bcz it is strong oxidizing agent... then y it doesn't oxidize hydrogen produce during this reactions????

Introduction:
Nitric acid (HNO3) is a strong acid that is commonly used in various chemical reactions. It is known to react with certain metals, such as magnesium (Mg) and manganese (Mn), to produce hydrogen gas (H2). However, it does not react in the same way with all metals.

Explanation:
When nitric acid reacts with magnesium or manganese, it undergoes a redox reaction in which the metal is oxidized and the nitric acid is reduced. This reaction can be represented by the following equations:

1. Reaction with Magnesium:
2HNO3 + Mg → Mg(NO3)2 + H2

2. Reaction with Manganese:
2HNO3 + 3Mn → 3Mn(NO3)2 + 2H2

The key factor that allows nitric acid to react with magnesium and manganese is the presence of the oxidizing agent, nitrate ion (NO3-), in the acid. Nitric acid is a strong oxidizing agent itself, and it readily donates oxygen atoms to the metal during the reaction. This results in the metal being oxidized and the acid being reduced.

However, not all metals can undergo this reaction with nitric acid. This is because some metals have a higher affinity for oxygen than hydrogen. When these metals come into contact with nitric acid, they form a protective oxide layer on their surface. This oxide layer prevents further reaction with the acid, inhibiting the production of hydrogen gas.

The formation of the oxide layer is a result of the metal's ability to resist oxidation. Metals such as aluminum, chromium, and iron readily form this protective oxide layer when exposed to nitric acid. As a result, the acid is not able to oxidize these metals and produce hydrogen gas.

Summary:
In summary, nitric acid reacts with magnesium and manganese to produce hydrogen gas because these metals can be oxidized by the acid. The oxidizing agent, nitrate ion, present in nitric acid facilitates this reaction. However, other metals that have a higher affinity for oxygen than hydrogen, such as aluminum, chromium, and iron, form a protective oxide layer when exposed to nitric acid, preventing further reaction and the production of hydrogen gas.