The best chemical test for acids is the litmus test. Litmus is a vegetable dye that can change color in the presence of acids or bases. From the standard of chemists acids are considered any substance that releases the hydrogen proton in aqueous solution. Since protons are much too reactive to exist by themselves, they always attach themselves to something else.
Water can receive the additional protons given off by an acid.
This forms the hydronium ion. This causes the characteristic properties of an acid. No discussion of acids is complete without the discussing of bases. Bases are any compound that release the hydroxide ion, usually in an aqueous solution. Bases have a whole set of properties dependent on the hydroxide ion.