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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 helpsus 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 otherproperties of matter, suggests that it probably is more than just a theory.
Broadly, the Kinetic Theory of Matter says that:
[link] summarises the characteristics of the particles that are in each phase of matter.
| 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 andtherefore 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.
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 [link] below the molecules in the solid phase are represented by single spheres, but they wouldin 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 smallamount of space between them and to make the drawing simpler.
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 eachother. 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 ableto 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 themolecules 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 heldtogether. [link] shows the changes in phase that may occur in matter, and the names that describe these processes.
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