Question

Heat is added to a substance, but its temperature does not rise. Which one of the following statements provides the best explanation for this observation?

Group of answer choices

The substance has unusual thermal properties.

The substance undergoes a change of phase.

The substance must be a non-perfect solid.

The substance must be cooler than its environment.

The substance must be a gas.

Answer 1

The best explanation for the observation that heat is added to a substance but its temperature does not rise is:

The substance undergoes a change of phase.

When a substance undergoes a change of phase, such as melting or boiling, the added heat energy is used to break the intermolecular forces holding the substance together rather than increasing the temperature. During this phase change, the temperature remains constant until the entire substance has undergone the phase transition. Only after the phase change is complete will the temperature start to rise again.

Therefore, the statement "The substance undergoes a change of phase" provides the best explanation for the observation that heat is added to a substance but its temperature does not rise.

Answer 2

The substance undergoes a change of phase.

Step-by-step explanation:

Intramolecular forces (IMFs) are the forces that hold substances together and form the state of matter.

Phase Changes

Phase changes occur when a substance changes its state of matter. For example, when a liquid changes to a gas. Phase changes happen because the IMFs that hold the atoms together in the substance break apart. It takes energy to break IMFs. So, for an endothermic phase change to happen energy, in the form of heat, must be added to the substance.

Temperature Changes

As stated above, heat must be absorbed to break IMFs. So, during phase changes the temperature does not rise. This is because all of the heat goes into overcoming the IMFs, not raising the temperature. The best explanation for the situation in the question is that it was in the middle of a phase change. We can see this phenomenon when looking at a phase change graph.