Heterogeneous system: components are not all in the same state of matter, e.g. $CaCO_3$ is solid, decomposes to $CaO$(solid) and $CO_2$(gas)
For such an equilibrium, if we were to double the volume of the system, the concentration of $CO_{2}$ is unchanged, however as the volume is doubled the moles of $CO_{2}$ is also increased!
