This is a deprotonation or acid-base reaction. This is a ligand exchange reaction. On heating it loses oxygen to give Cr 2 O 3 . Both of these oxides are basic. OH - groups round each cobalt ion in the solid state.
Zinc oxide goes yellow on heating and returns to white again on cooling.
This is result of movement of ions within the crystal lattice which is reversible. Peter Hughes is in the Chemistry Department at Westminster School, where he was formerly Head of Science.