In describing reactions that occur in solution, it is often desirable to write the equation for the reaction in ionic form, indicating the ionic species that actually exist in solution.
For example, if we were describing the reaction of a solution of BaCl₂ with a solution of Na₂SO₄ to form the insoluble solid BaSO₄ we would write
2Na¹⁺_(aq) + SO₄^2-_(aq) + Ba²⁺_(aq) + 2Cl^1-_(aq) -----> 2Na¹⁺_(aq) + 2Cl^1-_(aq) + BaSO_4(s)
In writing a total ionic equation for reactions in water:

We indicate all soluble ionic materials as ions, followed by (aq).
All substances that react with water to form ions are written as ions followed by (aq).
All insoluble ionic solids are written with (s) following their formula.
All soluble unionized species are written with their molecular formula followed by (aq).
In the above equation, we note that the sodium and chloride ions are unchanged and are present on both sides of the equation. Since they are not undergoing chemical reaction, they can be referred to as spectator ions. If we subtract the spectator ions from each side of the equation, we then have a net ionic equation. The net ionic equation for the above reaction is:

Ba²⁺_(aq) + SO4^2-_(aq) -----> BaSO_4(s)
The net ionic equation is a statement of the chemistry that occurred, namely, aqueous barium 2+ ion reacted with aqueous sulfate ion to form solid barium sulfate.
Net ionic equations may also be written for replacement reactions.

Practice writing net ionic equations
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Gwen Sibert, at the
Roanoke Valley Governor's School
gsibert@rvgs.k12.va.us

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