The specifications of a product refer to its properties and its purity. Before a drug can be tested or sold, it must meet product quality specifications. One of the most challenging aspects of chemical development is dealing with impurities observed in the final product. If an impurity is known to occur as a result of a side reaction, it may be possible to minimize its occurrence by varying the reaction conditions. Impurities sometimes occur depending on the order in which reagents are added to the reaction vessel. In some reactions, impurities are formed as a natural result of the reaction mechanism. If these are difficult to remove, it may be necessary to change the reaction or the reagent such that less troublesome by-products are formed. Sometimes an impurity may prove so troublesome that it may be necessary to develop a completely different route.