A double displacement reaction is a type of reaction in which two reactants exchange ions to form two new compounds. Double displacement reactions typically result in the formation of a product that is a precipitate. Double displacement reactions may be classified into several categories, including counter-ion exchange, alkylation, neutralization, acid-carbonate reactions, aqueous metathesis with precipitation (precipitation reactions), and aqueous metathesis with double decomposition (double decomposition reactions). A double replacement reaction will occur between two solids, resulting in a colour change. The lead ions combine with the iodide ions to make the yellow compound of lead iodide. The potassium ions combine with the nitrate ions to make the compound potassium nitrate which is white and cannot be seen because the yellow compound covers up the white compound.