Question

q1 .find the missing variable given as X in the following
a.CaCO3(s)--X-->CaO(s)+Co2(g)

Answer 1

heat is the answer as provided heat it brak down as rxn called decomposition rxn

Answer 2

The following reaction is a decomposition reaction thus heat is required for decomposition.

The missing variable is heat energy which is represented with ∆.

So ∆/ heat energy is the missing variable.
And this reaction is endothermic.

Hope this helps...