An acid is strong if it completely dissociates (ionizes) in water, releasing all of its hydrogen ions (H+). A weak acid only partially dissociates, meaning some of the acid molecules remain intact in the solution.
How to Tell if an Acid is Strong or Weak
- Strong acids fully dissociate in water, meaning 100% of the acid molecules break apart into ions (H+ and the conjugate base). This makes the solution highly conductive and acidic.
- Weak acids partially dissociate, so an equilibrium exists between the dissociated ions and the undissociated acid molecules. The solution is less conductive and less acidic compared to a strong acid solution.
Common Strong Acids to Know
There are seven classical strong acids often memorized in chemistry:
- Hydrochloric acid (HCl)
- Hydrobromic acid (HBr)
- Hydroiodic acid (HI)
- Nitric acid (HNO3)
- Sulfuric acid (H2SO4)
- Perchloric acid (HClO4)
- Chloric acid (HClO3)
Any acid not on this list is generally considered weak.
Other Ways to Identify Acid Strength
- pKa Values: Lower pKa values indicate stronger acids. Strong acids have very low or negative pKa values, while weak acids have higher pKa values.
- Electrical Conductivity: Strong acids conduct electricity strongly due to the high concentration of ions, while weak acids conduct weakly.
- Bond Strength: Strong acids have weaker bonds between hydrogen and other atoms, which allows full dissociation. Weak acids have stronger bonds and dissociate less.
In summary, an acid can be identified as strong if it is on the classical strong acid list, fully dissociates in water, has a low pKa value, and exhibits strong conductivity. Otherwise, it is weak.
