Bonding and structure — common mistakes

Graphite is a crucial case study. You must be able to explain its properties. It conducts electricity because of delocalised electrons between its layers. It is soft and used as a lubricant because the layers, held by weak van der Waals' forces, can slide over each other easily.

Graphite has a layered structure. Within each layer, carbon atoms are held by strong covalent bonds, making the layers themselves very strong. However, the layers are held to each other by only weak van der Waals' forces. These layers can easily slide over one another, which gives graphite its soft, slippery feel and makes it useful as a lubricant.

While many ionic compounds dissolve in water, it's not a universal rule. For a substance to dissolve, the energy released when ions are hydrated by water molecules must be greater than the energy required to break apart the ionic lattice (the lattice enthalpy). For compounds like AgCl, the lattice enthalpy is very large and the hydration enthalpy is not sufficient to overcome it, so it remains insoluble.

Ice has a simple molecular structure. It is composed of individual $H_2O$ molecules. In the solid state, these molecules are arranged in a regular lattice held together by hydrogen bonds, which are a type of intermolecular force. Because these are intermolecular forces (albeit the strongest type), they are much weaker than covalent bonds, which is why ice melts at a relatively low temperature ($0\,^\circ\text{C}$) compared to a giant covalent substance like diamond.