The purpose of this lesson is to familiarize the trainees with the basic chemical symbols and structure and the properties of different cargoes and/or groups of cargoes. The instructor should start with a description of atoms and explain the periodic system and the periodic table. A chemical reaction and the forming of simple molecules can then be explained, using the most common examples.

The Hydrocarbon Structure The aim here is to try and keep things simple and avoid complication. The essential message is that petroleum is made up of several different hydrocarbon molecules with molecular weights (or molecular masses) ranging from light to heavy. Although the structure consists of hydrogen atoms linking into carbon atoms, for simplicity it is only necessary to mention carbon atoms at this basic level to present a picture of a whole range of petroleum molecules, with the lightest molecules (such as methane, propane and butane, which are gaseous at atmospheric pressure) containing only small numbers of carbon atoms, with of course, an appropriate number of linked hydrogen atoms.

As more carbon atoms (with an appropriate number of linked hydrogen atoms) are used to form the petroleum molecule, the molecule will become heavier and have a higher boiling temperature at atmospheric pressure. As it leaves the well bore, the petroleum will comprise a whole range of molecules from light to heavy. The lightest molecules, methane, are stripped off at the well head together with any earthy solids, the remaining petroleum is termed CRUDE OIL.

The process of refining the crude oil will produce a number of FRACTIONS. Each fraction will consist of a range of petroleum molecules, which at atmospheric pressure and temperature will be gaseous, liquid or solid.