Question

question 5.a
a student heats a container of liquid and observes the behavior of the molecules.
which statement best explains what happens to the kinetic energy of the molecules as the temperature increases?
a. the molecules lose kinetic energy and move slower.
b. the molecules gain kinetic energy and move faster.
c. the molecules gain mass as their kinetic energy increases.
d. the molecules stop moving completely at higher temperatures.
question 5.b
which observation would best support your answer to part a?

Explanation:

According to the kinetic - molecular theory of matter, when a liquid is heated, the temperature (which is a measure of the average kinetic energy of the molecules) increases. Kinetic energy is related to the motion of molecules (\(KE=\frac{1}{2}mv^{2}\), where \(m\) is the mass of the molecule and \(v\) is the velocity). As the temperature increases, the average speed of the molecules (\(v\)) increases. Since the mass (\(m\)) of the molecules does not change (unless there is a chemical reaction, which is not indicated here), the kinetic energy (\(KE\)) of the molecules increases.

• Option A: When temperature increases, molecules gain, not lose, kinetic energy. So, this option is incorrect.
• Option B: As temperature increases, molecules gain kinetic energy. Since \(KE=\frac{1}{2}mv^{2}\) and \(m\) is constant (for non - nuclear, non - chemical change situations), an increase in \(KE\) means an increase in \(v\) (molecules move faster). This option is correct.
• Option C: The mass of the molecules does not change when the temperature of the liquid is increased (in the absence of a chemical reaction). So, this option is incorrect.
• Option D: Molecules are in constant motion (as per the kinetic - molecular theory), and they do not stop moving completely even at higher temperatures. Absolute zero (\(T = 0\space K\)) is the theoretical temperature at which molecular motion would be at a minimum, but we are not at absolute zero here. So, this option is incorrect.

Answer:

B. The molecules gain kinetic energy and move faster.