According to the Brønsted-Lowry definition of an acid, what happens to an acid when dissolved in water?

According to Bronsted-Lowry, when dissolved in water, an acid can be defined as a proton donor while a base can be defined as a proton acceptor.

Explanation:

The proton is usually in the form of hydrogen ions (H+). Consider:

I. HI (aq) + H2O (l) → H30 + (aq) + I - (aq)

II. H2O (l) + NH3 (aq) → NH4 + (aq) + OH - (aq)

In (I), HI donates the H + to H2O hence HI is a Bronsted-Lowry acid while H2O is a Bronsted-Lowry base. The I - (aq) is called the conjugate base of the acid HI while the H30 + is called the conjugate acid of the base H2O.

In (II), H2O donates the proton to NH3 so H2O is the Bronsted-Lowry acid and NH3 is the Bronsted-Lowry base. The conjugate base is OH - while the conjugate acid is NH4+.

This means that using the Bronsted-Lowry definition, an acid differs from its conjugate base pair only by a proton.

Notice that H2O can function both as a Bronsted-Lowry acid and Bronsted-Lowry base depending on the type of solution as seen above.