A single displacement reaction is a chemical reaction in which one element is substituted for another element in a compound, generating a new product. For example, 2HCl(aq)+Zn(s)→ZnCl2(aq)+H2(g) is an example of a single-displacement reaction. The hydrogen atom in HCl is replaced by Zn atoms, and in the process hydrogen gas is formed. A double-displacement reaction occurs when parts of two ionic compounds are exchanged, making two new compounds. A characteristic of a double-displacement equation is that there are two compounds as reactants and two different compounds as products. An example is CuCl2(aq)+2AgNO3(aq)→Cu(NO3)2(aq)+2AgCl(s)