Final answer:
A conjugate acid is the species that forms when a Brønsted-Lowry base gains a proton, while a conjugate base is what remains after a Brønsted-Lowry acid loses a proton.
Step-by-step explanation:
According to the Brønsted-Lowry theory, a conjugate acid is the species formed when a Brønsted-Lowry base gains a proton, while a conjugate base is the species that remains after a Brønsted-Lowry acid has lost a proton. In the reaction where hydrochloric acid (HCl) dissociates in water, HCl is the Brønsted-Lowry acid because it donates a hydrogen ion (proton) to water, which is acting as the Brønsted-Lowry base. After donating the proton, chloride ion (Cl-) becomes the conjugate base of HCl. Conversely, the water molecule that accepts the proton becomes hydronium ion (H3O+), which is the conjugate acid of water. Therefore, an acid-base reaction occurs when there is a transfer of a proton from a Brønsted-Lowry acid to a Brønsted-Lowry base resulting in the formation of a new conjugate base and a new conjugate acid respectively.