The graph below shows how the temperature of a pure substance changes as it is heated at a constant rate, transitioning from a liquid to a gas.
During the period represented by the horizontal section X X\,X of the graph, which of the following correctly describes the changes to the particles of the substance?
The mean kinetic energy of the particles increases, and their potential energy remains constant.
The mean kinetic energy of the particles remains constant, and their potential energy increases.
The mean kinetic energy of the particles remains constant, and their potential energy decreases.
The mean kinetic energy of the particles increases, and their potential energy increases.