Hydrogen Peroxide (H₂O₂) — Properties, Uses & Safety

Overview

Hydrogen peroxide (H2O2) is a pale blue liquid in its pure form, though it appears colorless in the dilute solutions most people encounter. With a molar mass of 34.01 g/mol, it is the simplest peroxide — a compound with an oxygen-oxygen single bond. Hydrogen peroxide is miscible with water and is commonly sold in concentrations of 3% (household antiseptic) to 35% (food grade) and even 90%+ (high-test peroxide for rocket propulsion). The molecule has a skewed, non-planar structure due to the dihedral angle between the two O-H bonds, which is an interesting topic in molecular geometry. Hydrogen peroxide is a powerful oxidizing agent that decomposes into water and oxygen gas (2 H2O2 → 2 H2O + O2). This decomposition can be catalyzed by manganese dioxide (MnO2), potassium iodide, or the enzyme catalase found in living cells — the famous elephant toothpaste demonstration uses this reaction with a catalyst and soap to create a dramatic foam eruption. In nature, hydrogen peroxide is produced in small amounts by nearly all living organisms as a byproduct of oxidative metabolism. White blood cells (neutrophils) use it to kill bacteria. It is also formed in the atmosphere by the action of sunlight on water vapor. Common uses include wound disinfection (3% solution), hair bleaching and textile whitening, water and wastewater treatment, electronics manufacturing (semiconductor cleaning), and as an oxidizer in rocket propellants (the HTP used in early space programs). Safety is a major consideration with hydrogen peroxide. Dilute solutions (3%) cause mild skin whitening and bubbling on wounds, but concentrated solutions (>30%) are corrosive, can cause severe burns, and are strong enough to be explosive when combined with organic materials. GHS hazard pictograms include the oxidizer and corrosion symbols. For chemistry students, hydrogen peroxide is an outstanding compound for exploring reaction kinetics (measuring decomposition rate), catalysis (comparing catalyzed vs. uncatalyzed reactions), oxidation-reduction chemistry, and molecular geometry. The elephant toothpaste experiment alone makes it one of the most exciting compounds in any chemistry classroom.

IUPAC Name

hydrogen peroxide

CAS Number

7722-84-1

PubChem CID

784

Molecular Formula

H₂O₂

Molar Mass

34.01 g/mol

State at STP

liquid

Appearance

Colorless liquid

Physical Properties

Melting Point

-0.43 °C

Boiling Point

150.2 °C

Density

1.45 g/cm³

ΔH°f

-187.8 kJ/mol

ΔG°f

-120.4 kJ/mol

Composition

Reactions