A spectator ion is an ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

 that exists as a reactant and a product

Product (chemistry)

Product are formed during chemical reactions as reagents are consumed. Products have lower energy than the reagents and are produced during the reaction according to the second law of thermodynamics. The released energy comes from changes in chemical bonds between atoms in reagent molecules and...

 in a chemical equation

Chemical equation

A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...

. Spectator ions can be observed in the reaction of aqueous solution

Aqueous solution

An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...

s of sodium chloride

Sodium chloride

Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...

 and copper (II) sulfate but does not affect the equilibrium:

2Na⁺_(aq) + 2Cl⁻_(aq) + Cu²⁺_(aq) + SO₄²⁻_(aq) → 2Na⁺_(aq) + SO₄²⁻_(aq) + CuCl2 (s)

Solid

Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

The Na⁺ and SO₄²⁻ ions are spectator ions since they remain unchanged on both sides of the equation. They simply "watch" the other ions react, hence the name. They are present in total ionic equations to balance the charges of the ions. In reaction stoichiometry

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

, spectator ions are removed from a complete ionic equation to form a net ionic equation. For the above example this yields:

So: 2Na⁺_(aq) + 2Cl⁻_(aq) + Cu²⁺_(aq) + SO₄²⁻_(aq) → 2Na⁺_(aq) + SO₄²⁻_(aq) + CuCl_{2 (s)} (Where x = Spectator ion)

= 2Cl⁻_(aq) + Cu²⁺_(aq) → CuCl_{2 (s)}

Spectator ions concentration only affect the Debye length

Debye length

In plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...

. In contrast potential-determining ions, whose concentrations affect surface potential (by surface chemical reactions) as well the Debye length.

Net Ionic Equation

A net ionic equation ignores the spectator ions that were part of the original equation.