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(ell) doublearrow 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(ell) + CO2(g) doublearrow H2CO3(aq)
H2CO3(aq) + H2O(ell) doublearrow H3O+(aq) + HCO3-(aq)
HCO3-(aq) + H2O(ell) doublearrow H3O+(aq) + CO32-(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


General Chemistry Online! Is distilled water a solution?

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