Why might you expect silicon dioxide and carbon dioxide to have similar melting and boling points?

Answer 1

You Ultimately would not expect them to have similar melting and boiling points.
This is because Carbon Dioxide is a gas, it might have strong covalent bonds between their atoms but between their molecules they have weak intermolecular forces, therefore these forces are easily overcome during heating in lower temperatures
However when you look at Silicon Dioxide, it has a macromolecular structure. In other words the whole solid is actually just one molecule which its atoms are each covalently bonded to each other. Therefore since it has strong covalent bonds between them you would expect them to have extremely high melting and boiling points.

Diamond is a macromolecular structure and it has a melting point of 3006 degrees centigrade. You could expect Silicon Dioxide's melting point (and boiling point) to be similarly high.

Answer 2

Silicon dioxide and carbon dioxide have similar melting and boiling points because both molecules are linear and nonpolar, which leads to weak intermolecular forces (van der Waals forces) between the molecules. These weak forces result in similar energy requirements for breaking the bonds in the solid and liquid states, leading to comparable melting and boiling points.