Explain the structure of silica ? How does it react with (a) NaOH and (b) HF ?

Structure of Silica :
Silica has a structure similar to diamond. In this each silicon is tetrahedrally bonded to four oxygen atoms by covalent bonds resulting in a giant three dimensional polymeric network structure.

a) NaOH :
Silica reacts with NaOH to form sodium silicate
${\mathrm{SiO}}_2+2\mathrm{NaOH}\to \underset{\mathrm{sodium} \mathrm{silicate}}{{\mathrm{Na}}_2{\mathrm{SiO}}_3+{\mathrm{H}}_2\mathrm{O}}$
b) HF :
Silica reacts with HF and gives silicon tetra fluoride.
${\mathrm{SiO}}_2+4\mathrm{HF}\to {\mathrm{SiF}}_4+2{\mathrm{H}}_2\mathrm{O}$