What is Entropy?
Entropy (S) measures the disorder or randomness of a system. Higher entropy means more disorder and more microstates (different molecular arrangements). The second law of thermodynamics states that entropy increases for all spontaneous processes — the universe tends toward disorder.
Entropy S is the measure of disorder in a system. ΔS = entropy change tells us how much randomness increases or decreases. Second law: the entropy of an isolated system increases for any spontaneous process (ΔS_universe > 0).
- •Solid ice
- •Organized molecular arrangement
- •Few possible microstates
- •Less randomness
- •Liquid water or steam
- •Random molecular arrangement
- •Many possible microstates
- •More randomness
Step-by-step worked examples
Ice melting to water at 0°C. Is ΔS_sys positive or negative?
Melting: solid → liquid Molecules become more disordered Disorder increases → ΔS_sys > 0 (positive)
Water evaporating. Entropy increases or decreases?
Evaporation: liquid → gas Gas is much more disordered than liquid ΔS_sys is large and positive
Gas cooling to liquid. Entropy change?
Condensation: gas → liquid Molecules become more ordered ΔS_sys < 0 (negative, disorder decreases)
Flashcards
Quick quiz
Q1.Melting ice, ΔS is…
Q2.Entropy measures…
Q3.Second law of thermodynamics states…
Q4.Gas diffusing into a room, ΔS…
The full card deck, worked steps and AI-tutor support for “What is Entropy?” are in Notek — study by hand before your exam.
Common mistakes
Confusing entropy with energy. — Correct: Entropy is disorder; energy is capacity to do work (related but different).
Thinking entropy only increases for all systems. — Correct: Entropy increases for the UNIVERSE; a system's entropy can decrease if surroundings increase more.
Higher temperature = lower entropy. — Correct: Higher temperature usually increases molecular motion → higher entropy.
Freezing increases entropy. — Correct: Freezing decreases entropy — solid is more ordered than liquid.
FAQ
What is entropy in simple terms?
How many ways a system's molecules can be arranged; higher S = more disorder.
Why does entropy increase?
Disorder has far more microstates than order — naturally most likely outcome.
Can entropy decrease?
Yes, in an open system; but universal entropy (system + surroundings) always increases.
Is life an exception to entropy?
No; life is an open system exchanging energy/matter with surroundings. Local order requires greater disorder elsewhere.