State one relevant observation for the following reaction : Zinc carbonate is heated strongly.

The original white colour turns yellow on heating. Colourless, odourless carbon dioxide gas is evolved.

$\begin{matrix} \text{ZnCO}$3 &\longrightarrow & \text{ZnO} & + & \text{CO}2 \ \text{[Zinc} & & \text{[Zinc oxide]} & & \text{[Carbon} \ \text{carbonate]} & & \text{[yellow - hot]} & & \text{dioxide]} \ \text{[white]} & & \text{[white - cold]} \end{matrix}