Question

question 5.a
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?
a. the liquid cools down and forms solid crystals.
b. the liquids molecules remain packed tightly and move slowly.
c. the number of molecules in the liquid decreases over time.
d. the molecules collide more frequently and spread farther apart.

Explanation:

• For Question 5.a:
• According to the kinetic - molecular theory of matter, when the temperature of a substance increases, the kinetic energy of its 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 its velocity). As temperature rises, the average speed of molecules (\(v\)) increases. Molecules do not gain mass with an increase in kinetic energy (mass of a molecule is a fundamental property at non - relativistic speeds, which is the case in normal thermal conditions). Also, molecules never stop moving completely (even at absolute zero, there is zero - point motion, and at higher temperatures, they are in constant motion).
• For Question 5.b:
• If the molecules gain kinetic energy and move faster (answer to 5.a), they will collide more frequently. In a liquid, as the kinetic energy of molecules increases (e.g., when heated), the increased motion causes them to spread farther apart (less dense packing compared to a cooler liquid). A liquid cooling down and forming crystals (Option A) is a case of molecules losing kinetic energy. Molecules remaining packed tightly and moving slowly (Option B) is characteristic of a cooler liquid. The number of molecules in a liquid not changing (assuming no chemical reaction or phase change that would add/remove molecules from the liquid phase, like evaporation which is a phase change) over time (Option C) is not related to the increase in molecular kinetic energy.

Answer:

• Question 5.a: B. The molecules gain kinetic energy and move faster.
• Question 5.b: D. The molecules collide more frequently and spread farther apart.