Covalent substances generally have low melting and boiling points because of the relatively weak intermolecular forces holding their molecules together. These forces, known as van der Waals forces, result from the temporary fluctuations in the electron distribution around molecules, creating transient attractive forces between them. While van der Waals forces are important for the behavior of gases and liquids, they are considerably weaker than the covalent bonds within molecules.

In covalent substances, the strength of the covalent bonds determines the melting point. The stronger the covalent bonds, the more energy is required to break them and separate the molecules, resulting in a higher melting point. However, since van der Waals forces are relatively weak in covalent substances, the energy required to overcome these intermolecular forces and allow the molecules to move past each other is comparatively small.

As a result, covalent substances tend to have lower melting points compared to ionic or metallic substances. Ionic and metallic substances have stronger intermolecular forces, such as electrostatic attractions between ions or metallic bonds, respectively. These stronger intermolecular forces require more energy to overcome, leading to higher melting points.

The melting point of a covalent substance also depends on the molecular weight and molecular structure. Larger molecules and more complex molecular structures generally have stronger van der Waals forces due to the increased surface area for intermolecular interactions. Therefore, they tend to have higher melting points compared to smaller molecules with simpler structures.

In summary, covalent substances have low melting points because the intermolecular forces holding their molecules together are relatively weak van der Waals forces. The strength of the covalent bonds within the molecules, molecular weight, and molecular structure also play a role in determining the melting point of a covalent substance.