Look at the reaction below. H2SO4(aq) + Ca(OH)2(aq).
→ CaSO4(aq) + 2H2O(1)
Which substance is the base in the reaction?
2H20 (1)
H2SO4 (aq)
CaSO4 (aq)
Ca(OH)2 (aq)

Ca(OH)2 (aq)

Explanation:

The balanced neutralization reaction i.e. a reaction between an acid and a base, in this question is given as follows:

H2SO4(aq) + Ca(OH)2(aq) → CaSO4(aq) + 2H2O(l)

According to Arrhenius in his definition of acid and base, a base is a substance that dissociates into hydroxide ions (OH-) when in an aqueous solution. In other words, a base increases the concentration of hydroxide ions when dissolved.

In this reaction, Ca(OH)2 releases the OH- (hydroxide ion) that combines with the hydrogen ion (H+) released by the acid, H2SO4, to form water (H2O). Hence, Ca(OH)2 (aq) is the BASE.