A CHEMICAL REACTION TAKES PLACE,whenever bonds between atoms are broken or made.In each case , atoms or groups of atoms rearrange , making new substances (products), from the original ones(reactants). Reaction happens naturally or can be made to happen ;they may take years or only an instant .A reaction usually involves a change in energy.In a burning reaction , for example , the making of new bonds between atoms release energy as heat and light.This type of reaction in which heat is given off,is an exothermic reaction .Many reactions , like burning is irreversible, but some can take place in either direction,and are said to reversible . Reactions can be used to form solids from solutions ;in a double decomposition reaction , two compounds in solutions break down and reform into two substances , often created a precipitate(insoluble solid)