Why is water a good negative control for chemical tests?

A control is used, in any experimentation, to check whether the obtained result is due to the test variable (a condition that is varied) or not. Both the positive and the negative controls are used for this purpose. A positive control allows us to assess the validity of the test. In other words, a positive control is an experiment where we test against something that we know works. In comparison, a negative control is an experiment that gives negative response or no response.
Any substance can be used as a negative control if we know that it will not interfere with the test or will not participate in it.
Water is commonly used as a negative control in chemical tests, especially distilled water. The distilled water is devoid of any minerals or salts, unlike regular water (or tap water) and hence is not likely to participate in any chemical reaction. This property makes it an ideal choice as a negative control in a number of chemical tests.
Another way to think about it is to consider that distilled water has only water molecules and has consistent properties. Due to this reason, it is safe to assume that any experimental observation is due to the change in the variable under study and not the addition of water.
Hope this helps.