Pyrogenic silicon dioxide powder and the dispersion

5~600m a silicon dioxide powder produced by the thermal decomposition method with carbon content of less than the specific surface area and 500ppm of The silicon dioxide powder, a vaporous tetramethoxysilane and / or tetraethoxysilane together with air, and separately supplying the hydrogen to the burner, and the mixture in the reaction chamber connected in series to the burner of the gas are reacted in a flame, and then separated from the gas stream by known means the reaction product of a solid, in which the lambda value in the burner is 0.95 to 1.5, and this time, in the reaction chamber is produced by feeding a sufficient secondary air into the reaction chamber for the lambda value is 0.8 to 1.6. The present invention also provides the use of dispersions and powders and the dispersion containing the silicon dioxide powder.