The chemical formula for oil of vitriol, also known as sulfuric acid, is H2SO4. It is made up of the three elements hydrogen (H), sulfur (S), and oxygen (O). This is one of the most potent acids ever discovered. It’s also a dangerous dehydrator, as it dissolves quickly in water and in any concentration. Sulfuric acid solubility in water is so energetically favored that it could even cause death or injury. Its high polarity, ease of ionization, stabilization through hydrogen bonding, and the symmetry of its anion all contribute to its high solubility in water.
“Like dissolves like” refers to the fact that water, a highly polar solvent, will dissolve other polar substances. The dielectric constant of a substance, or the charge handling capability of a material, is closely related to polarity. The dielectric constant of 100% sulfuric acid is around 100, which is higher than the dielectric constant of water, which is around 80. This constant specifies the amount of charge reduction a substance can achieve when a particle is completely engulfed within it. This process clearly favors sulfuric acid solubility energetically.
Sulfuric acid’s ionization is so strong that it can do it to itself, a process known as auto-protolysis. The reaction is 2 H2SO4 H3+SO4 + HSO4 mechanistically. Because ionization is the process of breaking down a substance into its charged constituents, sulfuric acid solubility in water is not only complete, but also quick. The H3+SO4 component ionizes even more.
Hydrogen bonding is a type of “weak” bonding between electropositive hydrogen atoms and specific electronegative atoms such as oxygen, nitrogen, and fluorine. The negative ion, or anion, of sulfuric acid has a sulfur atom attached to four oxygen atoms. As a result, the structure has a lot of potential for forming strong and multiple hydrogen bonds. This type of bonding gives anions more stability, and once formed, it affects ion recombination. The process is energetically unfavorable, which provides yet another guarantee of sulfuric acid solubility.
SO42 is the anion of completely ionized sulfuric acid. The two charge electrons can easily reside on any of the four oxygen atoms at the perimeter because this ion is highly symmetrical. This resonance stability, which is part of the sulfuric acid solubility package, reduces the tendency of ion recombination even more. The surrounding water molecules that hydrogen-bond to this already large anion greatly increase its size. This spreads the minor negative charge across the hydrated anion’s much larger outer surface area, and can be compared to the hydration and hydrolysis of positive metal cations.