Answer the question on the basis of passage given below:

Nitrogen forms a large number of oxoacids such as $H_2N_2O_2$, (hyponitrous acid), $HNO_2$, (nitrous acid) and $HNO_3$, (nitric acid). Amongst these, $HNO_3$, is the most important. It is used in the manufacture of ammonium nitrate for fertilisers and other nitrates for use in explosives. It is also used to prepare many important organo-nitro compounds like nitroglycerine, trinitrotoluene etc. It acts as a strong oxidizing agent and used as an oxidiser in rocket fuels.

Nitric acid is commonly known to act as a strong oxidising agent. Which of the following metals will be easily oxidised by both dilute as well as cone. HNO₃?

Zn

The correct answer is Option (1) → Zn.

When considering the oxidation of metals by nitric acid (HNO₃), it’s important to understand how different metals interact with dilute and concentrated forms of the acid. Here’s a detailed breakdown of each option:

1. Zinc (Zn)

Reaction with Dilute HNO₃: When zinc reacts with dilute nitric acid, the reaction produces zinc nitrate and hydrogen gas:

\(\text{Zn} + 2\text{HNO}_3 \longrightarrow \text{Zn(NO}_3\text{)}_2 + \text{H}_2\)

Reaction with Concentrated HNO₃: With concentrated nitric acid, zinc is oxidized to zinc nitrate, and nitrogen oxides (like NO₂) are released:

\(\text{Zn} + 4\text{HNO}_3 \longrightarrow \text{Zn(NO}_3\text{)}_2 + 2\text{NO}_2 + 2\text{H}_2\text{O}\)

Conclusion: Zinc is easily oxidized by both dilute and concentrated HNO₃.

2. Gold (Au)

Reactivity: Gold is a noble metal and is very resistant to oxidation. It does not react significantly with either dilute or concentrated HNO₃:

\(\text{Au} + \text{HNO}_3 \longrightarrow \text{No Reaction}\)

Conclusion: Gold is not oxidized by HNO₃.

3. Aluminum (Al)

Reaction with Concentrated HNO₃: Aluminum can react with concentrated nitric acid, producing aluminum nitrate and nitrogen oxides:

\(2\text{Al} + 6\text{HNO}_3 \rightarrow 2\text{Al(NO}_3\text{)}_3 + 3\text{NO}_2 + 3\text{H}_2\text{O}\)

Behavior with Dilute HNO₃: Aluminum often forms a protective oxide layer in dilute nitric acid, making it less reactive:

\(\text{Al} + 3\text{HNO}_3 \rightarrow \text{No Reaction} \, \text{(due to passivation)}\)

Conclusion: Aluminum can be oxidized by concentrated HNO₃ but not effectively by dilute HNO₃.

4. Platinum (Pt)

Platinum is also a noble metal, showing very low reactivity towards both forms of nitric acid. It remains unreacted:

\(\text{Pt} + \text{HNO}_3 \rightarrow \text{No Reaction}\)

Conclusion: Platinum is not oxidized by HNO₃.

Summary

Zinc is the only metal in the list that reacts readily with both dilute and concentrated nitric acid, making it easily oxidized. The other metals (gold, aluminum, platinum) do not have this property. Therefore, zinc is the correct answer to the question.