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Pure hydrogen peroxide is a clear, syrupy liquid, colourless in small amounts, but having a bluish colour like water when in bulk. It has an odour like that of nitric acid. It evaporates spontaneously in the air, boils at 84°-85°/68 mm. or 69.2°/26 mm. When heated to 151°, the boiling point at 760 mm., the substance explodes violently. Its specific gravity is 1.4649 at 0°. The liquid has a strong acid reaction to litmus. In dilute solution hydrogen peroxide is completely neutral. The pure substance is fairly stable, and can be kept for several weeks in the absence of sunlight, provided the glass of the bottle is perfectly smooth. In contact with rough surfaces, or on shaking, decomposition occurs: 2H2O2 = 2H2O+ O2. Finely divided metals such as gold, silver, and platinum (but not iron) bring about explosive decomposition. Cotton-wool at once inflames. A mixture of magnesium or carbon powder with a trace of manganese dioxide at once inflames in contact with pure liquid H2O2.
On cooling 95-96 per cent, peroxide in solid carbon dioxide and ether, or in methyl chloride at -23°, it solidifies to a hard crystalline mass. If a little of this solid is placed in the 95 per cent, solution cooled to -10°, columnar prismatic crystals of pure solid hydrogen peroxide melting at -1.70°, are obtained. These crystals explode with a trace of platinum black; alone, they are fairly stable. By mixing the pure peroxide with water and cooling in a mixture of solid carbon dioxide and ether, the crystalline hydrate: H2O2,2H3O, m. pt. -51°, is obtained.
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