When iron is exposed to moist air, it gradually rusts. This is due to the formation of?

Analysis

The question asks for the chemical identity of the substance formed when iron undergoes the process of rusting in the presence of moisture and oxygen.

Concept Explanation

Rusting is a common electrochemical process known as corrosion. For iron to rust, it requires two essential components: oxygen and water (moisture).

1. Chemical Reaction: When iron ($Fe$) reacts with oxygen ($O_2$) in the presence of water ($H_2O$), it forms a reddish-brown flaky substance.
2. Oxidation State: During this process, iron is oxidized from its metallic state ($Fe^0$) to the Iron(III) state ($Fe^{3+}$).
3. Hydration: The resulting compound is not just a simple oxide; it incorporates water molecules into its crystal lattice. This is why it is referred to as a "hydrated" oxide.
4. Formula: The chemical formula for rust is $Fe_2O_3 \cdot xH_2O$. This is chemically named hydrated iron(III) oxide.

Step-by-Step Solution

• Step 1: Identify the oxidation state of iron in rust. Rust is an Iron(III) compound, which eliminates options C and D (which refer to Iron(II)).
• Step 2: Determine if the compound is anhydrous or hydrated. Since the process requires "moist air," water is chemically bonded to the oxide. This eliminates option B (anhydrous means without water).
• Step 3: Combine these facts. The substance is an Iron(III) oxide that contains water, making it hydrated iron(III) oxide.

Conclusion

The formation of rust is the chemical reaction of iron with oxygen and water to produce hydrated iron(III) oxide.

Exam Tip

Always remember that "anhydrous" means "without water" (think of a dry desert), while "hydrated" means "with water." Since rust only forms in the presence of moisture, it must be a hydrated compound!