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The kinetic molecular theory

Module by: Free High School Science Texts Project. E-mail the author

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:

  • Matter is made up of particles that are constantly moving.
  • All particles have energy, but the energy varies depending on whether the substance is a solid, liquid or gas. Solid particles have the least amount of energy and gas particles have the greatest amount of energy.
  • The temperature of a substance is a measure of the average kinetic energy of the particles.
  • A change in phase may occur when the energy of the particles is changed.
  • There are spaces between the particles of matter.
  • There are attractive forces between particles and these become stronger as the particles move closer together. These attractive forces will either be intramolecular forces (if the particles are atoms) or intermolecular forces (if the particles are molecules). When the particles are extremely close, repulsive forces start to act.

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

Table 1: Table summarising the general features of solids, liquids and gases.
Property of matter Solid Liquid Gas
Particles Atoms or molecules Atoms or molecules Atoms or molecules
Energy and movement of particles Low energy - particles vibrate around a fixed point Particles have less energy than in the gas phase Particles have high energy and are constantly moving
Spaces between particles Very little space between particles. Particles are tightly packed together Smaller spaces than in gases, but larger spaces than in solids Large spaces because of high energy
Attractive forces between particles Very strong forces. Solids have a fixed volume. Stronger forces than in gas. Liquids can be poured. Weak forces because of the large distance between particles
Changes in phase Solids become liquids if their temperature is increased. In some cases a solid may become a gas if the temperature is increased. A liquid becomes a gas if its temperature is increased. It becomes a solid if its temperature decreases. In general a gas becomes a liquid when it is cooled. (In a few cases a gas becomes a solid when cooled). Particles have less energy and therefore move closer together so that the attractive forces become stronger, and the gas becomes a liquid (or a solid.)

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

Figure 1

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 2 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 2: The three phases of matter
Figure 2 (CG10C2_008.png)

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 3 shows the changes in phase that may occur in matter, and the names that describe these processes.

Figure 3: Changes in phase
Figure 3 (CG10C2_009.png)

Summary

  • There are three states of matter: solid, liquid and gas.
  • Diffusion is the movement of particles from a high concentration to a low concentration. Brownian motion is the diffusion of many particles.
  • The kinetic theory of matter attempts to explain the behaviour of matter in different phases.
  • The kinetic theory of matter says that all matter is composed of particles which have a certain amount of energy which allows them to move at different speeds depending on the temperature (energy). There are spaces between the particles and also attractive forces between particles when they come close together.
  • Intramolecular force is the force between the atoms of a molecule, which holds them together. Intermolecular force is a force between molecules, which holds them together.
  • Understanding chemical bonds, intermolecular forces and the kinetic theory of matter can help to explain many of the macroscopic properties of matter.
  • Melting point is the temperature at which a solid changes its phase to become a liquid. The reverse process (change in phase from liquid to solid) is called freezing. The stronger the chemical bonds and intermolecular forces in a substance, the higher the melting point will be.
  • Boiling point is the temperature at which a liquid changes phase to become a gas. The reverse process (change in phase from gas to liquid) is called condensing. The stronger the chemical bonds and intermolecular forces in a substance, the higher the boiling point will be.
  • Density is a measure of the mass of a substance per unit volume.
  • Viscosity is a measure of how resistant a liquid is to flowing.

End of chapter exercises

  1. Give one word or term for each of the following descriptions.
    1. The property that determines how easily a liquid flows.
    2. The change in phase from liquid to gas.
    Click here for the solution
  2. If one substance A has a melting point that is lower than the melting point of substance B, this suggests that...
    1. A will be a liquid at room temperature.
    2. The chemical bonds in substance A are weaker than those in substance B.
    3. The chemical bonds in substance A are stronger than those in substance B.
    4. B will be a gas at room temperature.
    Click here for the solution
  3. Boiling point is an important concept to understand.
    1. Define 'boiling point'.
    2. What change in phase takes place when a liquid reaches its boiling point?
    3. What is the boiling point of water?
    4. Use the kinetic theory of matter and your knowledge of intermolecular forces to explain why water changes phase at this temperature.
    Click here for the solution
  4. Describe a solid in terms of the kinetic molecular theory.
    Click here for the solution
  5. Refer to the table below which gives the melting and boiling points of a number of elements and then answer the questions that follow. (Data from http://www.chemicalelements.com)
    Table 2
    ElementMelting pointBoiling point (°C°C)
    copper10832567
    magnesium6501107
    oxygen-218,4-183
    carbon35004827
    helium-272-268,6
    sulphur112,8444,6
    1. What state of matter (i.e. solid, liquid or gas) will each of these elements be in at room temperature?
    2. Which of these elements has the strongest forces between its atoms? Give a reason for your answer.
    3. Which of these elements has the weakest forces between its atoms? Give a reason for your answer.
    Click here for the solution

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