When is a control variable not needed to make an experiment valid?

For example, if two enzymes' activities were being measured by seeing how much O2 gas was produced over time, would a control of no enzyme added to substrate be needed to ensure it was the enzymes having the effect and not a variable e.g. temperature?|||Reply to your reply:


I believe you would want to control all the variables, but that's not the same as having a control experiment.


For example, if you were testing 2 enzymes that break down Hydrogen Peroxide, you'd want to mix 1 batch of hydrogen peroxide in water, and distribute that between two testing chambers, then add equal amounts of the two enzymes and run the experiments simultaneously so that environmental factors (light, etc) did not play a role. Having a control of just hydrogen peroxide water with no enzyme is optional, but not required, if you're only showing a comparison between the two enzymes. If you were trying to prove that the two enzymes were, for example, THE SAME enzyme, you WOULD want a control with no enzymes, and perhaps an additional test with a third enzyme, to allow you to compare the 4 reaction rates.|||You'd need a control when you're trying to determine if something occurs or not. In this case, you know something will occur, you're just trying to find out the different rates of its occurrence.