A hydrocarbon is a molecule that is composed of only hydrogen and carbon atoms. These molecules are traditionally a major focus of study in organic chemistry, the branch of chemistry that focuses on chemical compounds involving carbon. They can be described as pure or impure. A pure hydrocarbon is truly composed of only carbon and hydrogen atoms, while an impure one contains carbon and hydrogen atoms bonded to other atoms, such as nitrogen or sulfur.
There are many different forms of hydrocarbons, based on the number and arrangement of atoms. Some molecules are simply long chains of carbon and hydrogen that can contain hundreds of atoms, while others are arranged in elaborate rings and other shapes. Even the bonds between individual atoms in organic molecules can be different from each other, and many of the atoms are joined together by multiple bonds. Some organic molecules, such as fatty acids, are classified as saturated or unsaturated, based on the numbers of bonds between the atoms.
While there are many different uses for such molecules, fuels are by far the most prevalent. The bonds in many hydrocarbons contain a significant amount of energy that can be released when they are burned. Fossil fuels are made up of decayed organic matter, which is composed largely of molecules made up of hydrogen and carbon. Unfortunately, fossil fuels such as coal are considered nonrenewable resources since it can take organic matter millions of years to decay. Burning such fuels also releases gases into the atmosphere that can be harmful to the environment and to humans.
Hydrocarbon fossil fuels come in two primary forms: liquid fuels, known as petroleum or mineral oil, and gaseous fossil fuels, known as natural gas. Coal is a solid fossil fuel. All are very important because of their high energy content, but none are renewable as they are all formed through decomposition. As such, scientists are always seeking alternatives to fossil fuel energy, such as wind or solar energy.
The number of atoms and bonds included in a given hydrocarbon is usually closely related to its use. Those with few atoms and bonds are seldom used for more than simple heating or cooking fuels, while larger molecules are used for diesel fuels and even jet fuels. Some of these molecules can also be used as lubricants, though they share many common characteristics with fuels.