In the reaction shown below, acts as a base because . HCO3- + H2O à H3O+ + CO32-

In the reaction , acts as a base because it has accepted a hydrogen ion or proton.

Explanation:

According to Arrhenius, species which dissociate to give hydrogen ions when dissolved in a solvent like water are called acid.

A species which readily accepts a hydrogen ion or proton is called a base.

For example,

Here, is donating hydrogen ion. So, it is an acid whereas is accepting the hydrogen ion. Hence, is a base.

Thus, we can conclude that in the reaction , acts as a base because it has accepted a hydrogen ion or proton.