A gas expands isothermally and reversibly into a vacuum, doing no work and exchanging no heat. Which of the following best describes what happens to the entropy of the universe?
AEntropy of the universe decreases
BEntropy of the universe stays the same
CEntropy of the universe increases
DEntropy of the gas decreases while the surroundings increase
Irreversible free expansion into a vacuum increases the number of accessible microstates for the gas, so the entropy of the gas (and thus the universe) increases even though no heat is exchanged. ΔS_universe > 0 for all irreversible processes.
Question 2 True / False
It is possible for the entropy of a system to decrease without violating the Second Law of Thermodynamics.
TTrue
FFalse
Answer: True
A refrigerator decreases the entropy of its interior by removing heat. This is allowed because the entropy increase of the surroundings (heat dumped to the room) more than compensates. The Second Law only requires that ΔS_universe ≥ 0 — a subsystem can decrease in entropy at the expense of its environment.
Question 3 Short Answer
Why is entropy described as the 'arrow of time' rather than just another state variable like pressure or volume?
Think about your answer, then reveal below.
Model answer: Because entropy can only increase or stay constant for an isolated system — it never spontaneously decreases. This gives time a preferred direction: the past is the direction of lower entropy, and the future is the direction of higher entropy. Pressure and volume can reversibly return to prior values; entropy cannot.
All other classical thermodynamic state variables (pressure, volume, temperature, internal energy) can oscillate freely. Entropy's monotonic increase in isolated systems creates the thermodynamic asymmetry we experience as the flow of time from past to future.