An amphiprotic substance can serve either as an acid or a base, but it serves only one of these functions in any one equilibrium.
Acids react only with bases and bases react only with acids.
Acid-base reactions are always proton transfers between conjugate pairs. The proton transfer reactions of aqueous acids and bases are generally rapid and take place stoichiometrically whenever possible. In all cases, the reaction proceeds from strength to weakness, in that the resultant acid or base is always weaker than the species from which it originated. Thus, strong acids react with strong bases to give a neutral solution because the hydrogen ions and hydroxide ions neutralize each other to give water. A strong acid reacts with a weak base to yield the weak acid which is the conjugate acid of the weak base.