Two cylinders are connected by a small tube which has a needle valve in the middle. One cylinder contains ${?}^{4}He$ gas, and the other cylinder

contains $C{O}_2$ gas. The gases are at the same temperature, pressure and volume. The lids that seal the gases in are free to move up or down.

When the valve is opened, what happens to the volume in each cylinder?

The volumes do not change.

The $C{O}_2$ volume decreases and the ${?}^{4}He$ increases.

The $C{O}_2$ volume increases and the ${?}^{4}He$ decreases.

What happens depends on the initial temperature.

What happens depends on the initial pressure.