Which compound serves as a proton donor?

The correct answer is based on the definition of acids and bases within the Brønsted-Lowry theory, which characterizes an acid as a proton donor. In this context, a compound that contains hydrogen atoms bonded to a non-metal can effectively release protons (H+ ions) when it reacts in solution, making it a proton donor.

When analyzing the options, a compound represented by H X (where X is a non-metal) can dissociate in solution, releasing a hydrogen ion. Typical examples include acids such as hydrochloric acid (HCl), acetic acid (CH3COOH), and others, which fit this profile. This capability of releasing protons defines the compound as an acid under the Brønsted-Lowry classification, thus serving as a proton donor.

Looking at the other choices: HCO3- (bicarbonate) typically acts as a base in most reactions or can also act as an acid in certain situations by donating a proton, but it is not primarily characterized as a proton donor. Ca(OH)2 is a strong base that contributes OH- ions to a solution rather than donating protons. NH3 (ammonia) generally functions as a weak base by accepting protons (