4.5 - Physical properties
4.5.1: Compare and explain the properties of substances resulting from different types of bonding. Examples should include melting points, boiling points, volatility, electrical conductivity and solubility in non-polar and polar solvents .
These form giant structures with many strong electrostatic attractions. They have the following properties:
High melting points:- There are many strong forces to overcome before the ions can be freed from their positions in the lattice.
Volatility:- Very low for the reasons outlined above. This means that they tend to be odourless.
Electrical conductivity:- In the solid state they are insulators as they have no charged particles which are free to move, they are all locked into position in the lattice. However, when molten the charged ions can carry an electric current as they become free to move. Similarly, in aqueous solution the ions are free to move and the solution conducts.
Solubility: Charged particles such as ions can easily form bonds to polar solvent molecules, such as water. This means that they tend to dissolve in polar solvents, but are insoluble in non-polar solvents.
These are usually simple molecules, unless a giant covalent structure is formed, which is rare. The simple covalent comounds have the following properties:
Low melting points:- The bonds between molecules are very weak, especially when the molecules are small. It only requires a small quantity of heat energy to create enough molecular vibration to break the molecules attraction for one another and the solid melts. In fact, many small molecular compounds are gases or liquids at room temperature.
Volatility:- The compounds tend to be volatile for the reasons outlined above. This means that they tend to be smelly and form vapours easily.
Electrical conductivity:- In the solid, liquid and gas states they are insulators as they have no charged particles. Similarly in solution they so not conduct.
Solubility: Covalent compounds which are non-polar do not dissolve in polar solvents to any great extent, unless thay are able to form hydrogen bonds with water. This is the case for the sugars which have many -OH groups, all of which can bond to the water molecules. In general we can say that non-polar covalent compounds will dissolve only in non-polar solvents.
Giant covalent structures
Such as silicon dioxide, have extremely high melting points and low solubility in all solvents. They are non-volatile and conductivity depends on the availability of electrons in the valence shell (eg graphite). If all electrons are used in bonding they are insulators.