On an old Mythbusters episode, the silly hosts tried to investigate whether water can explode after being microwaved. They declared “myth true” after an experiment.

Normal tap water of course boils with vigour, but does not explode. It turns out that superheated water can indeed explosively evaporate, but that was said to require pure distilled water. One of the hosts then made an erroneous leap of logic, by claiming that such water is in fact widely available in households that purify their water by boiling.

But that is not so. Boiling water does not get rid of any impurities, except perhaps some dissolved liquids and gases. It simply kills most bugs that are swimming around in it. When the boiled water is cooled down, it will have about the same amount of particulate matter in it as it did before. It will boil just as vigorously as before.

What makes distilled water so pure is, well, distillation. Instead of keeping the unboiled water+gunk solution, one syphons off the freshly boiled water vapour, and collects its condensate. If the boiling temperature and syphoning schedule are carefully controlled, one can grab fairly pure water vapour, which turns into fairly pure water. Not many households will have this kind of equipment. Thus the mythbusters reached the wrong conclusion.

Distillation is of course also used in oil refining, to separate the many different compounds intermixed in crude oil, many of which have different boiling points.