Complex sentence: At room temperature, propene is an colourless, odourless gas, though when used as a fuel, it is mixed with minute quantities of foul-smelling sulfurous compounds (mercaptans) so that gas leaks can be readily detected.
Split into simpler sentences: At room temperature, propene is an colourless, odourless gas. When used as a fuel, it is mixed with minute quantities of foul-smelling sulfurous compounds (mercaptans) so that gas leaks can be readily detected.