CID 1118

H₂SO₄

Sulfuric acid

Q: Why is sulfuric acid a strong acid?

Its first ionisation in water is complete: OpenStax gives Kₐ₁ as more than 10², so every molecule hands one proton to water as H₃O⁺. The second step is weaker, with Kₐ₂ = 1.2 × 10⁻². The hydrogen sulfate ion left behind, HSO₄⁻, is only about 25 per cent ionised in a solution of one of its salts.

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7 atoms, 6 bonds, tetrahedral.

IUPAC name: sulfuric acid
Molar mass: 98.072(24) g/mol
Atoms: 7
Shape: tetrahedral
SMILES: OS(=O)(=O)O
InChIKey: QAOWNCQODCNURD-UHFFFAOYSA-N
PubChem CID: 1118

Made of

H S O

Structural formula of sulfuric acid. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Oil of vitriol was the name in use when Joseph Priestley, in a method dating to 1767, reacted it with chalk to make the carbon dioxide he dissolved in water. PubChem’s record describes a dense, oily liquid, colourless to dark brown depending on purity. The IUPAC name is simply sulfuric acid, and the molar mass is 98.07 g/mol.

Sulfur here carries an oxidation number of +6, and OpenStax gives that as the reason the acid is stronger than sulfurous acid. A higher oxidation number pulls harder on the electrons sulfur shares with oxygen, which weakens the O–H bond. The acid is diprotic: each molecule holds two hydrogen atoms that can leave as ions, in two separate steps.

The concentrated acid also has a great affinity for water, taking it from the air and from many organic substances, and that is why it chars sugar and wood.

Structure and bonding

Sulfuric acid’s sulfur atom is surrounded by four oxygen atoms, two of them carrying a hydrogen. OpenStax calls this molecular structure tetrahedral, and the four oxygens do point towards the corners of a tetrahedron. The corners are uneven: in this model the two doubly bonded oxygens open to 120.6° at the sulfur, while the two OH oxygens close to 103.0°.

Two sulfur–oxygen bonds are drawn as double bonds and two as single. The Stick view strips away the atom spheres, which makes the bond orders easiest to count, and Labels prints S, O or H on each atom. In the model the double bonds are shorter, 1.45 Å against 1.63 Å for the bonds to the OH groups. Space-filling swells every atom to full size, so the bonds vanish inside a compact knot of four oxygens around the sulfur.

The drawing is the neutral molecule, and water does not leave it intact. The first hydrogen leaves completely as a hydronium ion, H₃O⁺. The hydrogen sulfate ion left behind, HSO₄⁻, gives up its second hydrogen only in part.

Where it turns up

One everyday container is the lead acid battery under a car bonnet, the rechargeable type OpenStax says is commonly used in cars, with dilute sulfuric acid as its electrolyte. NIOSH, the US federal institute for occupational safety and health, lists “battery acid” among its names. PubChem’s consumer product data count 136 products containing it. Some household cleansers rely instead on sodium hydrogen sulfate, NaHSO₄, one of its salts. In chip factories the acid, at a standard 96 per cent, is a wet etchant, a liquid that dissolves away thin layers of material.

Outdoors, OpenStax traces some acid rain to sulfur dioxide reacting with water vapour and going on to form sulfuric acid. Jefferson Lab, on PubChem’s sulfur record, names sulfurous acid, H₂SO₃, from the same gas as a major component. Sulfuric acid and particulate sulfates in the air, the Hazardous Substances Data Bank records, discolour and wear away limestone, marble, roofing slate and mortar. Volcanic gas is a natural source.

Why it matters

OpenStax states that industry uses more sulfuric acid than any other manufactured compound. US makers reported 65 to 70 billion pounds for 2023 to the Environmental Protection Agency (EPA). On the US Geological Survey’s 2025 summary, about 90 per cent of US sulfur consumption in 2024 was as sulfuric acid. The contact process makes it, in outline, by burning sulfur to sulfur dioxide, oxidising that to sulfur trioxide over a vanadium pentoxide catalyst, and absorbing the trioxide into strong acid. It has almost completely replaced the older lead chamber processes.

On the Toxin and Toxin Target Database (T3DB) estimate, about 60 per cent goes to fertilisers, particularly superphosphates, ammonium phosphate and ammonium sulfates. Another breakdown on PubChem’s record gives phosphoric acid and fertiliser 32 per cent and 58 per cent as a basic chemical for synthesis, pigments and the oil industry. One commercial route to phosphoric acid, OpenStax notes, treats calcium phosphate rock with the concentrated acid. Hot and concentrated, it is an oxidising agent that attacks bromide and iodide, so it cannot make hydrogen bromide or iodide.

Properties

Properties of sulfuric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1118.
Property	Value	Unit	Source
Molecular formula (Hill order)	H₂O₄S
Molar mass	98.072(24) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	2
Sulfur atoms	1
Oxygen atoms	4
Bonds	6
Single bonds	4
Double bonds	2
PubChem molecular weight	98.08 g/mol	g/mol	PubChem

Safety

Sulfuric acid carries the Globally Harmonized System (GHS) signal word Danger, and nearly every notification to the European Chemicals Agency classes it as causing severe skin burns and eye damage. On the NIOSH card, the organs at risk are the eyes, skin, respiratory system and teeth. The International Chemical Safety Card calls mists of this strong inorganic acid carcinogenic to humans, a finding about mists rather than the liquid. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card are the sheets to consult.

Questions

What shape is a sulfuric acid molecule?

OpenStax calls the molecular structure tetrahedral: one sulfur atom in the middle with four oxygen atoms around it. Two of those oxygens also hold a hydrogen each. The tetrahedron is lopsided rather than regular. OpenStax also writes the formula as O₂S(OH)₂, which shows the two OH groups sitting on the sulfur beside two bare oxygens.

Why does sulfuric acid turn sugar black?

Concentrated sulfuric acid has so strong an affinity for water that it pulls water out of the air and out of many organic substances. That pull on water is what chars sugar and wood, turning them black. OpenStax names the same property when it calls the concentrated acid a good dehydrating agent, one that can dry gases passed through it. Mixing it with water gives off much heat, and the combined volume shrinks.

Why is sulfuric acid a strong acid?

Its first ionisation in water is complete: OpenStax gives K_a1 as more than 10², so every molecule hands one proton to water as H₃O⁺. The second step is weaker, with K_a2 = 1.2 × 10⁻². The hydrogen sulfate ion left behind, HSO₄⁻, is only about 25 per cent ionised in a solution of one of its salts.

Why do car batteries use sulfuric acid?

The lead acid battery is the rechargeable type commonly used in cars, and its liquid electrolyte is dilute sulfuric acid. OpenStax notes the drawbacks: the battery is heavy and the liquid inside is caustic. It is often still the battery of choice because it delivers a high current density. Batteries take only a small slice of the acid made, 0.8 per cent in one breakdown on PubChem's record.

Same shape (tetrahedral)

CCl₄ Carbon tetrachloride 5 atoms, 4 single bonds, tetrahedral.
CHCl₃ Chloroform 5 atoms, 4 single bonds, tetrahedral.
CCl₂F₂ Dichlorodifluoromethane 5 atoms, 4 single bonds, tetrahedral.
CH₂Cl₂ Dichloromethane 5 atoms, 4 single bonds, tetrahedral.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.

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An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.