Whats the difference between a controlled variable and a constant variable?!?!?

I'm working on my science fair project and need answers FAST!!!!!!!!!!!!!Whats the difference between a controlled variable and a constant variable?!?!?
A controlled variable is something that is part of the control group. A constant variable is something that can be altered, but is kept constant during the experimentation.





Let's say I have a group of chemicals that I want to test for reaction with, say, HCl. The controlled variable would be one that is not tested at all during the experiment - this would show that your experiment does have some effect (and not that the chemicals you're testing would have reacted whether you tested them or not.) The constant variable would be how much HCl you use to test with, and also how much of each chemical is being tested. You might discover that chemical A reacts at a level of 2 when tested with 5 ml of HCl and at a level of 4 when tested with 10 ml, but that chemical B reacts at a level of 1 when tested with 5 ml of HCl and at a level of 5 when tested with 10 ml. I'm just pulling those numbers out of thin air, but you get my drift.