The Particle Model What makes up all the matter around us? If we were able to break up a piece of steel into smaller and smaller parts, what would we get at the end? Would we get the same thing if we broke up a piece of plastic? Could we break up a droplet of water? Ice, water and steam are all the same chemical substance, yet they behave very differently. How can we explain this?

Models are used by scientists because they help to explain many observations. Scientists have developed a way of thinking about matter that explains its nature and behaviour. They believe that all matter is made up of tiny particles, called atoms. These are so small that they can only be seen with powerful microscopes. If you could see deep inside solids, liquids and gases, how do you think the particles that make them up might be arranged?

The theory that we have to explain the properties of solids, liquids and gases is called the particle model of matter. Solids, like ice, have constant shape because particles of solid are packed closely together. They cannot move around much, but mainly vibrate around one point. These particles in solids are held together by strong forces.

In liquids, such as water, particles are spaced further apart. They are held together by weaker forces. This means that they are able to move around more, and slide over each other. This explains why a liquid will take up the shape of part of the container it is in and can be poured from one container into another. The ease with which a liquid can be poured is called its viscosity. Honey is more viscous than water because its particles have stronger forces between them.

In gases, like steam, there are large spaces between the particles, and the particles are able to move freely at high speeds and in all directions. The particles collide with each other and the walls of their container. This explains why a gas spreads apart so the gas fills its container. Can you think of some examples of this? Since gas particles are spread far apart, one type of gas can flow through another. In a process known as diffusion, the particles that make up an odour can flow through the particles that make up the air. This is how smells spread easily.

The particle model of matter tells us that everything is made up of tiny particles called atoms, which are constantly moving. The behaviour of these particles is summarised in figure 2.9.

States of matter….summary There are many ways to think about solids, liquids and gases. Particles in a solid could be represented by a group of people packed onto a dance floor, listening to a band, all moving but unable to break through the crowd. A liquid could be represented by a break in the music and people moving around to get a drink or freshen up. At the end of the night, people move at higher speed through all exits, to public transport and cars, spreading in all directions to go home. Here, they have behaved like a gas. Activity… Working in a group, make up a similar description of your own to explain states of matter. Act this out for your class and then see if they can explain your actions. Ask your classmates if they can suggest any ways of improving your model. Make the changes that your group agrees with and then re-enact your role play.