Statement Analysis: Physical vs. Chemical Changes

The question asks us to identify the statement that is NOT correct regarding changes in matter. Let's break down the concepts of physical and chemical changes and analyze each statement.

Understanding Physical Change

A physical change affects the form or appearance of a substance but does not alter its chemical identity. During a physical change, the molecules themselves remain the same. Examples include:

• Changes in state (melting, freezing, boiling, condensation, sublimation, deposition).
• Changes in shape or size (cutting, bending, dissolving).

Understanding Chemical Change

A chemical change results in the formation of new substances with different chemical properties. The chemical composition of the molecules changes. Examples include:

• Burning
• Rusting
• Cooking
• Digestion

Analyzing the Statements

Statement 1: Interconversion of states of matter is a physical change

This statement is correct. When water boils to become steam, or ice melts into water, the substance is still H₂O. Only the arrangement and energy of the molecules change, not the molecules themselves. This is a classic example of a physical change.

Statement 2: Burning of a candle, both chemical and physical changes take place

This statement is correct. When a candle burns:

• Physical Change: The solid wax melts into liquid wax, which then vaporizes. These are changes of state.
• Chemical Change: The wax vapor reacts with oxygen and burns, producing carbon dioxide, water vapor, and soot. This forms new substances.

Therefore, both types of changes occur.

Statement 3: During a physical change, the chemical composition of molecules changes

This statement is NOT correct. As defined earlier, a physical change involves altering the form or appearance without changing the fundamental chemical identity. The chemical composition of molecules does NOT change during a physical change. For example, when ice (solid H₂O) melts into water (liquid H₂O), the molecule remains H₂O.

Statement 4: Rusting of iron is a chemical change

This statement is correct. Rusting is a process where iron reacts with oxygen and moisture to form iron oxides (like Fe₂O₃·nH₂O). This is a new substance with different properties, making it a chemical change.

Conclusion

Based on the analysis, the statement that incorrectly describes changes in matter is the one claiming that the chemical composition of molecules changes during a physical change.