Question

substances changing colour on heating in closed vessel but gain colour on
cooling

Answer 1

The crystals of hydrated salts are heated, they lose their water of crystallization which is responsible for its color. when it is left to cool down, it absorbs moisture from air and again gains its water of crystallization and thus, also its color.

For example- FeSO4. 7H2o ====== when heated

                       forms FeSO4 + 7H2O

On heating Iron(II)sulfate, it loses the water of crystallization and the original green substances are converted into brown anhydrous substance. And again when added water or in other words cooled down it regains its color of crystallization

Hope it helps....