What does a substance need to act as a Bronsted-Lowry acid?

A substance needs to donate a proton to act as a Bronsted-Lowry acid. This definition is fundamental to the Bronsted-Lowry theory of acids and bases, which identifies acids specifically as proton donors and bases as proton acceptors. When a substance donates a proton (H⁺ ion), it effectively increases the concentration of protons in the solution, which leads to acidic properties.

For example, when hydrochloric acid (HCl) dissociates in water, it donates a proton to a water molecule, forming hydronium ions (H₃O⁺). This behavior exemplifies the role of Bronsted-Lowry acids in chemical reactions.

The other options, while related to certain chemical behaviors, do not define a Bronsted-Lowry acid. Accepting electrons aligns more with Lewis acid theory, producing hydroxide ions is characteristic of Arrhenius bases, and being ionic does not inherently determine whether a substance can act as a Bronsted-Lowry acid. Thus, the requirement of donating a proton is essential for identifying a substance as a Bronsted-Lowry acid.