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Hydrogen peroxide is always present as an aqueous solution of varying concentration. Aqueous solutions of hydrogen peroxide are colourless and odourless liquids. The pure, undiluted substance (i.e. 100% w/w hydrogen peroxide) has a melting point of -0.43 °C and a boiling point of 150.2 °C. At 25 °C the undiluted hydrogen peroxide has a relative density of 1.443 g/cm3, a vapour pressure of 300 Pa and a dissociation constant of 11.62. The vapour pressure at 20 °C is 214 Pa, the surface tension and viscosity of undiluted hydrogen peroxide are 80.4 mN/m and 1.249 mPa*s, respectively. QSAR estimations of the octanol-water partition coefficient indicate a log KOW of -1.57. Hydrogen peroxide itself is not flammable. The substance decomposes with increasing temperature and increasing contamination and is very reactive with transition metals, organic materials and other contaminations. This decomposition may cause an explosion of containers. Hydrogen peroxide releases oxygen when decomposing. Thus, it may induce spontaneous combustion of flammable materials and give continued support of the combustion. Vapour concentrations in excess of 26% v/v at atmospheric pressure can decompose spontaneously in an explosive fashion. The lowest liquid concentration which can generate such a vapour concentration is 74% w/w which must be heated to its boiling point of 130°C. At concentrations ≥86% w/w the liquid itself can be made to explode using extreme initiation energies. A solution containing 87% w/w did, however, not fulfil the criteria for classification as an explosive. Hydrogen peroxide is a strong oxidiser and may act also as a reducing agent.