Which of the following is $NOT$ a reversible process?

$A$ container with insulating walls is divided into two equal parts by a partition fitted with a valve. One part is filled with an ideal gas at a pressure $P$ and temperature $T$,whereas the other part is completely evacuated. If the valve is suddenly opened,the pressure and temperature of the gas will be

Which of the following processes is reversible?

In a thermodynamic process of uncontrolled change satisfying the equation $Q=W=0$,where $Q$ is heat supplied and $W$ is work done,the process is:

$A$ container with rigid walls is covered with perfectly insulating material. The container is divided into two parts by a partition. One part contains a gas while the other is fully evacuated (vacuum). The partition is suddenly removed. The gas rushes to fill the entire volume and comes to equilibrium after a little time. If the gas is not ideal,then

What is an irreversible process and what is a reversible process?