Question

a cup of hot chocolate with a temperature of 80°c is placed on a table in a 20°c room. what happens to the molecular motion of the hot chocolate when it is left on the table.
a the molecules move quickly as the velocity increases.
b the molecules stop moving and the average velocity becomes 0.
c the molecules move more slowly as the velocity decreases.
d the molecules do not change because this is a physical change.

Explanation:

The problem is about molecular motion in hot chocolate as it cools. According to the kinetic theory of matter, temperature is related to the average kinetic energy of molecules. When the hot chocolate (80°C) is in a 20°C room, it loses heat, so its temperature decreases. As temperature decreases, the average kinetic energy of molecules decreases, meaning they move more slowly (since velocity is related to kinetic energy). Option A is wrong because cooling would decrease molecular speed, not increase. Option B is wrong because molecules never stop moving (absolute zero is theoretical and not reached here). Option D is wrong because molecular motion does change with temperature change, and the type of change (physical) doesn't mean motion is unchanged. So the correct reasoning leads to option C.

Answer:

C. The molecules move more slowly as the velocity decreases.