Introduction: Nitric acid is a strong oxidizing agent that has the ability to oxidize various substances. However, it reacts only with magnesium and manganese to produce hydrogen gas. This phenomenon can be explained by considering the reactivity of these metals and the unique properties of nitric acid.

Reactivity of Magnesium and Manganese:
- Magnesium is a highly reactive metal that readily reacts with many substances, including acids. It has a strong tendency to lose electrons and form positive ions.
- Manganese, although less reactive than magnesium, can also undergo oxidation reactions with certain oxidizing agents.

Properties of Nitric Acid:
- Nitric acid (HNO3) is a strong oxidizing agent due to the presence of the nitrate ion (NO3-) in its structure. The nitrate ion has a high affinity for electrons and can accept them from other substances, causing oxidation.
- In the presence of reducing agents, such as magnesium and manganese, nitric acid can act as an oxidizing agent by accepting electrons from these metals.

Explanation of the Reaction:
1. Magnesium and manganese have a strong tendency to lose electrons, making them good reducing agents.
2. When magnesium or manganese reacts with nitric acid, the metals undergo oxidation, losing electrons to the nitrate ions in the acid.
3. The oxidation of magnesium or manganese results in the formation of magnesium or manganese ions and hydrogen gas.
4. The nitrate ions in nitric acid act as the oxidizing agent, accepting the electrons released by the metals.
5. The hydrogen gas is produced as a byproduct of the reaction.

Why Only Magnesium and Manganese?
- The reactivity of different metals varies based on their position in the reactivity series. Magnesium and manganese are both relatively high in the reactivity series, making them more prone to oxidation reactions.
- Nitric acid is a strong oxidizing agent, capable of oxidizing only those metals that are more reactive than hydrogen.
- Magnesium and manganese, being more reactive than hydrogen, can readily lose electrons to nitric acid and undergo oxidation to produce hydrogen gas.
- Other metals that are less reactive than hydrogen, such as copper or silver, do not react with nitric acid because they are not easily oxidized by it.

Conclusion:
Nitric acid reacts with magnesium and manganese to produce hydrogen gas because these metals are more reactive than hydrogen and can be oxidized by the strong oxidizing agent. The reaction involves the transfer of electrons from the metals to the nitrate ions in nitric acid, resulting in the formation of metal ions and hydrogen gas. Other metals that are less reactive than hydrogen do not undergo this reaction with nitric acid.