Prepn. of porous silica gel powder of low density comprises: (a) adding an acid soln. to a basic aq. silicate soln. to cause neutralisation; (b) forming a hydrosol from the above reaction mixt.; (c) gelling the hydrosol to give a hydrogel and then destroying this to give a silica gel slurry; (d) sepg., washing, drying and decomposing the hydrogel slurry. In step (a) mixing the solns. is carried out by spraying, in order to achieve partial neutralisation of the reaction mixt. at pH 11.6 or more, and then further adding acid soln. to give a reaction mixt. of pH 10.8-11.4.
