Hydrocarbons that have one triple bond between carbon atoms in a chain are called alkynes, and their general formula is \mathrm{C}_n\mathrm{H}_{2n-2}. The simplest alkyne is ethyne, \mathrm{C_2H_2}, which is usually called acetylene.
The 4 atoms of acetylene lie on a straight line.
Acetylene is a colourless gas. Pure acetylene has almost no smell, but acetylene made from calcium carbide has an unpleasant smell because of impurities.
Acetylene is made by adding water to calcium carbide.
\mathrm{CaC_2} + 2\mathrm{H_2O} \longrightarrow \mathrm{C_2H_2} + \mathrm{Ca(OH)_2}
The reaction is vigorous, so water is dripped in a little at a time from a dropping funnel. Saturated salt solution is sometimes used instead of water to make the reaction gentler. Acetylene dissolves only slightly in water, so it is collected over water.
Lamps that burned acetylene made in this way were once used by miners and on bicycles.
In air, acetylene burns with a bright, smoky flame, because it contains a high proportion of carbon.
2\mathrm{C_2H_2} + 5\mathrm{O_2} \longrightarrow 4\mathrm{CO_2} + 2\mathrm{H_2O}
When acetylene is burned in pure oxygen, the flame reaches more than 3000 °C. This oxyacetylene flame is used for welding and cutting metals.
Like ethene, acetylene undergoes addition reactions. With a catalyst, one molecule of hydrogen adds to it to give ethene, and a second gives ethane. Hydrogen chloride adds to it to give chloroethene (vinyl chloride), the monomer of poly(vinyl chloride).
\mathrm{C_2H_2} + \mathrm{HCl} \longrightarrow \mathrm{CH_2{=}CHCl}
When water adds to acetylene with a catalyst, ethenol (vinyl alcohol) forms first, but it is unstable and quickly changes into ethanal (acetaldehyde).
\mathrm{C_2H_2} + \mathrm{H_2O} \longrightarrow \mathrm{CH_3CHO}
Comparing ethane, ethene and ethyne, the more bonds there are between the two carbon atoms, the closer together the atoms are: the distance between them is about 0.154 nm, 0.134 nm and 0.120 nm.
The shape around the carbon atoms changes too, from tetrahedral in ethane, to flat in ethene, to a straight line in ethyne.
Ethane, which has only single bonds, undergoes substitution reactions, while ethene and ethyne, which have a double or triple bond, undergo addition reactions and turn bromine water colourless.