A cleaned aluminium foil was placed in an aqueous solution of zinc sulphate. When the aluminium foil was taken out of the zinc sulphate solution after 15 minutes, its surface was found to be coated with a silvery grey deposit. From the above observation it can be concluded that

According to the principles of chemical reactions mentioned in the text, a change in the nature and identity of the initial substance indicates a reaction. In this case, the formation of a 'silvery grey deposit' on the aluminium foil is a 'change in colour' and 'change in state' of the surface, which are observations identifying a chemical reaction (Section 1.1). This specific reaction is a displacement reaction where a more reactive metal ($Al$) displaces a less reactive metal ($Zn$) from its aqueous salt solution ($ZnSO_4$). The reaction is: $2Al(s) + 3ZnSO_4(aq) ightarrow Al_2(SO_4)_3(aq) + 3Zn(s)$.