Which of the following is a chemical change?

Rusting of iron

The correct answer is option 2. Rusting of iron.

Rusting of iron is a chemical change because it involves a chemical reaction that results in the formation of a new substance. Here's an explanation of why rusting of iron is a chemical change:

1. Chemical Reaction:

Rusting is a chemical reaction that occurs when iron reacts with oxygen and water in the presence of air. The chemical equation for the rusting of iron is generally represented as:
\[ \text{4Fe} + \text{3O}_2 + \text{6H}_2\text{O} \rightarrow \text{4Fe(OH)}_3 \]

2. Formation of a New Substance:

The reaction results in the formation of a new substance called iron(III) hydroxide (\(\text{Fe(OH)}_3\)), commonly known as rust. This substance has different properties than the original iron, indicating a change at the molecular level.

3. Irreversibility:

Chemical changes are often irreversible. Once iron undergoes rusting, it cannot be easily converted back to its original form. Physical changes, on the other hand, are usually reversible.

4. Change in Properties:

Rust has different physical and chemical properties than iron. For example, rust is brittle and flaky, while iron is malleable and ductile. The change in properties is an indication of a chemical transformation.

In contrast, the other options involve physical changes:

Conversion of water to ice:  This is a physical change as it involves a change in the state of matter (from liquid to solid) without altering the chemical composition of water.

Crumpling a sheet of aluminum foil:  This is a physical change as it only affects the shape and form of the aluminum foil without changing its chemical composition.

Casting silver:  This is a physical change related to the shaping of silver without altering its chemical composition. The silver retains its elemental identity.