Which substance is an example of a weak base?

The chosen answer, ammonia (NH₃), is indeed an example of a weak base. Ammonia is known for its ability to partially ionize in water, producing hydroxide ions (OH⁻) through the following equilibrium reaction:

[ \text{NH₃ (aq)} + \text{H₂O (l)} \rightleftharpoons \text{NH₄}^+ (aq) + \text{OH}^- (aq) ]

This partial ionization characterizes it as a weak base because it does not completely dissociate in solution, resulting in a relatively low concentration of hydroxide ions compared to strong bases. Strong bases, such as sodium hydroxide (NaOH), lithium hydroxide (LiOH), and strontium hydroxide (Sr(OH)₂), fully dissociate in water to release hydroxide ions, leading to a higher pH.

Therefore, NH₃ stands out as a weak base due to its incomplete dissociation and the establishment of an equilibrium, marking it as less basic compared to the strong bases listed.