Perchloric acid is listed as one of the six strong acids. It is considered a strong acid due to the fact that it completely breaks down into a positive hydrogen ion (a proton) and a negative ion when dissolved in water.
The other strong acids are hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid and sulphuric acid. All other acids are weak acids, including acetic acid, carbonic acid and hydrofluoric acid. These do not dissociate fully when dissolved in water. Thus, a weak acid solution in water contains a mixture of positive hydrogen ions, negative ions and unbroken acid molecules.