The boiling point of a substance depends on the strength of the intermolecular forces between its constituent particles. Giant covalent substances have strong covalent bonds between their atoms, but these bonds are non-directional. This means that the molecules are not held together in a regular lattice, and the intermolecular forces between them are relatively weak. As a result, giant covalent substances tend to have low melting and boiling points.

For example, the boiling point of diamond, a giant covalent substance, is 3550 °C. This is much higher than the boiling point of water (100 °C), which is a molecular substance with strong hydrogen bonds, but it is much lower than the boiling point of ionic compounds such as sodium chloride (1413 °C), which have strong ionic bonds.

The following are some general trends in the boiling points of giant covalent substances:

* The boiling point increases with increasing molecular weight. This is because the heavier the molecule, the more energy it takes to overcome the intermolecular forces and cause it to boil.

* The boiling point decreases with increasing branching. This is because branching reduces the regularity of the molecular structure and weakens the intermolecular forces.

* The boiling point increases with increasing polarity. This is because polar molecules have stronger intermolecular forces than non-polar molecules.