If a substance has a high melting point, it is probably an ionic compound. Ionic compounds often have high melting and boiling points because ionic substances are held together by very strong intermolecular forces. By contrast, the atoms in covalent substances are held together by weaker intermolecular forces which require less energy to disrupt. This enables covalent substances to melt and boil at lower temperatures.

Another property that is different between ionic and covalent substances is “hardness." Ionic substances tend to be hard and brittle because of their crystal lattice structure. On the other hand, covalent substances tend to be softer because they are held together by much weaker intermolecular forces.

Therefore, if you have a substance that has a high melting point, you can predict that it is probably ionic. Since ionic substances tend to be hard, you can also predict that your substance would be hard.