Can a substance contract on heating? Give an example.

Yes, certain substances can contract on heating under specific conditions. This behavior is known as anomalous expansion.

Most materials expand when heated because their particles gain kinetic energy and move farther apart. However, some substances behave differently in a limited temperature range.

Example: Water (0°C to 4°C)
When water is heated from 0°C to 4°C, it actually contracts instead of expanding. This happens because:

• At 0°C, water has a more open (less dense) structure due to hydrogen bonding.
• As temperature increases to 4°C, this structure becomes more compact.
• This leads to a decrease in volume and increase in density.

At 4°C, water reaches its maximum density. Beyond this temperature, it starts expanding normally like other liquids.

Why is this important?

• It prevents lakes and ponds from freezing completely in winter.
• Ice forms on the surface while liquid water remains below, supporting aquatic life.

Key Concept (for exams):

• Anomalous expansion of water occurs between 0°C and 4°C
• Water contracts on heating in this range, unlike most substances.