dilemma with english extention 1. i knew that if i put mountainous amounts of effort into it, i could probably do well, but it really wasn't worth it for me. i'm doing 10 units now, after dropping it, and i'm really glad i didn't keep it. like you said, it was just there bugging me.
of course you can get into a science course without chem. they do not require you to study certains subjects in high school, but it's stated that those subjects are assumed knowledge, which means it would help you a great deal to have studied them, so that you won't have to do bridging courses, etc.
i'm not 100% sure on the definition of a bridging course, but it's like making up for a subject you didn't do in high school, that you need for your uni course.