On heating strongly above 340 degree C, the white solid ammonium chloride, thermally decomposes into a mixture of two colourless gases ammonia and hydrogen chloride.
On cooling the reaction is reversed and solid ammonium chloride reforms.
This is an example of sublimation but here it involves both physical and chemical changes.
When a substance sublimes it changes directly from a solid into a gas without melting and on cooling reforms the solid without condensing to form a liquid.
But, more importantly in this section, it is an example of a reversible reaction.
Ammonium chloride->heat ammonia+hydrogen chloride
NH4Cl(s)->NH3(g) + HCl(g)
So the chemical change in the reaction is production of ammonia and HCl from NH4Cl. And the physical change is change of solid state of NH4Cl to gaseous state (i.e. sublimation)