When you mix Epsom salts and rubbing alcohol together, some of the salt may dissolve. The amount that dissolves depends on the composition of the rubbing alcohol. Rubbing alcohol is a solution of water and denatured alcohol -- isopropyl alcohol, ethanol or a mixture of these two. Common household formulas are generally no more than 70 percent alcohol. Epsom salts are a hydrated form of a mineral salt called magnesium sulfate heptahydrate (MgSO4 * 7H20), which remains inert when mixed with isopropyl alcohol but will dissolve to a small extent in ethanol. Epsom salts dissolve very easily in water.
Anhydrous Magnesium Sulfate and Isopropyl Alcohol
Baking Epsom salts at a high temperature will remove water from the salt, creating anhydrous magnesium sulfate. In the laboratory, anhydrous magnesium sulfate is used as a drying agent. When added to a solution of 70 percent isopropyl alcohol, magnesium sulfate quickly rehydrates, absorbing molecules of water out of the solution. Hydrated magnesium sulfate crystals settle to the bottom of the solution, leaving behind a higher concentration of isopropyl alcohol.