It is a Sublimation reaction.
Usually Solids on heating first change to liquids and then to gases but there are some compounds which change directly from solids to gases in the presence of heat.

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thnq dear siso
err .. actually Sublimation is not reaction ! .. it's a physical change
its ok mistake
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It is called Sublimation.
But a few substances go directly from the solid to the gaseous state without ever becoming a liquid. Scientists call this process sublimation.Like how camphor does!
The reverse of sublimation is deposition — going directly from a gaseous state to a solid state.


Dry ice also undergoes sublimation.