The fire point is usually several degrees above the flash point for hydrocarbons. The minimum autoignition temperature is the temperature at which a material will autoignite when it contacts a surface at that temperature. The procedure consists of heating a glass flask and squirting small quantities of sample into it at various temperatures until the vapours autoignite. The only source of ignition is the heat of the surface. For the smaller hydrocarbons the autoignition temperature is inversely related to molecular weight, but it also increases with carbon chain branching.
Supercritical fluids have some very unusual properties.
In particular, the solubility behaviour changes. The behaviour is neither that of the liquid or that of the gas. The transition between liquid and gas can be completely smooth. The pressure-dependant densities and corresponding Hildebrand solubility parameters show no break on continuity as the supercritical boundary is crossed. Physical properties fall between those of a liquid and a gas.