Both of the above answers r correct, but i think they r still missing an important part of double replacement. In double replacement, the charge in each separate ion stays the SAME AFTER the reaction has occured. This is because there is no oxidation-reduction occuring. Example: AgNO3 + NaCl ---> AgCl + NaNO3 Ag on the left is +1, on the right it is also +1 NO3 on the left is -1, same on the right Na on the left is +1, same on the right Cl on the left is -1, same on the right The easier part of double replacement is just flip flopping the metal and nonmetal, but when you are given the reactants and told to write the products, knowing that the charges stay the same is very important.
A double replacement is AB + XY ---> AY + XB Example FeS + HCl ---> FeCl2 + H2S Where Fe=A S=B H=XCL=Y
Think of the metal and nonmetal part of the compounds as letters.. AB + CD -----> AD + CB The compound just change partners ;-) so as an example I'll use an neutralization which in this case is a Double Replacement reaction sodium carbonate + hydrogen chloride-----> sodium chloride + hydrogen carbonate
