Is distilled water a solution?

Rachel Kim

	Vocabulary
	autoprotolysis distillation hydronium ion hydroxide ion pure substance solution

Yes, Rachel, this is one of the many dirty little secrets of chemistry: There isn't any such thing as pure liquid water. Distilled water is a very dilute solution of hydroxide and hydronium ions, because (like many other solvents) water reacts with itself:

2 H₂O() H₃O⁺(aq) + OH^-(aq)

At room temperature, about 1 in every 10 000 000 water molecules has dissociated by donating a hydrogen atom to another water. Distilled water under these conditions is actually 10^-7M H₃O⁺ and 10^-7M OH^-.

Hydroxide ion is the caustic component of lye, and hydronium ion makes acid solutions corrosive. However, the concentrations in water are extremely low and not at all dangerous. (For an opposing opinion about the toxicity of water, click here.)

Distilled water also reacts with atmospheric carbon dioxide according to

H₂O() + CO₂(g) H₂CO₃(aq)
H₂CO₃(aq) + H₂O() H₃O⁺(aq) + HCO₃^-(aq)
HCO₃^-(aq) + H₂O() H₃O⁺(aq) + CO₃^2-(aq)

which brings the pH of distilled water in an open-air container down to about 5.8.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/matter/faq/is-distilled-water-a-solution.shtml