Understanding Iron Rusting and Chemical Reactions

Let's analyze the given Assertion (A) and Reason (R) regarding the rusting of iron.

Analyzing Assertion (A): Iron rusts when exposed to water.

This statement is a well-known fact. Iron is a reactive metal that undergoes oxidation, commonly known as rusting, when exposed to certain environmental conditions. One of the key conditions required for iron to rust is the presence of water (moisture). In dry air, iron rusts very slowly or not at all. In humid air or when in contact with water, rusting occurs much more readily.

Therefore, Assertion (A) is true.

Analyzing Reason (R): Iron reacts with the oxygen molecules present in water.

Rusting is essentially the oxidation of iron. This oxidation requires an oxidizing agent, which is typically oxygen. Oxygen is present in the air, and it also dissolves in water. The process of rusting involves iron (Fe) reacting with oxygen ($\text{O}_2$) in the presence of water ($\text{H}_2\text{O}$) to form hydrated iron(III) oxide ($\text{Fe}_2\text{O}_3 \cdot \text{nH}_2\text{O}$), which is rust.

The reaction is complex but can be summarized as:

Anode: $\text{Fe(s)} \rightarrow \text{Fe}^{2+}\text{(aq)} + 2\text{e}^-$

Cathode: $\text{O}_2\text{(aq)} + 2\text{H}_2\text{O(l)} + 4\text{e}^- \rightarrow 4\text{OH}^-\text{(aq)}$ (in neutral or basic water)

or $\text{O}_2\text{(g)} + 4\text{H}^+\text{(aq)} + 4\text{e}^- \rightarrow 2\text{H}_2\text{O(l)}$ (in acidic water)

Overall: $\text{2Fe(s)} + \text{O}_2\text{(aq or g)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{Fe(OH)}_2\text{(s)}$

Further oxidation: $\text{4Fe(OH)}_2\text{(s)} + \text{O}_2\text{(aq or g)} + 2\text{H}_2\text{O(l)} \rightarrow 4\text{Fe(OH)}_3\text{(s)}$

Dehydration of $\text{Fe(OH)}_3$ forms rust: $\text{Fe(OH)}_3 \rightarrow \text{Fe}_2\text{O}_3 \cdot \text{nH}_2\text{O}$

The statement in Reason (R) says iron reacts with oxygen molecules *present in water*. Oxygen from the air dissolves in water, and this dissolved oxygen ($\text{O}_2\text{(aq)}$) acts as the primary oxidizing agent in the electrochemical process of rusting. Therefore, the reaction of iron with oxygen molecules present in water is a crucial step in the rusting process.

Therefore, Reason (R) is also true.

Evaluating the Relationship between A and R

Assertion (A) states that iron rusts when exposed to water. Reason (R) states that iron reacts with oxygen molecules present in water. As explained above, the reaction of iron with oxygen (which is present in water, either dissolved from the air or directly from reactions involving water) is the fundamental chemical process that causes rusting. Water acts as a medium and electrolyte, and dissolved oxygen acts as the oxidizing agent. Thus, the reaction described in Reason (R) is the direct cause of the phenomenon described in Assertion (A).

Therefore, Reason (R) is the correct explanation for Assertion (A).

Conclusion

Based on the analysis:

• Assertion (A) is true.
• Reason (R) is true.
• Reason (R) correctly explains why Assertion (A) is true.

This corresponds to the option where both A and R are true, and R is the correct explanation of A.