Ibn Sina in his Canon said,
“Purification of water.
Bad water may be purified by sublimation and distillation. If that is not feasible, boiling will suffice, for boiled water, as the learned know, is less likely to cause inflammation and passes more rapidly through the body.
Ignorant persons believe that when water is boiled the attenuated part is dispersed, and that therefore it is made denser; hence they think it is better not to boil water. But as you know, the very nature of "water" means that its particles are alike in attenuation and density. It is pure, simple [in the scholastic sense], and will not thicken by boiling except in virtue of a cold quality being dominant in it, and of earthy particles being plentiful in it, which, although extremely minute, are not easily separated out or precipitated. They are not plentiful enough to break the continuity of the water and are too small to separate out by standing. Hence they are bound to remain admixed with the substance of the water.
Boiling removes the density which the quality of coldness produced; the particles of water are then forcibly rarefied and the substance of the water becomes more and more rare, until the heavy earthy particles hitherto suspended burst loose and fall down and sink to the bottom. A nearly quite pure water remains behind.”