This is a GREAT introduction to variables in chemistry. I designed this PowerPoint for 5th-8th graders, but it could easily be used to refresh high school students' minds as well.
There are 16 slides in this PowerPoint, which explains four simple variables and how they can change the rate of a chemical reaction:
(time and distance/length are mentioned also, but not explained)
Each variable is explained with words and with an embedded video, so you don't just explain the concept, the students get to SEE it!