Sulfuric Acid
Formula: H₂SO₄ — Oil of vitriol, battery acid
Appearance: Colorless oily liquid (concentrated); clear solution (dilute)
Hazard: Corrosive · Severe burns · Highly exothermic with water
Properties
Strong diprotic acid — fully dissociates its first proton in water; the second proton (pKa₂ = 1.99) dissociates almost completely in dilute solution. The concentrated acid (98%) is a dense, oily liquid (density 1.84 g/mL) with powerful dehydrating properties — it removes water from organic compounds, leaving carbon. Dissolving concentrated H₂SO₄ in water is violently exothermic (~880 kJ/mol); always add acid to water, never the reverse. Dilute solutions (10–30%) behave as a typical strong acid and are far safer. The sulfate ion is the counterion for many common salts (copper sulfate, iron sulfate, zinc sulfate). Used as the electrolyte in lead-acid batteries.
Historical Context
“Oil of vitriol” — concentrated sulfuric acid — was described by the 8th-century Arab alchemist Jabir ibn Hayyan and later by European alchemists, who produced it by dry distillation of iron sulfate (green vitriol). The name “vitriol” referred to various metal sulfates; blue vitriol is copper sulfate, white vitriol is zinc sulfate, and the “oil” was the acid derived from their distillation.
The industrial production of sulfuric acid was central to the 18th-century chemical revolution. The lead-chamber process (1746) enabled mass production for the first time, making sulfuric acid cheap enough to use as a commodity chemical. By the 19th century, a nation’s sulfuric acid production was considered a direct measure of its industrial power. The contact process, developed in the 1870s, further reduced costs and improved purity.
Today, sulfuric acid remains the world’s most produced chemical — roughly 250 million tons per year, used primarily for fertilizer production (converting phosphate rock to soluble superphosphate).
Experiments
Sugar Dehydration: Place a small amount of sucrose (table sugar) in a heat-resistant container in a fume hood. Carefully add a few mL of concentrated sulfuric acid. The acid dehydrates the sucrose instantly — C₁₂H₂₂O₁₁ → 12C + 11H₂O — and the heat from the reaction causes the water to steam and the carbon to foam upward dramatically as a black, porous column. A vivid demonstration of dehydration chemistry. Performed with concentrated acid; strictly adult supervision required.
Electrochemistry Electrolyte: Dilute sulfuric acid (10–15%) is the standard electrolyte for water electrolysis demonstrations, providing excellent conductivity. Connect a 9V battery to carbon or platinum electrodes immersed in the solution — hydrogen gas evolves at the cathode (2H⁺ + 2e⁻ → H₂), oxygen at the anode (2H₂O → O₂ + 4H⁺ + 4e⁻). Collect and compare volumes (2:1 ratio, hydrogen:oxygen).
Acid Dilution: Demonstrate the exothermic dilution of concentrated acid safely (teacher demonstration only): add a few drops of concentrated acid to a large volume of water in a beaker on a stir plate. Measure temperature rise. Then incorrectly add water to acid in a smaller container to demonstrate the danger — the localized heating can cause spattering.
Experiments using this chemical:
- Water Electrolysis — Conducting electrolyte for H₂ and O₂ generation
Safety
High hazard — severely corrosive; causes immediate, deep tissue burns on contact.
Always add acid to water, never water to acid — adding water to concentrated acid causes violent spattering as steam and heat are generated locally. Wear gloves, eye protection, and a lab coat when handling concentrated solutions. Work with ventilation — hot concentrated acid releases SO₃ fumes. Dilute solutions (≤10%) are far safer but still corrosive to eyes. Spills: neutralize immediately with sodium bicarbonate, then flush with large amounts of water.
Incompatible with: Water — concentrated acid reacts violently, never add water to concentrated acid; bases (vigorous exothermic neutralization); reactive metals — zinc, iron, magnesium (H₂ evolved; dilute H₂SO₄ reacts moderately, concentrated H₂SO₄ reacts differently, producing SO₂); organic materials (dehydration and possible ignition with concentrated acid); potassium permanganate (explosive mixture); sodium chlorate; chlorates