Free expansion (Joule expansion) occurs when a gas expands into a vacuum through a valve.

1. Work: Since there is no external pressure ($P_{ext}=0$) for the gas to push against, the work done is zero ($W=0$).
2. Heat: The container is typically insulated, so $Q=0$.
3. Internal Energy: From First Law $\Delta U=Q-W=0-0=0$. For an ideal gas, since $U$ is purely a function of temperature ($T$), $\Delta U=0$ implies the temperature of the gas remains constant ($T_i=T_f$).