Silica is the hard, glassy material that is not soluble in water. If it accidently mixed with sodium chloride, it can be easily separated out using the following activity.
Take the mixed sample of sodium chloride and silica into the glass beaker. Add sufficient amount of water to it. Since sodium chloride is completely soluble in water, it will be dissolved, but silica will not. This solution can then be passed through the funnel containing the filter paper. Silica will accumulate on the filter paper, whereas the dissolved sodium chloride will pass along with the water as the supernatant. Filter paper can then be dried to obtain the silica in its pure form and same can be done with the solution containing sodium chloride.