ABSTRACT
Mass exists in three forms, solids, liquids and gases. Structurally, the main difference between solids, liquids and gases is in the strength of their intermolecular bonds. Most substances become less stable (the intermolecular bonds become progressively weaker) with increase in temperature and vice versa, such that a substance may change its state from one form to another. For example, water can exist as a solid (ice), liquid or gas (steam). In general, the stronger the intermolecular bonds, the more stable the structure, and the less likely it is to deform in response to load. Whereas the strength of the intermolecular bonds is different in different solids, i.e. some solids deform more easily than others in response to load, a solid is characterised by a fixed volume and a fixed shape when subjected to no external force other than its own weight. In contrast, a liquid will tend to deform under its own weight and is characterised by a fixed volume and variable shape. In comparison with solids and liquids, the intermolecular bonds in gases are very weak, such that in response to external pressure, a gas is likely to change not only its shape, but also its volume, i.e. gases are characterised by variable volume and variable shape.
