Difference Between Sulfuric Acid and Sulfurous Acid

Key Difference – Sulfuric Acid vs. Sulfurous Acid


Sulfuric acid (H2SO4) and sulfurous acid (H2SO3) are two inorganic acids containing sulfur, hydrogen, and oxygen as elements. The key difference between sulfuric acid and sulfurous acid is in the oxidation number of Sulfur. Moreover, when we compare two acids in terms of acidity, sulfuric acid is more acidic than sulfurous acid. In other words, sulfuric acid is a very strong acid, and sulfurous acid is relatively weak.

What is Sulfuric Acid?

Sulfuric acid is a very strong diprotic mineral acid that is completely miscible with water in any proportion. Dissolving sulfuric acid in water is an exothermic reaction. It is a corrosive and harmful liquid and causes so many injuries such as acid burns on the skin or eyes. This includes both short and long term effects depending on the concentration of the acid and the contact time. Sulfuric acid is highly corrosive due to several factors; acidity, oxidizing ability, dehydration caused by concentrated solutions and the heat released by the exothermic reaction.

Difference Between Sulfuric Acid and Sulfurous Acid

What is Sulfurous Acid?

Sulfurous acid has the chemical formula of H2SO3, where oxidation number of sulfur is equal to +4. It is a clear, colorless, weak and unstable acid. It has a pungent burning sulfur odor. It forms when sulfur dioxide dissolves in water and the pure anhydrous form of sulfurous acid has never been isolated or detected. Sulfurous acid decomposes and dissociates into its chemical constituents quickly; since it is thermodynamically unstable. The decomposition reaction is,

H2SO3(aq)  →  H2O(l)   +   SO2 (g)

Key Difference - Sulfuric Acid vs Sulfurous Acid

What is the difference between Sulfuric Acid and Sulfurous Acid?

Structure and Chemical Formula:

Sulfuric Acid: The chemical formula of sulfuric acid is H2SO4 where the oxidation number of sulfur is +6. The geometrical structure of this molecule is tetrahedral.

Sulfurous Acid: The chemical formula of sulfuric acid is H2SO3 where the oxidation number of sulfur is +4. The geometrical structure of this molecule is trigonal pyramidal.


Sulfuric Acid: Sulfuric is one of the strongest acids, and it is a diprotic acid. Acid dissociation constants of sulfuric acid are; K1 = 2.4×106(strong acid)and K2 = 1.0×10−2.

Sulfurous Acid: The acidity of sulfurous acid on the pH scale in equals to 1.5. It is not considered as a very strong acid, but it is not a too weak acid too. 


Sulfuric Acid: Sulfuric acid has both oxidizing and reducing characteristic in addition to its acidic properties. Therefore, it reacts with both metals and non-metals; it reacts as other acids with metals producing Hydrogen gas and the relevant salt of the metal.

Reactions with metals:

Fe (s) + H2SO4 (aq) → H2 (g) + FeSO4 (aq)

Cu + 2 H2SO4 → SO2 + 2 H2O + SO42− + Cu2+

Reactions with non-metals:

C + 2 H2SO4 → CO2 + 2 SO2 + 2 H2O

S + 2 H2SO4 → 3 SO2 + 2 H2O

Sulfurous Acid: Sulfurous acid does not exist as a solution in anhydrous formlike sulfuric acid. However, there is evidence that sulfurous molecules exist in gaseous phase. Unlike H2SO4, Sulfurous acid shows a very limited number of chemical reactions.

CaCO3(s) + H2SO3(aq) → CO2(g) + H2O(l) + CaSO3(aq)


Sulfuric Acid: Sulfuric acid is widely used all over the world in a wide range of industries. For example, it is used to produce fertilizers, explosives, papers, detergents, dyes and coloring materials. In addition, it is very important in chemical synthesis, surface treatments, petroleum, and textiles industry.

Sulfurous Acid: Sulfurous acid is used as a very strong reducing agent. It has some bleaching properties and used as a bleaching agent.


Image Courtesy:

1. Sulfuric acid chemical structure By DMacks (Own work) [Public domain], via Wikimedia Commons

2. Sulfurous-acid-2D By Benjah-bmm27 (Own work) [Public domain], via Wikimedia Commons