Question

Explain with the help of a graph, why, at some specific temperatures, even after adding heat, there is no change in the temperature of the substance.

Answer 1

The horizontal flat lines (plateaus) on the graph show the specific temperatures where the temperature does not rise despite adding heat. This happens due to two phases:

1. Melting Phase (First Plateau at 0°C):

• The heat added is absorbed as Latent Heat of Fusion.
• It is entirely used to break intermolecular bonds to turn solid ice into liquid water.
• Therefore, the temperature remains constant at 0°C.

2. Vaporisation Phase (Second Plateau at 100°C):

• The heat added is absorbed as Latent Heat of Vaporisation.
• It is entirely used to overcome molecular forces to turn liquid water into gas (steam).
• Therefore, the temperature remains constant at 100°C.