Oleum (Latin oleum = "oil"), or fuming sulfuric acid refers to a solution of various compositions of sulfur trioxide in sulfuric acid or sometimes more specifically to disulfuric acid (also known as pyrosulfuric acid).
Oleums can be described by the formula ySO3.H2O where y is the total molar sulfur trioxide content. The value of y can be varied, to include different oleums. They can also be described by the formula H2SO4.xSO3 where x is now defined as the molar free sulfur trioxide content. Oleum is generally assayed according to the free SO3 content by weight.
A value for x of 1 gives us the empirical formula H2S2O7 for disulfuric (pyrosulfuric) acid. Pure disulfuric acid itself is a solid at room temperature, melting at 36 °C and rarely used either in the laboratory or industrial processes.
