Hydrogen Peroxide (H2O2) is a colourless liquid that resembles water in many respects. Its physical properties are very similar to those of water, except that it is 40% denser. The main difference between hydrogen peroxide and water, however, is in its chemical behaviour. The single bond between the two oxygen atoms is weak, so that H2O2 readily fragments into either H and HO2 or two OHs. Either way, the resulting species are free radicals, which means they are very reactive, and this makes H2O2 a very powerful oxidising agent. For this reason it has been utilised in rocket propulsion, when it is used to oxidise the hydrazine fuel, liberating hot gases (steam and oxygen) which propel the rocket forward.