Changes of state

If you've ever taken a run or bike ride in freezing conditions before, you may have experienced that the water in your water bottle started to have small chunks of ice in it. What happened there was a change of state of the water in your bottle! Parts of your water went from being liquid to solid because it was so cold. In this article, we will explain what changes of state there are and how they occur.

Meaning of a change of state

Let's start by definining a state!

Now that we know what is a state, we can study the meaning of change of state.

Materials will change state depending on how much energy they receive or lose. With an increase of energy in a material, the average kinetic energy of the atoms begins to increase, causing the atoms to vibrate more, pushing them apart to the point that they change their state. The fact that kinetic energy changes the state of materials makes this a physical process, rather than a chemical one, and no matter how much kinetic energy is put into or taken away from the material, its mass will always be conserved and the material will always stay the same.

Changes of state and thermodynamics

So we know what happens when Materials change their state, but why does this actually happen? Let's look into the thermodynamic aspects of changing states, and how energy plays a part in this.

More energy put into material will result in it turning into a liquid or a gas, and energy being taken out of material will result in it turning into a liquid or a solid. This of course depends on whether the material is starting as a solid, liquid, or gas, and what the exact environmental conditions are. For example, if a gas loses energy, it can turn into a liquid, and if a solid gains energy, it can turn into a liquid as well. This energy is typically introduced into a material via an increase in temperature or an increase in Pressure, and both of these variables can cause different changes of state.

Fig. 1: An example of the molecular structure of a solid, liquid, gas.

Examples of changes of state

Below is a list of all changes of state we need to know about, and a short explanation describing what each one is.

Freezing

A good example of this is when water turns into ice. As the temperature decreases, the water will begin to lose energy until each water molecule no longer has the energy to move around other water molecules. Once this occurs, the molecules form a rigid structure that is kept rigid by the attraction that occurs between each molecule: we now have ice. The point at which freezing occurs is known as the freezing point.

Melting

Melting is the opposite of freezing. Using our previous example, if the ice was subjected to higher temperatures, it would begin to absorb the energy from its warmer surroundings, which would, in turn, excite the molecules within the ice and give them the energy to move around each other again: we now have a liquid again. The temperature at which a material melts is known as the melting point.

Evaporation

When a material is a liquid, it is not entirely bound by the force of attraction between molecules, but the force still has some hold over them. Once a material has absorbed enough energy, molecules are now capable of freeing themselves from the force of attraction entirely and the material turns into a gaseous state: molecules fly around freely and are not affected by each other as much anymore. The point at which a material evaporates is known as its boiling point.

Condensation

Condensation is the opposite of evaporation. When a gas enters an environment of a lower temperature or encounters something of a lower temperature, the energy within the gas molecules begins to be sapped by the cooler environment, causing the molecules to become less excited as a result. Once this happens, they begin to be bound by the forces of attraction between each molecule, but not entirely, so the gas then becomes a liquid. A good example of this is when a piece of glass or a mirror fogs up in a hot room. The vapour or steam in a room is a gas, and the glass or mirror is a colder material in comparison. Once the vapour hits the cold material, the energy within the vapour molecules is sapped out and into the mirror, warming it slightly. As a result, the vapour turns into liquid water that ends up directly on the cold mirror surface.

Fig. 2: An example of condensation. The warm air in the room hits the cold window, turning the water vapour into liquid water.

Sublimation

Sublimation is different from the other changes of state that we have previously gone over. Usually, a material needs to change state 'one state at a time': solid to liquid to gas, or gas to liquid to solid. However, sublimation forgoes this and has a solid turn into a gas without having to turn into a liquid!

This occurs through the increase of energy within the material to the point where the forces of attraction between the molecules are entirely broken, with no in-between phase of having to be a liquid. In general, the temperature and pressure of the material would have to be very low for this to occur.

Fig. 3: The process of sublimation. The white fog is the consequence of condensation of water vapour on the cold, sublimated carbon dioxide gas.

Changes of state and the particle model

The particle model of matter describes how molecules within a material will arrange themselves, and the movement in which to arrange themselves. Each state of matter will have a way in which they are formed.

Solids have their molecules lined up against one another, the bond between them strong. The molecules in liquids have a looser bond between each other but are still bound, just not as rigidly, allowing for a wider degree of movement: they slide over each other. In gases, this bond is entirely broken, and individual molecules are able to move completely independently of each other.

Diagram of changes of state

The figure below shows the entire process of how all the changes of state relate to one another, from solid to liquid to gas and back.

Fig. 4: The states of matter and the changes they go through.