Hydrogen Peroxide

H₂O₂ inorganic

Molar Mass

34.014 g/mol

Properties

State	Liquid (pale blue when pure; colorless when dilute)
Color	Colorless (dilute); pale blue (concentrated)
Solubility	Miscible with water in all proportions
Melting Point	-0.43°C
Boiling Point	150.2°C

About Hydrogen Peroxide

Hydrogen peroxide (H2O2, 34.014 g/mol) is structurally just water with one extra oxygen — but that O-O peroxide bond is the source of all the chemistry. The bond is weak (only 142 kJ/mol versus 498 kJ/mol for the O=O double bond in O2) and homolytically cleaves to give two hydroxyl radicals when triggered by heat, light, or trace transition metals (especially Fe(2+), the basis of Fenton chemistry). That makes H2O2 a versatile two-electron oxidant in basic solution and a one-electron radical source in acidic or metal-catalyzed conditions. The 3% drugstore solution is the form most people know — it's the historical antiseptic, though current wound-care guidance has moved away from it because it damages fibroblasts as much as bacteria. The 30-35% reagent grade is the standard lab oxidant: epoxidation of alkenes (with mCPBA-like chemistry), Baeyer-Villiger oxidations, sulfide-to-sulfoxide conversions, and the workup oxidant for hydroboration (H2O2/NaOH converts R-BH2 to R-OH). At 50-70% concentrations, H2O2 is industrial-scale bleach for paper pulp, textiles, and wastewater treatment — appealing because the only byproducts are water and O2. At 90-98% (high-test peroxide, HTP) it's a rocket monopropellant: a silver-mesh catalyst decomposes it to steam and oxygen at ~600 °C with enough thrust to drive the steam catapults on aircraft carriers and the rocket-belt of 1960s science fairs. Concentrated H2O2 mixed with anything organic is a serious explosion hazard.

Where you'll encounter it

If you've ever poured 3% peroxide on a scraped knee, bleached your hair, watched the elephant-toothpaste demo at a science museum, or driven a fuel-cell car (whose fuel-cell membranes are tested with peroxide-stress protocols), you've encountered H2O2. The brown drugstore bottle of 3% solution has been the household antiseptic in American medicine cabinets for a century, even though current wound-care guidance has moved toward gentler alternatives because peroxide damages fibroblasts as much as bacteria. Hair salons use 6-9% developer mixed with bleach powder to lift melanin out of the cortex during blonde processing. Aseptic juice and milk packagers (Tetra Pak lines) sterilize cardboard cartons with vaporized 35% H2O2 immediately before fill, leaving only water and oxygen behind. And every undergraduate organic synthesis course runs hydroboration-oxidation with H2O2/NaOH workup to convert alkenes into anti-Markovnikov alcohols.

Common Uses

Epoxidation, Baeyer-Villiger, and sulfoxide oxidations in organic synthesis
Workup oxidant for hydroboration-oxidation (R-BR2 → R-OH)
Bleaching agent for kraft paper pulp, cotton textiles, and hair
Disinfectant for medical instruments via vaporized hydrogen peroxide (VHP)
Etchant for printed circuit boards (with sulfuric acid)
Monopropellant for spacecraft thrusters at 90%+ concentrations
Oxygen source in advanced oxidation processes for wastewater treatment

Safety Information

Concentration changes everything. The 3% drugstore solution is mildly irritating; 30% reagent grade causes serious skin burns (immediately rinse with copious water — the burn turns white as oxygen gas forms under the epidermis); 50%+ is corrosive and presents an explosion hazard if it contacts organic matter (paper, wood, cotton, skin). OSHA PEL for H2O2 vapor is 1 ppm (8-hr TWA). GHS for >50%: H271 (oxidizer that may cause fire or explosion), H302+H332 (harmful if swallowed or inhaled), H314 (severe burns), H335 (respiratory irritation). Store at low temperature in vented containers — even reagent-grade slowly decomposes and pressurizes a sealed bottle. Never store near reducing agents, acetone (forms explosive acetone peroxide), or transition metal contaminants. Always dilute in glass or HDPE, never in metal.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of hydrogen peroxide?

H2O2 is 34.014 g/mol — 2 H (2.016) + 2 O (31.998). The 30% reagent-grade stock is roughly 9.8 M with density 1.11 g/mL, so 1 mL of 30% H2O2 gives you about 9.8 mmol of oxidant. For comparison, the 3% drugstore version is 0.88 M — a useful number when calculating how much hydrogen peroxide is in a typical mouthwash or contact-lens cleaner.

What happens when hydrogen peroxide decomposes?

It splits cleanly into water and oxygen: 2 H2O2 → 2 H2O + O2, with a release of 196 kJ per mole of H2O2. The reaction is thermodynamically very favorable but kinetically slow at room temperature in pure solution. Anything that lowers the activation barrier — manganese dioxide, iron salts, iodide ion, the enzyme catalase (turnover number 4 × 10^7 per second, one of the fastest enzymes known), or a piece of rusty steel — speeds it up dramatically. The elephant-toothpaste demo uses KI or yeast catalase to flash-decompose 30% H2O2 with surfactant, giving a tower of warm O2 foam in seconds.

Why is hydrogen peroxide stored in dark brown bottles?

UV and visible light promote homolytic cleavage of the O-O bond, generating hydroxyl radicals that catalyze further decomposition. Dark amber glass blocks the relevant wavelengths (under 400 nm) and slows the loss to a manageable few percent per year for reagent-grade. The same brown HDPE is used for industrial drums. Refrigeration helps further — the rule of thumb is that reaction rate roughly halves for every 10 °C reduction. Always check the assay before relying on an old bottle for a quantitative reaction.