How is silica gel made?
Sand may be dissolved in concentrated and hot solutions of NaOH. The dissolution is slow, and pressure may help this action. At the end of the reaction, a viscous solution of so-called sodium silicate is obtained. The formula of the solute is not well defined, and can be described as $(Na_2SiO_3)_n$. It is made of a long polymeric chain similar to the formula $HO-Si-O-(Si-O)_n-Si-O-Si-OH$, with supplementary negatively charged Oxygen atoms above and under each Silicon atoms. The whole chain is negatively charged. And of course, there are Sodium ions $Na^+$ in the vicinity of each negatively charged Oxygen atoms. To make the description more complete, one should mention that from time to time there are side branches of Si-O-Si-O chains, which get fixed on the Si atoms of the main chain. Furthermore there are no $Si=O$ bonds in this formula.

If you add some $HCl$ solution, you produce the precipitation of an insoluble gel whose formula has the same skeleton as the previous solution $HO-Si-O-(Si-O)_n-Si-O-Si-OH$. But the Oxygen atoms above and under the main chain are now holding a H atom, and the whole chain is neutral. 

This gel can be filtrated and washed with large amounts of water. This removes the ions $Na^+$ and $Cl^-$. The result is a gel containing more water than silica. Usually this gel is then heated and the water is evaporated. An amorphous white substance is obtained with a formula between $H_2SiO_3$ and $SiO_2$. This is the common "silicagel" used in the labs for dehydrating purposes, because it has a tendency to adsorb water from the atmosphere.  

The fresh gel can also be dried in a vacuum, instead of by heating. In this case, the silica gel is transformed into an extremely light substance, lighter than snow, which has a tendency to float in the air. It is a dangerous substance if you breeze it, because it will stay in the nose and the lung, without being able to get out. It may induce cancers, like asbestos.