In a science project or experiment, a fair test has only one variable. All other aspects of the experiment are kept the same for each test; these are called "controls." Distilled water is used in science projects because it is purified and and can be considered a standard control.
Tap and bottled water contain many different minerals, such as iron and calcium, which vary from one source to another. These minerals may alter the results of a specific test by contaminating or reacting with other materials in the experiment. Therefore, tap and bottled water are not suitable controls for science projects.
Distilled water is purified of minerals by being heated until it evaporates and then recondensed. The minerals in the water do not evaporate with it but remain in the original container, while the vapor is collected to condense in a separate container. Distilled water has a noticeably bland taste because of this purification process and the subsequent lack of minerals. Since distilled water contains only water molecules, its properties are consistent, and it can be assumed that any changes observed in an experiment will be a result of the variable and not the water.