WAEC 2022 Physics · Question 16

Which of the following statement(s) is/are correct about a fixed mass of gas compressed in an inexpansible…

Question

Which of the following statement(s) is/are correct about a fixed mass of gas compressed in an inexpansible container; I. The average speed of the molecules increases II. The temperature of the gas increases III. The molecules hit the walls of the container more often than before the compression

A.I only
B.I and II only
C.I, II, and III only
D.I and III

Answer: C. I, II, and III only

Explanation written with AI and checked against the answer key

Analyzing the question

A fixed mass of gas is compressed in a container. The question asks which of three statements are correct. This is about the kinetic theory of gases and the gas laws.

Key concepts

When a gas is compressed (volume reduced), work is done on it. If this happens quickly (adiabatically, with no time for heat to escape), the internal energy of the gas increases.
For an ideal gas, temperature is proportional to the average kinetic energy of the molecules: KE = (3/2)kT. So higher temperature means higher average molecular speed.
Reduced volume means the molecules are closer together and travel shorter distances between collisions with the walls, so the collision rate with the walls increases. This is why pressure rises.

Checking each statement

I. The average speed of the molecules increases. Compressing the gas does work on it and raises its internal energy. This increases the average kinetic energy, so the average speed goes up. ✔
II. The temperature of the gas increases. Temperature is a measure of average kinetic energy, so it rises too. ✔
III. The molecules hit the walls more often. The volume is smaller, so the molecules reach the walls more frequently. The faster speed adds to this effect. ✔

All three statements are consistent with compressing the gas (the standard WASSCE assumption is that the compression is rapid, so the gas is not kept at constant temperature).

Caution: If the compression were slow and isothermal (temperature held constant), I and II would not hold. The question's wording, with statements I and II included alongside III, implies the usual adiabatic treatment.

Answer: C. I, II, and III only

Exam tip: Remember the chain: compress → work done on gas → internal energy up → temperature up → faster molecules → more frequent wall collisions → pressure up. If a question says "slowly" or "constant temperature," only the collision frequency (III) increases. Look for those keywords.

AI can make mistakes. Report anything that looks wrong.