In the examiner reports, it is recorded that silicon dioxide has very high boiling point, the reason being that strong covalent bonds are broken in the boiling process. I find it hard to accept this explanation as boiling is a process whereby a compound changes from the liquid state to a vapour state and silicon dioxide does not have a liquid state. Sublimation is usually explained from phase diagram. I greatly appreciate your help. Silicon dioxide is commonly called silica, and it is widely found as sand and quartz.
Carbon can form double bonds and hence carbon dioxide is a discrete molecule and is a gas.
Silicon cannot form double bonds in this way using orbitals. Thus silicon dioxide forms an infinite three-dimension structure with high melting point. Silicon dioxide exists in many different forms. The main ones are quartz, tridymite and cristobalite, each of which has different structures at high and low temperatures. At high temperatures, the three-dimension structure breaks down as it melts.