In a free expansion of ideal gas (into vacuum), the temperature:
- Stays the same
- Becomes zero
- Decreases
- Increases
Show answer and solution
Correct answer: A. Stays the same
**Free expansion:** no work done (against vacuum), no heat exchange (rapid).
So ΔU = 0. For ideal gas, ΔU = 0 implies ΔT = 0 — temperature unchanged.
(Real gases show Joule-Thomson effects.)