Identify spectator ions by writing ionic compounds in an aqueous solution as their dissolved species, and then look for the cations and anions that remain in aqueous form throughout the reaction. According to Chemlab, spectator ions remain in solution and do not change ionic charges.
Ionic compounds, such as sodium chloride, dissociate into ions in water, according to Reference.com. NaCl becomes Na+ ions and Cl- ions. In an aqueous reaction of sodium chloride and copper (II) sulfate, copper chloride forms as a precipitate, but sodium and sulfate ions remain in solution, unchanged as shown by the following balanced reaction: 2Na + (aq) + 2Cl ? (aq) + Cu 2+ (aq) + SO 4 2? (aq) ? 2Na + (aq) + SO 4 2? (aq) + CuCl 2 (s). In other words, the sodium and sulfate ions are on both sides of the reaction arrow. Removing the spectator ions from the equation gives the net ionic equation: 2Cl ? (aq) + Cu 2+ (aq) ? CuCl 2 (s).
Concerning the sodium ions and sulfate ions in this reaction, Anne Helmenstine, Ph.D. writes on About.com, "These ions just 'spectate' while the other ions form the copper chloride." In other reactions, the ions in the net ionic equation show the formation of a gas, such as carbon dioxide, indicated by the formation of bubbles, while spectator ions remain in solution, according to ChemLab.