By the term impurities, we mean substances out of place. Pure water is oxide of hydrogen, HaO. If water has salt dissolved in it, for instance, the salt is an impurity for the water, though we do not think of salt as being an impure substance in itself. The mineral impurities in drinking water are seldom a source of danger, although if the amount is large, such water may not "agree" with persons not used to it. Mineral impurities will usually make the water hard, and therefore troublesome for laundry work and to some extent in cooking.