Final answer:
In a double-replacement reaction, 'spectator ions' are often left out as they do not take part in the actual chemical reaction.
Step-by-step explanation:
In a double-replacement reaction, the ions that are often left out are known as spectator ions. These ions do not participate in the actual chemical reaction and remain unchanged in solution both before and after the reaction. An example would be a reaction where the Ag+ and Cl- ions combine to form AgCl(s), leaving the Na+ ions and NO3- ions in solution as spectator ions. These can be canceled out when writing the net ionic equation, which only shows the elements, compounds, and ions that are directly involved in the chemical reaction.