Question

Is `H_2O` a base or acid?

Answer 1

Well... either-or. It's amphoteric.

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Water can act as either a base or acid in the Bronsted sense. That is, it can accept proton (base) or donate a proton (acid), depending on the other species in the solution.

For example, ammonia is a weak Bronsted base, so when water dissolves ammonia, ammonia acts as a base, and water acts as a Bronsted acid:

`overbrace("H"_2"O"(l))^("Bronsted "color(red)bb"acid") + overbrace("NH"_3(aq))^"Bronsted base" rightleftharpoons overbrace("NH"_4^(+)(aq))^"Conj. acid" + overbrace("OH"^(-)(aq))^("Conj. "color(red)bb"base")`

(Note, however, that this equilibrium lies heavily to the left, since `"NH"_4^(+)` is a stronger acid than `"H"_2"O"`, with a `K_a` about 4 orders of magnitude larger than `K_w`.

The equilibrium lies on the side with the weaker acid.)

But, if water dissolves, for example, acetic acid (which is obviously a Bronsted acid), acetic acid acts as, well, the acid, and water behaves as a Bronsted base...

`overbrace("H"_2"O"(l))^("Bronsted "color(red)bb"base") + overbrace("CH"_3"COOH"(aq))^"Bronsted acid" rightleftharpoons overbrace("CH"_3"COO"^(-)(aq))^"Conj. base" + overbrace("H"_3"O"^(+)(aq))^("Conj. "color(red)bb"acid")`