What does a "control" mean in an experiment?

Question

I'm an AS level student and I was wondering what exactly is the control of a chemical experiment. There's an example of an experiment which involves extracting potato juice from a potato and adding it to glucose and adding iodine to discover that the potato juice has turned the glucose into starch. Apparently the control of the experiment is an enzyme. But why is it an enzyme?

Answer 1

Perhaps it is a positive control. In other words: you KNOW the enzyme will catalyze the reaction from glucose into starch. So then you can compare that to the potato juice to make sure the reaction has proceeded in the experimental trial.

Answer 2

The control experiment is one in which nothing is done. For example, take glucose, iodine, and "more water." Put them together for awhile. Nothing happens. The starch-iodine does not turn blue. There is something in potato juice. It's not necessarily an enzyme (although I most strongly believe that it is LOL!) What you are trying to do is to refute the null hypothesis. The null hypothesis is that it's all chance, and there was no effect. If glucose and water can somehow produce ablue color sometimes, then you/ve proved nothing.

Answer 3

Without knowing the intent of your "example experiment" one can not really answer why enzyme was the control. But for a good definition of a scientific control check wiki:

A scientific control augments integrity in experiments by isolating variables as dictated by the scientific method in order to make a conclusion about such variables. In a controlled experiment, two virtually identical experiments are conducted. In one of them, the treatment, the factor being tested is applied. In the other, the control, the factor being tested is not applied. Effects of the treatment are isolated by comparing outcomes in the two cases.

For example, in testing a drug, it is important to carefully verify that the supposed effects of the drug are produced only by the drug itself. Doctors achieve this with a double-blind study in a clinical trial: two (statistically) identical groups of patients are compared, one of which receives the drug and one of which receives a placebo. Neither the patients nor the doctor know which group receives the real drug, which serves both to curb researchers' bias and to isolate the effects of the drug.