Concentrated nitric acid does not attack or dissolve which of the following metals under normal conditions?

Introduction / Context:
This question relates to the reactivity of metals with acids, a core topic in inorganic chemistry. Concentrated nitric acid is a strong oxidising agent and can react with many metals, often passivating them by forming protective oxide layers. However, some noble metals are resistant to attack by nitric acid alone and require special mixtures such as aqua regia for dissolution. Identifying which metal among the options is not attacked by nitric acid illustrates the concept of noble metals and chemical stability.

Given Data / Assumptions:

• The acid considered is concentrated nitric acid, HNO3.
• Metals listed include gold, copper, zinc, iron and aluminium.
• We are considering reaction under typical laboratory conditions, not in special mixtures.
• Nitric acid can act both as an acid and as an oxidising agent.

Concept / Approach:
Gold is a noble metal with very low reactivity. It does not dissolve in most single acids, including concentrated nitric acid. Instead, gold requires aqua regia, a mixture of concentrated nitric acid and concentrated hydrochloric acid, to dissolve. In contrast, base metals such as copper, zinc and iron do react with nitric acid, often producing metal nitrates and nitrogen oxides. Aluminium can be passivated by nitric acid but is still not considered completely inert the way gold is. The key point is that nitric acid alone does not dissolve gold, so gold is the correct answer.

Step-by-Step Solution:
Step 1: Recall that gold is a noble metal, highly resistant to corrosion and chemical attack. Step 2: Nitric acid is a strong oxidising acid but is unable to dissolve gold by itself under normal conditions. Step 3: Aqua regia, a mixture of concentrated nitric acid and concentrated hydrochloric acid, is required to dissolve gold, highlighting its resistance to single acids. Step 4: Copper reacts with concentrated nitric acid to form copper nitrate, nitrogen oxides and water, so it is attacked by nitric acid. Step 5: Zinc and iron also react with nitric acid, though sometimes they may become passivated due to protective oxide or nitrate layers. Step 6: Therefore, among the listed metals, gold is the one that nitric acid does not react with under ordinary conditions.

Verification / Alternative check:
This fact is emphasised in many general chemistry textbooks: gold and platinum are classic examples of noble metals that resist attack by single mineral acids. The historical name aqua regia, meaning royal water, comes from its ability to dissolve royal metals like gold and platinum, unlike nitric or hydrochloric acid alone. Laboratory demonstrations often show copper reacting vigorously with nitric acid, producing brown fumes of nitrogen dioxide, while a piece of gold remains unchanged in the same acid. These observations confirm that gold is the metal not attacked by nitric acid alone.

Why Other Options Are Wrong:
Copper is well known to react with concentrated nitric acid, forming a blue solution of copper nitrate and releasing nitrogen oxides. Zinc, being a more reactive metal, also dissolves in nitric acid, though concentrated acid can sometimes produce passivation effects. Iron reacts with nitric acid but may likewise become passivated, especially with concentrated acid, yet it is still chemically attacked. Aluminium can form a protective oxide layer, but nitric acid does interact with its surface; it is not as inert as gold. Thus, none of these metals exhibits the complete resistance to nitric acid that gold does.

Common Pitfalls:
Students sometimes confuse passivation, where a protective layer slows or stops further reaction, with complete inertness. Metals like aluminium and iron can appear to resist further attack after an initial reaction, but that does not make them equivalent to gold. Another mistake is to assume that strong acids must react with all metals, ignoring the special stability of noble metals. To avoid confusion, remember that gold and platinum are classic examples of metals that do not dissolve in nitric acid alone and require aqua regia for dissolution.

Final Answer:
Concentrated nitric acid does not attack or dissolve gold under normal conditions.