The decay of metals by atmospheric oxygen and moisture is called corrosion. Few metals do not react with atmospheric gases and water and do not undergo corrosion, while others develop a dull layer on them or a coloured layer to spoil their shiny appearance or turn black or are converted into powder. few elements, on the other hand are protected due to presence of this dull layer of oxide on them.
Q. Iron reacts with air in presence of water and forms a brown powder which is called rust. chemically rust is

Options

A. iron oxide
B. hydrated iron oxide
C. iron sulphate
D. iron carbonate

Explanation

To answer the question about the chemical composition of rust formed when iron reacts with air and moisture, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding Rusting**: Rusting is a chemical process that occurs when iron reacts with oxygen in the presence of water. This process leads to the deterioration of iron, resulting in a brown powder commonly known as rust.

2. **Chemical Reaction**: The rusting of iron can be represented by the following chemical reaction:
\[ 4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3 \]
Here, iron (Fe) reacts with oxygen (O2) and water (H2O) to form iron(III) hydroxide.

3. **Formation of Rust**: The iron(III) hydroxide (Fe(OH)3) can further dehydrate to form rust, which is primarily composed of hydrated iron(III) oxide. The chemical formula for rust is often represented as:
\[ Fe_2O_3 \cdot nH_2O \]
where \( n \) represents the number of water molecules associated with the iron oxide.

4. **Conclusion**: Therefore, chemically, rust is a hydrated form of iron oxide, commonly referred to as hydrated iron(III) oxide.

### Final Answer:
Rust is chemically known as hydrated iron(III) oxide, represented by the formula \( Fe_2O_3 \cdot nH_2O \).

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