Therefore, the words vapor and gas are often used interchangeably. Vapor is more frequently used for a substance which, although present in the gaseous phase at 20⁰C, exists as a liquid or solid at this temperature and normal atmospheric pressure. We can also say that SO₂ (157⁰C), Cl₂ (144⁰C), NO₂ (158⁰C), and CO₂ (289⁰C) can be obtained as “vapor” since at 20⁰C they can be condensed as a liquid by increasing the pressure. Their critical temperature (given in parenthesis) is above 20⁰C. However, since they are normally purchased in steel cylinders under pressure we usually refer to them as gases.