Which of the following substances can act as both an acid and a base?

Water is classified as an amphoteric substance, meaning it can act as both an acid and a base depending on the situation. As an acid, water can donate a proton (H+) to a base, forming hydroxide ions (OH−). For instance, when water reacts with ammonia (NH3), it donates a proton to ammonia, resulting in ammonia ions and hydroxide ions in solution.

Conversely, water can also function as a base by accepting a proton from an acid, leading to the formation of hydronium ions (H3O+). For example, when water interacts with hydrochloric acid (HCl), it accepts a proton from HCl, thus acting as a base.

The other substances listed do not possess both acidic and basic properties. Hydrochloric acid and sodium hydroxide are strong acid and strong base respectively, meaning they can only act in one capacity in aqueous solutions. Ammonium nitrate, on the other hand, is a salt derived from a weak base (ammonia) and a strong acid (nitric acid), and does not demonstrate amphoteric behavior. Therefore, water is the substance that demonstrates the ability to act as both an acid and a base.