Change of State: Melting MCQs Quiz | Class 9

Understanding Melting (Fusion)

Melting, also known as fusion, is the physical process where a substance changes from a solid state to a liquid state. This phase transition occurs when the internal energy of the solid increases, typically by the application of heat or pressure.

Key Concepts

• Melting Point: The specific temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. For pure substances, this is a sharp, defined temperature.
• Latent Heat of Fusion: The amount of heat energy required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• Particle Behavior: As a solid is heated, its particles gain kinetic energy and vibrate more vigorously. At the melting point, they possess enough energy to overcome the forces of attraction holding them in fixed positions.

Why Temperature Remains Constant

During the process of melting, the temperature of the system does not rise even though heat is being supplied continuously. This heat is absorbed by the solid to break the inter-particle bonds. Since this heat is hidden (it does not raise the temperature), it is called “Latent Heat.”

Comparison of Energy

Particles in water at 0 degrees Celsius have more energy than particles in ice at the same temperature. This is because water particles have absorbed extra energy in the form of latent heat of fusion.

Melting Points of Common Substances

Quick Revision Notes

• Unit of Temperature: The SI unit is Kelvin (K). T(K) = T(C) + 273.15.
• Pressure Effect: Generally, increasing pressure raises the melting point, but for water (ice), increasing pressure lowers the melting point slightly.
• Impurity Effect: Adding impurities (like salt to ice) lowers the melting point.