Recall why the nitrate ions in this reaction are called spectator ions.

In a chemical reaction, spectator ions are ions that do not participate in the reaction itself. They are present in the same form on both the reactant and product side of the chemical equation.

In the case of a reaction involving nitrate ions, if the nitrate ions appear in the same form (as NO3-) in both the reactants and the products, they are termed as spectator ions. They are essentially 'watching' the reaction take place without getting involved.

The reason they are called 'spectator' ions is because they do not change, react, or get consumed in the reaction. They remain in the solution unchanged, just like a spectator at a sports event.