A metal can displace another metal from its salt solution if it is more reactive, which means it must have a lower standard reduction potential (higher oxidation potential). According to the electrochemical series, Aluminium (Al) is more reactive than Zinc (Zn) because the standard reduction potential of Al ($E^{\circ}_{Al^{3+}/Al} = -1.66\text{ V}$) is lower than that of Zn ($E^{\circ}_{Zn^{2+}/Zn} = -0.76\text{ V}$) . Therefore, an Aluminium container will react spontaneously with the $ZnCl_2$ solution ($2Al + 3Zn^{2+} \rightarrow 2Al^{3+} + 3Zn$), causing the concentration of the solution to change. On the other hand, Copper ($E^{\circ}_{Cu^{2+}/Cu} = +0.34\text{ V}$) and Silver ($E^{\circ}_{Ag^{+}/Ag} = +0.80\text{ V}$) have higher reduction potentials and are less reactive than zinc, so they will not react with the $ZnCl_2$ solution .