Question

why co2 is a gas while sio2 is a solid

Answer 1

Sio2 has Definite shape,Definite volume,Its particles do not have intermolecular spaces and have more force of attraction whereas in the case of Co2 viceversa

Answer 2

SiO2 forms network structure leading to a definite shape and volume. That's why it is a solid. Whereas in CO2, there is no bonding of any two molecules. Hence it remains as gas.