Chemistry - Why is SiO2 a solid while CO2 is a gas?
Solution 1:
The reason why carbon dioxide is a gas and silicon dioxide is a solid is because their chemical structures are different.
Carbon dioxide is a linear structure with two double bonds between carbon and oxygen. It is a small molecule and non-polar with only weak bonds between the molecules. Hence it is a gas.
Silicon dioxide is not formed of small molecules. It consists of an infinite array of silicons where each silicon is bonded to four separate oxygens (and each oxygen is shared between two silicons). This creates a strong refractory solid (glass and sand are mostly silicon dioxide aka silica). So the same apparent overall formula doesn't describe the actual structure of the compounds at all. But the structures explain the difference in behaviour.
Of course this doesn't explain why silicon prefers to bond with four oxygens when carbon prefers just two. This is not completely simple and results from the relative bond strengths of carbon-oxygen bonds, carbon-oxygen double bonds and the equivalent bonds for silicon and oxygen. The simple version is that silicon oxygen bonds are strong relative to their double-bond equivalents whereas carbon-oxygen double bonds are strong relative to their single bond equivalents. Or, more precisely, if we could make a carbon-oxygen network solid with the equivalent structure to silica, it would tend to fall apart into carbon dioxide. If we could make silicon dioxide molecules, they would react with the release of energy to give silica.
Deeper explanations would need to look at why the relative strengths of double and single bonds turn out that way, but that would get into molecular quantum mechanics and would not be much more useful as an explanation.
The simplest explanation is the fact that the structures are different.
Solution 2:
If you look at the crystal structure of silicon dioxide ($\ce{SiO2}$), you will see it is comprised of tetrahedra (one silicon surrounded by 4 oxygen atoms) and these tetrahedra are in turn connected to each other, and subsequently form a 2-dimensional network which is large enough to make this substance a solid.
Also, the factor of 2.5 in mass makes a big difference, but it is not relevant to this discussion. I would recommend reading any inorganic chemistry book where such topics are discussed in depth. Van der Vaals forces only play significant roles in long-chain carbohydrates, such as fats, and are not really observed in "inorganic" molecules.
$\ce{CO_2}$ molecules cannot form a crystal in the way $\ce{SiO2}$ units do, and I do not think I have to explain why. Also, it has no dipole moment and no van der Waals forces between the molecules.
Solution 3:
Elements of the answer are also contained in other answers here but this needs some more teasing out. It has to do with the bond energies of C-O, Si-O, C=O, and Si=O. I don't have the numbers at hand, but work it out and it will point you to carbon being the most stable in the form of $\ce{CO2}$ and silicon the most stable as a tetrahedral network (similar to diamond.)
The reason why $\ce{CO2}$ is a gas can easily be explained using intermolecular bonding principles, which you can look up easily.