In chemistry, a strong acid is an acid which ionizes (splits) completely in a solution of water. It always loses a proton (A H₊) when put in water. A weak acid does not always lose a proton, though; It can lose its proton, but not all the time. They also have a very low pH, usually between 1 and 3. Many strong acids have a negative pKa value, which means they are very strong.

The six common strong acids are: (Acronym:CBSPIN)

1. HydroChloric acid HCl
2. HydroBromic acid HBr
3. Sulfuric acid H₂SO₄
4. Perchloric acid HClO₄
5. HydroIodic acid HI
6. Nitric acid HNO₃

Almost strong acids include:

1. Chloric acid HClO₃