# Why can't gases be compressed?

### 3.3 The physical states of the substances

Attempt:
A piece of ice is heated slowly.
During this process, the solid ice slowly changes into the liquid water. If this liquid is brought to the boiling point by heating, then the liquid water changes into a gaseous state, water vapor is produced.

The three states of solid, liquid and gaseous are called the three physical states of substances.
Depending on their physical state, substances have different properties that can be explained with the help of the particle model.

the particle model can be used to explain many properties of substances.

a. Solid state.
With a lot of effort you can certainly deform a steel beam slightly with a sledgehammer, but this is only possible with a great deal of force.
The particle model shows us that the particles occupy fixed positions and that there are great forces of attraction between the particles. If you deform a piece of steel, you have to overcome these attractive forces, so the particles are difficult to move.

If you want to cut through a steel beam, you have to use a hacksaw. Again you have to overcome the forces of attraction between the particles.

It is practically impossible to compress a solid. In the particle model, the particles are arranged very close to one another, so they can hardly be compressed.

As soon as the temperature is above absolute zero, the particles move slightly in the fixed place they occupy in the solid. An improved particle model for a solid is therefore:

b. Liquid state
The properties of liquids can also be explained with the help of the particle model. Liquids are very easy to separate, for example by pouring them into several portions.

The particle model shows us that the particles do not occupy fixed positions and that they can move relative to one another. This also explains why liquids take the shape of a vessel. The relatively weak forces of attraction between the particles allow them to move freely within the volume occupied by the liquid.

However, if you want to compress a liquid, you will quickly discover that this is very difficult. Here, too, the particle model gives a simple explanation. As can be seen from the model, the particles are also arranged relatively close to one another in liquids. The distances between the particles are only slightly larger than in a solid. It is therefore hardly possible to compress them.
In liquids, particles are in constant motion. An improved particle model for a liquid is therefore:

c. Gaseous state
In contrast to solids and liquids, gases can be compressed very easily. If you look at the particle model of gases, you will notice that the distances between the particles are very large. Therefore it is also easy to compress a gas, one only reduces the distances between the particles.

The gas particles move chaotically and very quickly, which is why gases take up any space that is made available to them. An improved particle model for a gas is therefore:

The particle model also allows to explain how a substance can change from one physical state to another:
Transitions between the states of aggregation