Yes, dilute nitric acid is indeed considered a strong acid.
A strong acid is defined by its ability to completely dissociate (ionize) in water, meaning that every molecule of the acid breaks apart into its constituent ions. For nitric acid (HNO$_3$), this means it fully separates into hydrogen ions (H$^+$ or H$_3$O$^+$) and nitrate ions (NO$_3^-$) when dissolved in water, even in dilute solutions.
Understanding Strong Acids
Strong acids are electrolytes that ionize almost entirely in aqueous solutions. This high degree of ionization releases a large concentration of hydrogen ions, which is characteristic of strong acidity.
Key characteristics of strong acids include:
- Complete Dissociation: They break apart 100% into ions in water.
- High Conductivity: Due to the abundance of ions, their solutions conduct electricity very well.
- Low pH: Solutions of strong acids typically have very low pH values (e.g., pH 1-2).
Common examples of strong acids include:
- Hydrochloric Acid (HCl)
- Sulfuric Acid (H$_2$SO$_4$)
- Perchloric Acid (HClO$_4$)
- Nitric Acid (HNO$_3$)
Nitric Acid's Strength in Dilution
Nitric acid maintains its characteristic strength even in dilute concentrations, generally defined as solutions less than 2 molar (<2M). Its complete dissociation in water makes it a potent source of H$^+$ ions, enabling it to react vigorously.
Reactions with Metals: A Key Indicator
The behavior of nitric acid with metals clearly demonstrates its strength. In diluted solutions, nitric acid acts as a strong acid in its reactions with various metals. However, the nature of the reaction products can vary due to nitric acid's additional property as a powerful oxidizing agent.
While strong acids typically react with reactive metals to produce hydrogen gas (H$_2$), nitric acid's oxidizing nature often leads to the formation of nitrogen oxides (e.g., NO, NO$_2$) instead of hydrogen.
| Metal | Reaction with Dilute HNO$_3$ | Gas Liberated | Notes |
|---|---|---|---|
| Magnesium | Mg(s) + 2HNO$_3$(aq) → Mg(NO$_3$)$_2$(aq) + H$_2$(g) | Hydrogen (H$_2$) | Uniquely, only in the reaction of dilute nitric acid with magnesium is hydrogen gas itself liberated. This is because magnesium is highly reactive, and under dilute conditions, the acid acts predominantly as a strong acid rather than an oxidizing agent to produce hydrogen. |
| Copper | 3Cu(s) + 8HNO$_3$(dilute) → 3Cu(NO$_3$)$_2$(aq) + 2NO(g) + 4H$_2$O(l) | Nitrogen Monoxide (NO) | With less reactive metals like copper, the strong oxidizing property of nitric acid dominates, leading to the reduction of nitrogen to nitrogen oxides rather than the release of hydrogen gas. |
| Zinc | 4Zn(s) + 10HNO$_3$(dilute) → 4Zn(NO$_3$)$_2$(aq) + N$_2$O(g) + 5H$_2$O(l) | Nitrous Oxide (N$_2$O) | Depending on the dilution and metal, different nitrogen oxides can be formed. The oxidizing power is still evident. |
| Iron | Fe(s) + 4HNO$_3$(dilute) → Fe(NO$_3$)$_3$(aq) + NO(g) + 2H$_2$O(l) | Nitrogen Monoxide (NO) | Similar to copper, iron reacts to produce nitrogen oxides. Concentrated nitric acid can even passivate iron (and aluminum), forming a protective oxide layer that prevents further reaction. |
This table illustrates that while dilute nitric acid is a strong acid, its additional role as an oxidizing agent influences the specific products of its reactions with most metals, making the liberation of hydrogen gas a rare exception, primarily seen with magnesium.
Why This Matters
The strong acidic nature of dilute nitric acid means it is highly corrosive and reacts readily with many substances. Its strong oxidizing properties also make it useful in various industrial applications, such as the production of fertilizers, explosives, and dyes, but also necessitate careful handling due to safety concerns.
For further reading on acid strength and nitric acid, you can refer to resources like Wikipedia's page on Nitric Acid or Khan Academy's explanation of Strong and Weak Acids.
