Up to this point we have confined our discussion to covalently bonded substances consisting of small discrete molecules. The forces between these molecules are quite weak compared with covalent bonds. There are some covalently bonded substances, however, in which no small individual molecules exist. These substances contain extremely large molecules, and so they are called macromolecular substances. They are invariably solids at room temperature. Quite often we can regard each crystal of the substance as a single molecule.