The equation you're referring to is likely the decomposition of mercury(II) oxide:

2 HgO → 2 Hg + O₂

In this equation:

* Mercury(II) oxide (HgO) is the reactant. It's the substance that is being broken down.

* Mercury (Hg) and oxygen gas (O₂) are the products. They are formed as a result of the decomposition reaction.

So, to answer your question directly: mercury(II) oxide is the only reactant in this chemical equation.