Medical College Admission Test (MCAT) General Chemistry Practice Exam 2026 - Free MCAT Chemistry Practice Questions and Study Guide

A basic hydride is defined as a hydride that can act as a base by accepting protons or donating electrons during a chemical reaction. In this case, calcium oxide (CaO) is the correct choice, as it is considered a basic hydride.

CaO is formed from the combination of calcium and oxygen and has the ability to react with acids to produce salts and water. In the presence of water, CaO can form calcium hydroxide (Ca(OH)₂), which is a strong base. The basic nature of CaO is due to the tendency of calcium to donate electrons, facilitating reactions with protons from acids.

On the other hand, hydrochloric acid (HCl) is an acid and does not act as a basic hydride. Ammonia (NH₃), while it can accept protons and is a Lewis base, is better classified as a molecular compound rather than a hydride in this context. Water (H₂O) is amphoteric; it can act as both an acid and a base, but it is also not considered primarily a basic hydride. Therefore, among the given options, CaO is the only hydride that fits the definition of being basic.