Heat is added to a substance at a constant rate. The substance starts as a solid and is melted; the liquid is heated and vaporized; finally, the vapor is heated. This process is shown in the graph. The latent heat of vaporization can be found by

Question

Quizplus · Accepted Answer

The latent heat of vaporization is the amount of energy needed to change a substance from a liquid to a gas (i.e., during the phase change from D to E on the graph). During this phase change, the temperature of the substance remains constant despite the addition of heat. Therefore, the energy added during this phase change goes towards breaking the intermolecular bonds holding the liquid together, rather than increasing the temperature.

By multiplying the length of phase change D in seconds by the rate at which heat is added, and dividing by the mass of the substance, we can find the total energy added during the phase change. This is because the rate of heat added is constant and the mass of the substance is also constant. Therefore, choice B is the correct answer.

Choices A, C, and E are incorrect because they do not take into account the fact that the substance undergoes a phase change during the heating process. Choice D is also incorrect because the slope of phase change C only represents the rate of temperature increase during that phase, not the amount of energy added.

Heat Is Added to a Substance at a Constant Rate