Question

Which part of the graph shows the greatest amount of energy required to change the phase?

(A) A
(B) B
(C) C
(D) D
(E) E

Answer 1

The heat required to change the phase of any substance from the solid to liquid phase or the liquid to the gaseous phase is called the “latent heat”. This is also known as the heat of transformation and is the energy released or absorbed by a body or a thermodynamic system during a constant temperature process.

Complete Step by Step Answer
In the given graph, the different phases that take place relate to the phase changes of water from the formation of ice to the formation of water to the formation of water vapour. In the first change, i.e., A, the temperature rises slowly and the ice starts to melt and the heat due to the rise in the temperature is high enough to break the bonds in the molecules of ice, it converts to water at $\text{0}{{\text{ }}^{\text{0}}}\text{C}$ , the phase B. Again the temperature rises, the liquid molecules of water start to move far apart from each other represented by phase C and at the point when the temperature is high enough, and the water gets converted to water vapour, phase D. still when the temperature rises the water vapour molecules gain higher kinetic energy to move apart farther represented by phase E. As can be seen from the graph, the highest amount of energy is required for the conversion of the liquid water to the water vapour represented by the phase D which is the heat of vaporization of water. This is because higher energy is required for the conversion of the liquid to the gaseous phase than the solid to the liquid phase.
Hence the correct option is D.

Note
The amount of heat required to change the phase of any substance or the enthalpy change for the conversion of the liquid to the gaseous state is defined as the heat of vaporization or the enthalpy of vaporization.