Two gas jars are connected to each other, but they are separated by a closed valve. One gas jar contains oxygen, and the other is empty. What will happen when the valve is opened?

When the valve is opened the gas from the filled jar will moved through the valve toward the empty jar. After some time the empty jar will filled with oxygen also. This process is called Diffusion.

Diffusion is the net movement of molecules or atoms from a region of high concentration to a region of low concentration as a result of random motion of the molecules or atoms.