States of Matter

Ice, water, steam - three different forms of the same molecule, H₂O. Despite having the same chemical formula and being made of the exact same elements, these three species have very different structures, intermolecular forces and characteristics. They're great examples of states of matter.

• This article is about states of matter in physical chemistry.
• We'll start by defining states of matter before looking at the three main states of matter: solid, liquid and gas.
• We'll also investigate ideal gases and take a deep dive into plasma.
• After that, we'll explore changes in state of matter.
• Finally, we'll look at some examples of state of matter.

States of matter definition

In 1742, the Swedish astronomer Anders Celsius invented a way of measuring temperature. He noticed that the melting point of water was almost entirely independent of its pressure, and labelled this point as 100 on his new scale. On the other hand, he showed that the boiling point of water did depend on its pressure, and labelled its boiling temperature at sea level as 0. In the following years, various scientists reversed his system until we ended up with the familiar scale we know today: the Celsius scale. Now, 0 indicates the melting point of water whilst 100 indicates its boiling point at sea level, and the units are named degrees Celsius, °C. Unlike other temperature scales, such as the Fahrenheit scale, it is based on defined and measurable states of matter.

There are three main states of matter. They're characterised by their structure, arrangement of particles, intermolecular forces and relative energy, and can be represented by the particle model, in which particles are shown as spheres. These three states are:

• Solid
• Liquid
• Gas

We'll look at them all in turn, starting from solids.

Solid state of matter

The first state of matter we'll explore today is solid. In solids, the particles are held together very closely in a regular pattern. There are very strong intermolecular forces between particles, and because of this, the particles can't move freely but instead vibrate around a fixed spot. This means that solids maintain a certain shape and volume, no matter their container. The particles also have a low energy.

Fig. 1 - The arrangement of particles in a solid

Liquid state of matter

If you heat up a solid, it eventually turns into a liquid. In liquids, the particles are randomly arranged. They're still held together closely by intermolecular forces, but these forces are partially overcome and so the particles are able to move around more freely. This means that liquids flow to take the shape of their container. However, they still have a definite volume. Because we've heated the particles, they have more energy than those in a solid.

Fig. 2 - The arrangement of particles in a liquid

Gas state of matter

The third main state of matter is gas. This is produced when you heat a liquid to an even higher temperature. In gases, the particles are randomly arranged and are spaced very far apart. There are (almost) no intermolecular forces between particles, and this means that they move freely in all directions at high speeds and have a lot of energy. Gases always fill their containers and don't have a fixed volume - instead, they can be compressed or expand.

Fig. 3 - The arrangement of particles in a gas

Ideal gases

Ideal gases are useful because they obey a certain law relating pressure (P), temperature (T) and volume (V), where PV = nRT. Here, n represents the number of moles of the gas, and R represents the universal gas constant, a value equalling 8.134 J mol^-1 K^-1. This is known as the ideal gas law, and it means that one mole of any ideal gas occupies the same volume at the same temperature and pressure. Although no gas is perfectly ideal, many gases are close enough for this law to be used in chemical calculations.

Comparing states of matter

To help consolidate your learning, we've created a handy table comparing the three main states of matter:

Fig. 5 - A table comparing the three main states of matter

Changes in states of matter

Now that we know what the different states of matter are, let's look at changes in states of matter. As the name suggests, this involves switching from one state of matter to another.

If you heat a solid, its temperature increases. However, at some point, its temperature stops increasing. Instead, the solid starts to melt. The thermal energy supplied is used to increase the kinetic energy of the particles and overcome the intermolecular forces holding them tightly together. This point is known as the substance's melting point.

Once all of the substance has melted, its temperature increases again. But as before, it reaches a plateau at a certain point. The substance starts to boil. Once again, the thermal energy supplied is used to increase the kinetic energy of the particles even further and overcome the remaining intermolecular forces between them. This is known as the substance's boiling point. Its temperature remains the same until all of the substance has turned into a gas; only then does it increase again.

The opposite is also true. If you take a gas and cool it down, it eventually condenses into a liquid. Cool it even further, and it freezes into a solid. Some solids can go straight from a solid to a gas, skipping the liquid state altogether. This is known as sublimation. The reverse process, turning from a gas to a solid, is known as deposition.

Here's a handy diagram showing you the names of the changes from one state of matter to another:

Fig. 6 - Changes in states of matter

States of matter examples

To finish, let's explore some common examples of states of matter:

• The typical example of states of matter is water, H₂O. At atmospheric pressure, it boils at 100°C and freezes at 0°C. We call gaseous water steam and solid water ice.
• Dry ice, as it is known, is the solid form of carbon dioxide, CO₂. CO₂ doesn't have a liquid state but instead sublimes directly from solid to gas.
• The only two elements on the periodic table that are liquid at room temperature and atmospheric pressure are bromine, Br, and mercury, Hg. The rest of the metals are solids whilst the non-metals are a mixture of solids and gases.
• Found at room temperature and atmospheric pressure, sand, flour and wood are all examples of solids. Milk, oil and syrup are examples of liquids whilst oxygen, ammonia and chlorine are examples of gases.