Question

a blue solid is changed into a white substance and a colorless gas. Was the blue solid an element or a compound?

Answer 1

A colour change was observed, so we know that a chemical reaction occurred........

........and we further know that the blue solid was a compound and NOT an element. The blue substance evolved (i) a gas, and (ii) a new white substance. Given the two new species formed, the starting material could not have been an element. Elements are the simplest chemical substances, and an element as the sole starting material cannot be be chemically transformed into new species.