Here's how to determine the spectator ions in the reaction between Rb₂O and CaCl₂:

1. Write the balanced chemical equation:

Rb₂O(aq) + CaCl₂(aq) → 2RbCl(aq) + CaO(s)

2. Identify the soluble ionic compounds:

* Rb₂O (Rubidium oxide) is soluble in water.

* CaCl₂ (Calcium chloride) is soluble in water.

* RbCl (Rubidium chloride) is soluble in water.

3. Write the ionic equation:

2Rb⁺(aq) + O²⁻(aq) + Ca²⁺(aq) + 2Cl⁻(aq) → 2Rb⁺(aq) + 2Cl⁻(aq) + CaO(s)

4. Identify spectator ions:

Spectator ions are ions that appear on both sides of the ionic equation unchanged. In this case, they are:

* Rb⁺(aq)

* Cl⁻(aq)

Therefore, the spectator ions are Rb⁺ and Cl⁻.