MCAT® ExamChemical and Physical Foundations of Biological SystemsMedium
A student is investigating the concept of entropy in thermodynamics. Which of the following processes would result in a decrease in the entropy of the system?
- AMelting of ice at 0°C.
- BEvaporation of water at 100°C.
- CSublimation of dry ice at -78.5°C.
- DCondensation of steam at 100°C.
Show answer & explanationAnswer & explanation
Correct answer: D. Condensation of steam at 100°C.
Entropy is a measure of the disorder or randomness of a system. Condensation of steam into liquid water involves molecules moving from a more disordered gaseous state to a more ordered liquid state, thereby decreasing the entropy of the system. All other options describe processes that increase disorder.
Why the other options are wrong
- A. Melting (solid to liquid) increases disorder, thus increasing entropy.
- B. Evaporation (liquid to gas) increases disorder, thus increasing entropy.
- C. Sublimation (solid to gas) significantly increases disorder, thus increasing entropy.
Entropy (ΔS)
A thermodynamic property that is a measure of the disorder or randomness of a system. An increase in disorder corresponds to a positive change in entropy (ΔS > 0), while a decrease in disorder corresponds to a negative change (ΔS < 0).
- Gases have higher entropy than liquids, which have higher entropy than solids.
- Systems tend towards higher entropy (Second Law of Thermodynamics).
- Entropy increases with increasing temperature and volume.
Memory trick: Entropy's Easy: More Mess, More Entropy. Less Mess, Less Entropy.