Scientifically, energy is defined as the ability to do work. While there are many forms of energy, they can be grouped into two categories: potential energy, or stored energy; and kinetic energy, or energy of motion. Chemical energy is a form of potential energy and it is possessed by things such as food, fuels and batteries.
Within each category of energy, there are many different forms of energy. Chemical energy is one form of potential energy, along with mechanical energy, gravitational energy, nuclear energy and electrical energy. All of these forms of energy are stored within an object and are converted to forms of kinetic energy when a force or change is applied. The different forms of kinetic energy are radiant energy, which includes light, x-rays and radio waves, heat, motion and sound.
As stated by the first law of thermodynamics, energy can neither be created nor destroyed; it can only be converted from one form to another. During chemical reactions, molecules can be created or destroyed. If a product is created, the chemical energy is stored in the bonds that make up the molecules. If something is broken down, the chemical energy is released, usually as heat. If a reaction releases energy, it is called exothermic, and if it absorbs energy, it is called endothermic.
One example of chemical energy is that found in the food that we eat. Energy is stored in the bonds of the molecules that make up food. When we eat the food, the large molecules are broken down into smaller molecules that can be used by the cells of the body. The process of breaking down and using the food by our cells is called respiration. During respiration, the chemical energy is converted to heat, kinetic energy, and other forms of chemical energy, like that stored in the fat cells in our body.
Food is just one example of a fuel — it is how animals, including humans, fuel their bodies. Other forms of fuel include wood and chemicals, such as petroleum. When wood is burned, the chemical energy within the cells of the wood break and heat is released. In the engine of a car or truck, the energy in the gasoline is converted to heat and motion, to make the car move.
Kinetic energy can also be converted to potential energy. During the process of photosynthesis carried out by plants, radiant energy, or the light from the sun, is converted into chemical energy, which is stored within the plant. When animals eat the plants, the reverse reaction takes place. The bonds are broken, which releases the stored chemical energy for the animals to use.