When a gas is released upon heating a substance, the same material can not be recovered by reversing the physical conditions. The chemical composition of the material also changes. Hence this change describes a chemical change.
Such a situation is represented by heating sdium bicarbonate:
`2NaHCO_3 stackrel Delta rarr Na_2CO_3 +CO_2(g) +H_2O`
Both `NaHCO_3` and `Na_2CO_3` have the same appearnce, but definitely differ in their reactivity. Sodium carbonate is extremely stable towards heat, while sodium bicarbonate upon heating gives off `CO_2(g)` and `H_2O` readily.