Two cylinders $A$ and $B$ of equal capacity are connected to each other via a stop clock. A contains an ideal gas at standard temperature and pressure. $B$ is completely evacuated. The entire system is thermally insulated. The stop clock is suddenly opened. The process is:
The entire system is completely insulated and the free expansion of the gas will take place, the temperature of the gas will remain constant. So the process will be adiabatic.