Which statement best explains the plateau seen in the cooling curve of naphthalene?

A) The energy of the system decreases

B) The temperature of the system increases

C) the temperature of the system decreases

D) The energy of the system remains constant

Answer: option A) The energy of the system decreases.

Explanation:

1) A cooling curve shows the evolution of the temperature (vertical axis) as the time goes (horizontal axis)

2) Then, in the curve are shown the different stages (phases) of the substance: solid, liquid, or gas.

3) The cooling curve comes decreasing from left to right: it starts high in the left and starts decreasing: in this region the substance is gas. The temperature decraeses as the substance loses energy.

4) Then, it comes a flat stage (plateu) : this signals that the temperature remains constant, while the system releases energy. The energy is released while the particles of gas get closer to each other and form a liquid. During, this process (condensation) the gas does not change the temperature but decreases it kinetic energy.

5) After all the gas has condensated (become liquid) the temperature of the liquid starts to decrease until the particles come so close that they form the solid. This change of phase (freezing) is also featured by the constant temperature (flat of plateau) and release of energy.

That explains the question.