When atoms join to form molecules, they are held together by chemical bonds. The type of bond, and the strength of the bond, depends on the atoms that are involved. These bonds are called intramolecular forces because they are bonding forces inside a molecule ('intra' means 'within' or 'inside'). Sometimes we simply call these intramolecular forces chemical bonds.

Definition 1: Intramolecular force

The force between the atoms of a molecule, which holds them together.

Examples of the types of chemical bonds that can exist between atoms inside a molecule are shown below. These will be looked at in more detail in Grade 11.

Intermolecular forces are those bonds that hold molecules together. A glass of water for example, contains many molecules of water. These molecules are held together by intermolecular forces. The strength of the intermolecular forces is important because they affect properties such as melting point and boiling point. For example, the stronger the intermolecular forces, the higher the melting point and boiling point for that substance. The strength of the intermolecular forces increases as the size of the molecule increases.

Definition 2: Intermolecular force

A force between molecules, which holds them together.

The following diagram may help you to understand the difference between intramolecular forces and intermolecular forces.

Figure 1: Two representations showing the intermolecular and intramolecular forces in water: space-filling model and structural formula.

It should be clearer now that there are two types of forces that hold matter together. In the case of water, there are intramolecular forces that hold the two hydrogen atoms to the oxygen atom in each molecule of water (these are the solid lines in the above diagram). There are also intermolecular forces between each of these water molecules. These intermolecular forces join the hydrogen atom from one molecule to the oxygen atom of another molecule (these are the dashed lines in the above figure). As mentioned earlier, these forces are very important because they affect many of the properties of matter such as boiling point, melting point and a number of other properties. Before we go on to look at some of these examples, it is important that we first take a look at the Kinetic Theory of Matter.

The kinetic theory of matter helps us to explain why matter exists in different phases (i.e. solid, liquid and gas), and how matter can change from one phase to the next. The kinetic theory of matter also helps us to understand other properties of matter. It is important to realise that what we will go on to describe is only a theory. It cannot be proved beyond doubt, but the fact that it helps us to explain our observations of changes in phase, and other properties of matter, suggests that it probably is more than just a theory.

Broadly, the Kinetic Theory of Matter says that:

Table 1 summarises the characteristics of the particles that are in each phase of matter.

The following presentation is a brief summary of the above. Try to fill in the blank spaces before clicking onto the next slide.

Let's look at an example that involves the three phases of water: ice (solid), water (liquid) and water vapour (gas). Note that in the Figure 3 below the molecules in the solid phase are represented by single spheres, but they would in reality look like the molecules in the liquid and gas phase. Sometimes we represent molecules as single spheres in the solid phase to emphasise the small amount of space between them and to make the drawing simpler.

Figure 3: The three phases of matter

Taking water as an example we find that in the solid phase the water molecules have very little energy and can't move away from each other. The molecules are held closely together in a regular pattern called a lattice. If the ice is heated, the energy of the molecules increases. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them together, and the molecules move further apart to form liquid water. This is why liquid water is able to flow, because the molecules are more free to move than they were in the solid lattice. If the molecules are heated further, the liquid water will become water vapour, which is a gas. Gas particles have lots of energy and are far away from each other. That is why it is difficult to keep a gas in a specific area! The attractive forces between the particles are very weak and they are only loosely held together. Figure 4 shows the changes in phase that may occur in matter, and the names that describe these processes.

Figure 4: Changes in phase