In any solution, the solvent is always the substance present in the largest amount. If the solvent contains ionizable protons it is said to be protonic , and if it is protonic, it will engage in acid-base reactions.
Many solvents are protonic, including water, liquid ammonia, and glacial acetic acid.
A protonic solvent is often amphiprotic, like water, and thus engages in acid-base reactions. All acids which are stronger than the hydronium ion will react with the essentially limitless supply of water to quantitatively produce hydronium ion, and so their strength will be leveled to that of the hydronium ion. This levelling effect is the reason why, in aqueous solution, the strongest acid which can exist is the hydronium ion. All others will be levelled to the hydronium ion by reaction. In aqueous solution hydrochloric acid, sulfuric acid, perchloric acid, and nitric acid are all equally strong.