$(i)$ Arrange the following metals in the order of their decreasing activities: Aluminium, Gold, Sodium, Copper.
$(ii)$ Give the reaction of aluminium powder with manganese dioxide on heating.

(N/A) $(i)$ The order of decreasing reactivity is: Sodium > Aluminium > Copper > Gold.
$(ii)$ When aluminium powder is heated with manganese dioxide, a thermite reaction occurs, producing manganese metal and aluminium oxide: $4Al(s) + 3MnO_2(s) \xrightarrow{\Delta} 3Mn(l) + 2Al_2O_3(s) + \text{Heat}$.