External conditions that affect reactions are usually types of energy that are put into a reaction, such as heat or light. Chemical reactions are always accompanied by an energy change. Either energy is released or it is acquired. When the amount of energy is changed, so is the amount of matter. This is called the Law of Conservation of Matter and Energy. However, ordinary chemical reactions involve such small matter changes that they go undetected and may be ignored.
Heat. Generally, heat is the form of energy we are most concerns us most. It may affect a reaction in one of two ways.
Light. Light is a form of energy that may cause many chemicals to decompose. For this reason, it is necessary to protect some drugs from contact with light by placing them in dark-colored or opaque containers. These containers prevent most or all of the outside light from coming into contact with the drug.