An acid is a substance that creates hydrogen ions when dissolved in water. The strength of any acid is determined by how readily it releases these hydrogen ions into the solution. Nitric acid is classified as a strong acid, meaning it is highly effective at releasing these ions and is considered one of the seven common strong mineral acids. This high degree of reactivity makes it a widely used chemical in industrial processes worldwide.
What Determines Acid Strength
The fundamental difference between a strong acid and a weak acid lies in the extent to which it ionizes, or dissociates, when mixed with water. Strong acids dissociate almost completely in a water solution, meaning nearly every molecule breaks down to release its hydrogen ion.
This near-total breakup results in a high concentration of free hydrogen ions in the water. Conversely, a weak acid only partially dissociates, with most of the acid molecules remaining intact in the solution. This partial dissociation means that weak acids release far fewer hydrogen ions, making the resulting solution less acidic.
Acid strength is formally quantified by the acid dissociation constant, or Ka value. Strong acids have a very large Ka because the equilibrium strongly favors the dissociated ions over the intact acid molecule.
The Behavior of Nitric Acid in Water
Nitric acid (HNO3) exhibits the complete dissociation characteristic of a strong acid when dissolved in water. When the acid is introduced into an aqueous solution, the bond holding the hydrogen atom breaks almost entirely, releasing a positively charged ion.
This free hydrogen ion immediately combines with a water molecule to form a hydronium ion, which is the species responsible for the acidic properties of the solution. The remaining part of the original nitric acid molecule forms a stable nitrate ion (NO3-). Because this dissociation is virtually 100% complete, nitric acid generates the maximum possible concentration of hydronium ions for its given concentration.
The near-perfect transfer of the hydrogen ion to water is why nitric acid is grouped with other strong acids like hydrochloric acid and sulfuric acid. This high degree of ionization results in a solution that is highly corrosive and reactive due to the abundance of free hydronium ions.
Common Uses of Nitric Acid
The potent, highly reactive nature of nitric acid, stemming from its strength, makes it invaluable across various industrial sectors. Its single largest application is in the production of nitrogen-based fertilizers, accounting for approximately 75 to 80% of its global use. Nitric acid is neutralized with ammonia to create ammonium nitrate, a compound widely used to enrich agricultural soils.
The ability of nitric acid to perform nitration—a chemical reaction that adds a nitro group to an organic molecule—makes it a foundation for producing explosives. It is used in the synthesis of powerful compounds such as nitroglycerin and trinitrotoluene (TNT).
Beyond fertilizers and explosives, nitric acid is also employed in metal processing. Its oxidizing properties are used to clean, etch, and purify precious metals like gold and platinum, often in combination with hydrochloric acid.