`SiO_2` is a solid while `CO_2` is a gas - explain.

Explanation

`""^(-)O - C equivO^(+)overset()harrO=C=O overset()harr ""^(+)O equiv C - O^(-)`
`CO_2` molecule is linear, carbon dioxide molecules are held together only by weak van der Waal's forces and hence it exists as a gas at ordinary temperature.
Silicon has a three-dimensional infinite structure, in which silicon atom is tetrahedrally bonded to two silicon atoms by covalent bonds. The whole crystal of silica thus exists as a giant molecule and hence silica is a high melting solid.