Final answer:
In aqueous solutions, acids and bases according to the Brønsted-Lowry theory are substances that respectively donate and accept protons.
Step-by-step explanation:
In aqueous solutions, an acid is defined as a substance that donates protons (H+ ions), and a base is a substance that accepts protons. This concept is encapsulated in the Brønsted-Lowry theory, which was proposed in 1923 and provides a more general description of acids and bases beyond the limitations of the Arrhenius definition, which is restricted to aqueous environments and the generation of hydronium (H3O+) and hydroxide (OH−) ions.
For example, when ammonia (NH3) is dissolved in water, a water molecule, acting as a Brønsted-Lowry acid, donates a proton to the ammonia, which acts as a Brønsted-Lowry base. This results in the formation of the conjugate base of water (OH−) and the conjugate acid of ammonia (NH4+). An acid-base reaction in this context is always synonymous with proton transfer from the acid to the base.