Try to put some mothballs in your closet and you will surely find out after a few days that they have gotten smaller and soon they disappeared The mothballs have just sublimated. They transformed into the gaseous state of matter without melting into the liquid first. To understand how this happens, it is best to know first the intermolecular forces between covalent molecules. There are three major types of intermolecular forces: Van derWaals, polar bonds, and hydrogen bonds. Van der Waals forces are the weakest attraction between molecules and occur between all atoms, molecules, and ions. Polar bonds is the intermolecular attraction which occur between polar or unsymmetrical covalent molecules.
Hydrogen bonds are the strongest of the intermolecular forces. It involves hydrogen. It occurs between hydrogen atoms in one polar covalent molecule. Polar covalent bond is a kind of bond when two different atoms are joined by a covalent bond the bonding electrons are shared unequally.
Besides the three major types of intermolecular forces are other important factor to consider is the kinetic molecular theory. According to the theory there are factor disruptive forces and there are also attractive forces between atoms and molecules. Disruptive forces increase with temperature, whereas attractive forces are not affected by temperature. Whether a substance exists as a solid, a liquid, or a gas at a particular temperature and pressure depends upon the balance between these attractive and disruptive forces. In solids its molecules moves very little because it is held in place by the intermolecular forces of its neighbor. The molecules in solid vibrate more strongly by adding heat. As the temperature of a solid is increased up to its melting point, its molecules vibrate with greater energies. When they gain enough energy to overcome the intermolecular attractive forces; they break loose from their fixed positions and move about more freely. There are some solids, whose intermolecular attractions are weak, so that they do not melt, but instead, convert directly in to a gas without first becoming liquid under normal condition as what had happened to mothballs.