(a) The reactivity series is a list of metals (and some non-metals) arranged in decreasing order of their reactivity (tendency to react). It is developed by comparing the ability of elements to displace each other from solutions or oxides and by observing reactions with water and acids.
Order (most to least reactive):
Calcium > Aluminium > Lead > Copper
(b) Balanced chemical equation for the reaction of Fe₂O₃ with aluminium (thermite reaction):
$$\text{Fe}_2\text{O}_3 + 2\text{Al} \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$$
Source: Chapter 1 (Chemical Reactions and Equations), Exercises Q.2
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