Aluminium powder burns in oxygen gas to form aluminium oxide, the compound responsible for the thin, protective coating that gives aluminium metal its natural resistance to corrosion. Which equation correctly represents this reaction, fully balanced?

IGCSE Chemistry 0620: Balancing the Equation for Aluminium Oxide Formation

Step 1: Recall what “balanced” means

A symbol equation is balanced only when the same number of atoms of every element appears on the left (reactants) and the right (products). Aluminium oxide has the formula $\text{Al}_2\text{O}_3$ , so any balanced equation forming it must give equal numbers of aluminium atoms and equal numbers of oxygen atoms on both sides.

Step 2: Count atoms in each option

Option A: $2\text{Al(s)} + 3\text{O}_2\text{(g)} \rightarrow \text{Al}_2\text{O}_3\text{(s)}$ Al: left $=2$ , right $=2$ (balanced). O: left $=3\times2=6$ , right $=3$ (not balanced).
Option B: $4\text{Al(s)} + 2\text{O}_2\text{(g)} \rightarrow 2\text{Al}_2\text{O}_3\text{(s)}$ Al: left $=4$ , right $=2\times2=4$ (balanced). O: left $=2\times2=4$ , right $=2\times3=6$ (not balanced).
Option C: $\text{Al(s)} + \text{O}_2\text{(g)} \rightarrow \text{Al}_2\text{O}_3\text{(s)}$ Al: left $=1$ , right $=2$ (not balanced). O: left $=2$ , right $=3$ (not balanced).
Option D: $4\text{Al(s)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{Al}_2\text{O}_3\text{(s)}$ Al: left $=4$ , right $=2\times2=4$ (balanced). O: left $=3\times2=6$ , right $=2\times3=6$ (balanced).

Step 3: Select the correctly balanced equation

Only option D has both aluminium and oxygen balanced on each side, using the state symbols $\text{(s)}$ for the solid metal and oxide and $\text{(g)}$ for oxygen gas.

Final answer

$\boxed{4\text{Al(s)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{Al}_2\text{O}_3\text{(s)}}$ , option D.