Chemical Reactions

Formation of a Precipitate in a double-displacement reaction

A precipitate forms in a double-displacement reaction when the cations from one of the reactants combine with the anions from the other reactant to form an insoluble ionic compound. 

For example,when aqueous solutions of potassium iodide and lead (II) nitrate are mixed,  a brilliant yellow precipitate of lead iodide is formed while,the other product of the reaction, potassium nitrate, remains soluble in the solution.
The equation can be written as follows
            
2KI(aq)+Pb(NO₃)₂(aq)  \( \to \)  2KNO₃(aq)+PbI₂(s)

Formation of a Gas in a double-displacement reaction

Some double-displacement reactions produce a gaseous product which then bubbles out of the solution and escapes into the air.

For example,when the solutions of sodium sulfide and hydrochloric acid are mixed, the products of the reaction are aqueous sodium chloride and hydrogen sulfide gas.
The equation can be written as follows

Na₂S(aq)+2HCl(aq)  \( \to \)   2NaCl(aq)+H₂S(g)