Which explains a process where a precipitate is formed?
1. Propane gas combusts to form CO2 and H2O.
2. A solution of H2SO4 mixes with a solution of NaOH to produce water and Na2SO4.
3. A solution of HCl mixes with a solution of Ca (OH)2 to produce H2O and CaCl2.
4. A solution of AgNO3 is added to a solution of CaCl2 to produce solid AgCl and a solution of CaNO3.