What state symbol would you use to show that a reacting substance was a gas?

Answer: (g)

Explanation:

A reacting substance can either be a solid, liquid or gas.

For instance:

A (s) + B (g) - --> AB (l)

In the reaction above, the reactants are "A", a solid with state symbol (s); while "B", a gas with state symbol (g); both form a product "AB", a liquid with state symbol (l).

Thus, (g) is the state symbol that shows the reacting substance was a gas.