The compound that decomposes when heated to form zinc oxide (ZnO) is zinc carbonate (ZnCO₃).

Here's the chemical equation for the decomposition reaction:

ZnCO₃(s) → ZnO(s) + CO₂(g)

When heated, zinc carbonate breaks down into zinc oxide (a solid) and carbon dioxide (a gas). This is a typical example of a thermal decomposition reaction.