The states of matter

Now that you have learned in The structure of matter how matter is constituted, let's see how molecules and ionic compounds can organize between themselves in three different states : gas, liquid and solid.

The gaseous state

A gas:

  1. is compressible;
  2. is fluid (it takes the shape of its container);
  3. fills the totality of its container.

Molecules in the gaseous state have a lot of energy, so they have a lot of space around themselves. This is the reason why gases are compressible. When you compress a gas, you simply reduce the space between each molecule.

Scheme of a gas

Note : "gas" is a synonymous of "vapor".

The liquid state

A liquid:

  1. is not compressible;
  2. is fluid;
  3. does not necessarily fill the totality of its container.

The liquid state has an energy between the gas state and the solid state. As a result, molecules in a liquid are very close to their neighbors, and they can move freely.

Scheme of a liquid

The solid state

A solid:

  1. is not compressible ;
  2. is not fluid.

There exist two different solid states : the crystalline and glassy state.

The crystalline state

A crystal is a solide whose atoms, molecules or ions are arranged in a very regular pattern.

Scheme of a crystal

It can be formed by slowly cooling a liquid, until the crystallization temperature of the liquid is reached.

The glassy state

A glass is a solid whose atoms, molecules or ions are little ordered.

Scheme of a glass

It can be obtained by cooling a liquid very fast, until the glass transition temperature of the liquid is reached.

Transitions of state

It is possible to change the state of any sample of matter, by changing some parameters such as pressure or temperature.

Each transition from a given state of matter to another has a particular name:

The six transitions of state

As we said earlier, the gas state has a high energy, the liquid state has an intermediate energy, and the solid state has a low energy.

In order to change the state of a sample of matter, one must change the energy it has.

To transform a sample of liquid water (intermediate energy) into solid water (low energy), i.e. ice, one must lower the energy of the sample. On the contrary, to transform a sample of liquid water (intermediate energy) into water vapor (high energy), one must higher the energy of the sample.

But how does one change the energy of a sample of matter ? One easy way is by changing its temperature. The higher the temperature of a sample of matter, the higher its energy.

To follow with our previous example, to lower the energy of our sample of liquid water and transform it into ice, we have to decrease its temperature, i.e. cool it. And to higher the energy of our liquid water and transform it into water vapor, we have to increase its temperature, i.e. heat it.

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