Question

Why does ammonia act like a base?

Answer 1

Ammonia acts like a base because of these two reasons:
it combines with H+ ions
it increases OH- concentration

Answer 2

Answer: It combines with H+ions.

Explanation: It combines with H+ions.

Ammonia releases $OH^-$ ions.

According to Arrhenius concept, a base is defined as a substance which donates hydroxide ions $(OH^-)$ when dissolved in water and an acid is defined as a substance which donates hydronium ions $(H_3O^+)$ in water.

According to the Bronsted Lowry conjugate acid-base theory, an acid is defined as a substance which donates protons and a base is defined as a substance which accepts protons.

According to the Lewis concept, an acid is defined as a substance that accepts electron pairs and base is defined as a substance which donates electron pairs.

$NH_3+H^+\rightarrow NH_4^+$

$NH_4OH(aq)\rightarrow NH_4^++OH^-$

Ammonia being a lewis base donates lone pair of electrons and thus combines with $H^+$. Also aqueous solution of ammonia gives $OH^-$ ions in water, thus act as Arrhenius base.