Precipitation is the formation of a solid as a result of the combining of solutions. 
    This is a chemical reaction in which a product is formed which has low solubility in water.

   Example: Silver nitrate solution  +  sodium chloride solution combine to form silver chloride precipitate  +  sodium nitrate solution.

AgNO₃(aq)   +   NaCl(aq)   -->   AgCl(s)   +   NaNO₃(aq)

REACTANTS
Aqueous solutions will dissociate in solution:  AgNO₃(aq)  -->  Ag¹⁺(aq) +  NO₃^1-(aq)
                                                                                 NaCl(aq)  -->   Na¹⁺(aq) +  Cl^1-(aq)

PRODUCTS
Using a solubility table, it could be determined that silver chloride will form the precipitate AgCl(s)
            and silver nitrate will remain in solution.  NaNO₃(aq)  -->   Na¹⁺(aq)  +  NO₃^1-(aq)

Ag¹⁺(aq) +  NO₃^1-(aq)  +  Na¹⁺(aq) +  Cl^1-(aq)  -->  AgCl(s)  +  Na¹⁺(aq)  +  NO₃^1-(aq)

[This is called an ionic equation] 

Ag¹⁺(aq)  +    Cl^1-(aq)  -->  AgCl(s)         [This is called the net ionic equation]    

This is an equation showing the only reaction that is taking place, and is called the net ionic reaction. 
The species represented are called the predominant reacting species. 

Those ions not taking an active part in the reaction are called spectator ions (Na⁺ and Cl^-).

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