Particle Model

Models in Science

In science, a model is an idea that explains certain behaviour. A model might not match exactly what is really going on, but it can be used to help us understand and predict what will happen in other situations, just like a model of a planned building or aircraft helps designers better understand the real thing.

To better understand different forms of water and other substances, we use the particle model.

The Particle Model

The particle model of matter explains the properties of solids, liquids and gases in terms of the packing and movement of the particles in a substance. We also use a description of bonds to understand the arrangement of particles in solids, liquids and gases.

Solids

In a solid, the particles are strongly bonded to each other. The particles in a solid move or vibrate, but not much when compared to liquids and gases. Solids have definite shape, do not flow, and are very difficult (virtually impossible) to compress. Solids do, however, have the potential to expand if heated but less than liquids and gases. The heat energy makes the particles move or vibrate faster.

Liquids

The particles in a liquid are weakly bonded to each other, but it doesn’t take much to break these bonds (which is just as well when you jump into a swimming pool). The particles in a liquid move around more than those in a solid. Liquids have no definite shape, but can flow to take the shape of the bottom of a container. Like solids, they are very difficult to compress—they are virtually incompressible.

Gases

There are no bonds between the particles in a gas. The particles in a gas have much more energy than those of a solid or liquid, and fly around, bouncing off each other. Gases have no fixed shape. Even a small amount of gas will spread (or diffuse ) to completely fill a container. Unlike solids and liquids, gases are compressible. This is because gases have large amounts of space in between the particles. It is this property that makes them useful in vehicle shock absorbers.

Explaining Properties Using the Particle Model

In the previous section we considered how the particle model is used to explain the basic properties that are used to define the states of matter. We can also use the particle model to explain some special properties of matter - diffusion, hardness and density.

Diffusion

Diffusion is the name for the spreading out of a substance. Perfume spreads out and can be smelt a long way away. Dye slowly spreads through water, even though the water is not stirred.

Diffusion is fast in gases. The particles in gas move very quickly. They travel a greater distance. Particles in a gas will diffuse quicker than particles in a liquid. The particles in a liquid do not move very far before colliding with another particle. They only move a short distance. Diffusion in liquids is slow. In a solid the particles cannot move. There is no diffusion in a solid.

Hardness

A steel nail will scratch a plastic ruler. This is because the nail is harder than the plastic. A nail will not scratch glass, because the glass is harder than the nail. A hard substance will scratch a softer substance. The particle theory explains hardness. Hardness depends on how tightly the particles hold on to each other. In a hard substance the particles hold on very tight, so they cannot be scratched or pulled off. In a soft substance the particles are held loosely, so they can be pushed off easily.

Density

The particle idea explains mass as being caused by the heaviness of each particle, and how close the particles are together. Heaviness also depends on size. A large object is often heavier than a small object.