simple molecules, which contain a set number of atoms joined by covalent bonds
giant covalent substances, which contain many atoms joined by covalent bonds
An example - silica
Silica is the main compound found in sand. It is an example of a giant covalent substance. It contains many silicon and oxygen atoms. These are joined together by covalent bonds in a regular arrangement, forming a giant covalent network or lattice structure. There is no set number of atoms joined together in this type of structure, so these covalent lattices are not classed as molecules. However, the atoms in the compound will be present in the ratio indicated by the chemical formula.
High melting points and boiling points
Giant covalent substances are solids at room temperature and have very high melting points and boiling points. Covalent bonds are strong, so a lot of energy is needed to break up these large structures during melting and boiling.
Conduction of electricity
Giant covalent substances have no overall charge, so most cannot conduct electricity. Graphite, a form of carbon, which can conduct electricity, is an exception.
Insoluble in water
A substance can dissolve in water if it forms strong enough attractions with water molecules. Giant covalent substances cannot form these strong attractions with water, so they are insoluble.