Question

what happens to the molecules in a gas as it is cooled inside a jar?
a. the molecules move more slowly.
b. the molecules move farther apart.
c. the molecules increase their vibrating motion.
d. the molecules collide more frequently with the walls of the jar.

Explanation:

To solve this, we recall the kinetic theory of gases: temperature is related to the average kinetic energy of gas molecules. Cooling a gas (decreasing temperature) decreases the average kinetic energy of its molecules. Kinetic energy ($KE$) is related to velocity ($v$) by $KE = \frac{1}{2}mv^2$, so lower kinetic energy means lower velocity (slower movement).

• Option A: As temperature (cooling) decreases, molecular speed decreases (since $KE$ and speed are related). This matches.
• Option B: Cooling a gas (at constant volume, like in a jar) leads to decreased pressure, but molecules move closer (if volume could change, but in a jar, volume is fixed; however, the key is speed, not distance apart here. Also, cooling generally causes contraction, but the question is about molecular motion, not spacing. So B is incorrect.
• Option C: Vibrating motion is more related to solids. Gas molecules move freely, and cooling decreases motion, so C is incorrect.
• Option D: Slower molecules collide less frequently with the jar walls, so D is incorrect.

Answer:

A. The molecules move more slowly.