Thermodynamic and chemical properties of simple fluids N(2), CO(2), and H(2)O and their binary and ternary mixtures have been studied using density functional theory simulations in a high pressure and high temperature regime. We show that N(2) and binary mixtures with N(2) follow an ideal behavior over a large temperature and pressure range. On the contrary, the water molecule is observed to dissociate as either pressure or temperature increases. Dramatic consequences are observed when water is mixed with carbon dioxide at extreme conditions. Indeed, a new molecule is formed, CO(3)H(2), and the thermodynamic behavior of the mixture strongly deviates from ideality. Chemistry occurring at extreme conditions is then discussed in the context of detonation product modeling.