Compounds made only of carbon and hydrogen are called hydrocarbons. The simplest of them is methane, \mathrm{CH_4}.
In methane, the carbon atom is at the centre of a regular tetrahedron and is bonded to 4 hydrogen atoms at its corners. The angles between the bonds are all about 109.5°.
Methane is a colourless, odourless gas and is the main component of natural gas. It is also produced by microorganisms that break down organic matter where there is no air, for example in marshes, in rice fields and in the stomachs of cattle.
Hydrocarbons whose carbon atoms form chains joined only by single bonds are called alkanes. After methane come ethane, propane and butane, each with one more carbon atom.
Each alkane has one more \mathrm{CH_2} than the one before, and the general formula of the alkanes is \mathrm{C}_n\mathrm{H}_{2n+2}.
A series of compounds like this, which have the same general formula and similar properties, is called a homologous series.
The more carbon atoms an alkane has, the higher its boiling point. This is because larger molecules attract one another more strongly through intermolecular forces.
At room temperature, alkanes with 1 to 4 carbon atoms are gases, those with about 5 to 16 are liquids, and larger ones are solids. Paraffin wax, used to make candles, is a mixture of solid alkanes.
Alkanes hardly dissolve in water and are less dense than water, so liquid alkanes float on water.
Alkanes do not react easily, but when a mixture of methane and chlorine is exposed to light, a hydrogen atom of methane is replaced by a chlorine atom.
\mathrm{CH_4} + \mathrm{Cl_2} \longrightarrow \mathrm{CH_3Cl} + \mathrm{HCl}
A reaction in which an atom of a molecule is replaced by another atom or group of atoms is called a substitution reaction. As the reaction goes on, more hydrogen atoms are replaced, giving dichloromethane, trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride) in turn.
Alkanes burn and give off a great deal of heat, so they are used as fuels. Natural gas, which is mainly methane, is supplied through gas pipes, and liquefied petroleum gas (LPG), which is mainly propane and butane, is supplied in cylinders.
\mathrm{CH_4} + 2\mathrm{O_2} \longrightarrow \mathrm{CO_2} + 2\mathrm{H_2O}
\mathrm{C_3H_8} + 5\mathrm{O_2} \longrightarrow 3\mathrm{CO_2} + 4\mathrm{H_2O}
Methane is less dense than air, while propane and butane are denser than air. So an alarm for natural gas leaks is fitted near the ceiling, and an alarm for LPG leaks is fitted near the floor.