When several chemicals are mixed together to form gunpowder
or dynamite, a violent explosion can occur if care is not taken
to prevent this. An explosion can do work against the force of
gravity, for example, by throwing pieces of material into the
air. A mixture of chemicals that can do work is said to have chemical
energy.
Molecules are formed in chemical reactions. Some molecules
give off a great deal of energy when they are formed from individual
atoms. Such molecules are very stable because all that energy
must be put back into them before they decompose. Other molecules
release very little energy when they are formed. Such molecules
are very unstable. They react easily to form more stable molecules.
During these reactions much energy is given off. Nitroglycerin--a
dense, oily liquid--changes readily to water, carbon dioxide,
nitrogen, and oxygen. This reaction is explosive because it occurs
very rapidly and because the suddenly formed gases take up much
more room than did the liquid nitroglycerin. Other chemical reactions
can produce energy but not be explosive. They may occur more slowly,
and the resulting molecules may take up the same amount of room
as the original molecules.
Food energy is a form of chemical energy. Plants absorb energy
from sunlight and store it in energy-rich chemicals, such as glucose.
This process is called photosynthesis. Animals that eat plants
use the chemicals created by photosynthesis to maintain life processes.
Other animals may eat plant-eating animals to gain the energy-rich
chemicals that the plant-eaters formed from the chemicals of plants.
