Silica is formed by the reaction of

Silicon tetrachloride when heated with water, it forms silicon dioxide along with HCl. The reaction is represented as: 

${\mathrm{SiCl}}_4\left(\mathrm{s}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{∆}{\to }{\mathrm{SiO}}_2+4\mathrm{HCl}$