Compression of gasses

Any compound that is a vapor at room temperature but has a boiling point near room temperature will be easier to compress than air. 1,2-butadiene, a gas at room temperature and pressure, has a boiling point of about 10C. At room temperature it is almost "compressing" to a liquid by itself. Butane is another example. Butane is so easy to compress that it can be contained as a liquid in a small plastic container (Bic). Some of the Freons (chlorofluorocarbons) are easier to compress as well.

What I have described are compounds that do not behave like an ideal gas near room temperature. At higher temperatures, these compounds behave more like ideal gases although not exactly like them.