Question

Substances A and B have the same freezing and boiling points. If solid samples of both substances are heated in the exact same way, substance A boils before substance B. Which of the following would NOT explain this phenomenon? a. Substance B has a higher specific heat. b. Substance B has a higher heat of vaporization. c. Substance B has a higher heat of fusion. d. Substance B has a higher internal energy.

Short answer

d. Substance B has a higher internal energy.

Step-by-step solution

Understand freezing and boiling points

Given: Substances A and B have the same freezing and boiling points. This means both substances change from solid to liquid at the same temperature and from liquid to gas at the same temperature.

Analyze the heating process

When solid samples of both substances are heated, substance A boils before substance B. This indicates that substance A reaches the boiling point faster or requires less energy in some manner to transition from a liquid to a gas.

Consider the effect of specific heat

Substance B having a higher specific heat (option a) implies it requires more energy to increase its temperature. This means B would heat up more slowly, which explains the phenomenon.

Consider the effect of heat of vaporization

Substance B having a higher heat of vaporization (option b) means it requires more energy to change from liquid to gas. This would also account for B boiling later than A.

Consider the effect of heat of fusion

Substance B having a higher heat of fusion (option c) means it requires more energy to change from solid to liquid. While this affects the melting process, it can indirectly delay boiling if the energy distribution is considered.

Consider the effect of internal energy

Substance B having a higher internal energy (option d) does not directly contribute to the particular heating process described, as internal energy involves the total kinetic energy of particles, not specifically the heating rate or phase change energy requirements.

Identify the correct answer

Given the explanations in the previous steps, the description that does NOT explain why substance A boils before substance B is option d.

Key concepts

specific heat

Specific heat is the amount of heat energy required to raise the temperature of a substance by 1 degree Celsius. It essentially measures how much energy a substance can absorb before its temperature increases.
If a substance has a high specific heat, it needs more energy to heat up compared to a substance with a low specific heat. Therefore, substance B having a higher specific heat means it would take longer to reach its boiling point than substance A.
This concept is essential in explaining why substance A boils before substance B when heated at the same rate.

heat of vaporization

The heat of vaporization is the amount of energy required to transform a substance from a liquid to a gas at its boiling point.
A higher heat of vaporization means a substance needs more energy to change into a gas.
Given substance B's higher heat of vaporization, it requires more energy to boil compared to substance A.
Therefore, substance B would reach its boiling point slower, explaining why substance A boils first under identical conditions.

heat of fusion

The heat of fusion is the energy required to change a substance from solid to liquid at its melting point.
If substance B has a higher heat of fusion, it needs more energy to melt compared to substance A.
Energy that goes into melting B delays the time it takes to begin heating the liquid phase, indirectly contributing to a slower progression to the boiling point.
This can be seen in the scenario where substance A boils before substance B under the same heating conditions.

internal energy

Internal energy refers to the total energy contained within a substance, including both kinetic and potential energy of particles. While this energy does affect the overall state of matter, it does not directly influence the rate at which a substance heats up or the energy needed for phase changes like boiling and melting.
In the context of the given problem, option d states that substance B has a higher internal energy. This does not explain the scenario where substance A boils before substance B, as internal energy doesn't impact the specific heating or phase transition processes.
Hence, this option is the correct answer to the problem.