While you may have been told you need zero science knowledge to answer any ACT Science questions, this is NOT true. There are typically around four questions per test that you cannot answer without previous background knowledge. That's the difference between a 31 and a 36 on the Science section!
In this article, I will give an overview of the 18 topics that will NOT be explained in the ACT Science passages that the ACT expects you to know. The ACT assumes you know these topics from school and science class. To gather this info, I dissected dozens of previous ACT Science test sections to find the concepts you have to know. As far as we can tell, we're the only ones who have taken the time to research this.
If you've gotten bad grades in science classes in school, this article will give you a refresher on the only concepts you need to know.