Chemical changes are those changes that result from a chemical reaction. A chemical reaction involves the breaking and/or formation of a chemical bond. Anytime chemical bonds change, the new chemicals that are produced have their own unique set of properties. The change from old to new is what we observe and refer to as a chemical change.
There are many different ways that a chemical reaction can occur. The one constant that is involved in them all is energy. Energy is required for a chemical reaction to take place. The energy needed to start a reaction is called the “activation energy”. Beyond that, individual reactions proceed in different ways. By no means will this be an exhaustive list, but a number of causes of chemical changes occur are provided below.
Light is a form of energy, and when it strikes a chemical, that energy can be absorbed. If the light provides enough energy, it may break a chemical bond. Some bonds are weaker than others, and break more readily on exposure to light. Chemicals that react easily when exposed to light are called “photo-sensitive”. Light is more energetic the shorter the wavelength, so while many chemicals are stable when exposed to visible light, ultraviolet (UV) light, x-rays, or gamma rays can break bonds more easily. If you’ve ever left something colorful out in the sun only to see it fade, it’s because UV from the sun has broken bonds in the dye molecules.
Heat too, is energy. Given enough heat, any chemical bond will break. Some chemicals are heat sensitive, and will break down even at room temperature. Others remain stable even at temperatures of hundreds of degrees. Heat also commonly provides the activation energy needed to allow two atoms to form a new bond.
When atoms, molecules, and/or ions collide, there is kinetic energy – the energy of motion – involved. This can supply the activation needed for bond breaking and/or formation. Since two atoms must be near one another to form a bond, collisions are considered a necessary part of every bond-forming reaction. Energy may still be acquired from other sources, when kinetic energy alone is insufficient. Odds are that you’ve seen a precipitation reaction in a beaker before, when two solutions are mixed, producing an insoluble product. The precipitate only forms when the two ions that make it up collide within the solution. At high concentrations, you can’t observe this well, but when more dilute solutions are mixed, the precipitate forms more slowly, as it takes time for random collisions to pair up the ions.
Sometimes outside influences change reaction conditions. Catalysts themselves are unchanged by a chemical reaction, but they can lower the activation energy required for a reaction to occur in another molecule. Catalysts vary in nature from biologic (enzymes) to metals. They usually function by adding or withdrawing electron density from the molecule of interest, either weakening an existing bond or making it easier to form the new bond. Enzymes often work to position the two reactants in the correct orientation for reaction, further helping to speed up the reaction process.
These are just a few of the driving forces behind chemical changes, but by now you’ve seen that energy is the central theme. If energy is not available, chemical changes do not occur. This is something that people are familiar with – we store perishable foods in cold, dark places. This reduces the energy available to power those chemical functions of life, keeping bacteria from spoiling tomorrow’s dinner.