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# 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.

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• 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

States of matter are one of the distinct physical forms in which matter can exist.

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.

You might also hear the term 'phase' being used when talking about states of matter. Although these terms are similar, they have slightly different meanings. A phase is defined as a chemically distinct, physically uniform region of a species. This means that each distinct phase has the same structure, density, index of refraction and magnetisation. States of matter are all examples of phases, but you can get different phases within states of matter. For example, solid ice has many different phases, differentiated by their unique crystal structures.

## 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

You'll look at different types of solids in the article "Lattice Structures". There, you'll be able to compare molecular, covalent, ionic and metallic lattice structures and their properties.

## 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

An ideal gas is a theoretical gas that doesn't have any intermolecular forces or interactions between molecules. Molecules are assumed to be particles with no volume, and no kinetic energy is lost when they collide.

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.

Gases that don't behave quite like ideal gases are known as real gases. We have an article all about ideal and real gases which should help you compare the two. Check out "Ideal and Real Gases" for more. And if you want to try your hand at calculations involving the ideal gas law, then head over to "Ideal Gas Law" for plenty of worked examples.

## Plasma state of matter

There is actually a fourth state of matter that is more common than you think. In fact, it plays a role in many everyday objects and phenomena. This state is called plasma.

Similar to how heating a liquid turns it into a gas, heating a gas turns it into plasma. Plasma can also be created using a laser, microwaves or any magnetic field. Much like gases, the particles in plasma are randomly arranged and spread far apart. They don't have a fixed shape or volume and expand to fill their container. However, unlike gases, plasma is made from charged particles. When you heat a gas to a high enough temperatures (or carry out one of the other methods of creating plasma), you separate some of the particles into negatively charged electrons and positively charged ions. These electrons are called free electrons. These charged particles mean that plasma can conduct electricity. If only some of the particles in plasma are ionised, the plasma is said to be partially ionised. But if all of the particles are ionised, the plasma is said to be fully ionised.

Fig. 4 - The arrangement of particles in plasma

You'll find plasma in stars, neon lights, plasma televisions and lightning.

## 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

We can expand on this to talk about changes of state when it comes to plasma. Turning from a gas to plasma is known as ionisation, whilst turning from plasma back to a gas is known as deionisation or recombination.

## 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.

## States of Matter - Key takeaways

• States of matter are distinct physical forms in which matter can exist.
• The three main states of matter are solid, liquid and gas. The particles in these states have different arrangements, speeds and energy levels, amongst other properties:
• The particles in a solid are held together very closely in a fixed position. They have low energy and vibrate on the spot.
• The particles in a liquid are held together closely but are able to move around. They have slightly higher energy than the particles in a solid.
• The particles in a gas are spread far apart and move around rapidly. They have very high energy.
• There is a fourth state of matter, known as plasma. Plasma is made by ionising a gas and so contains charged particles.
• Changing states of matter involves heating orcooling a substance. When a substance changes state, its temperature remains the same until all of the particles are in the new state.
• Examples of states of matter include water, H2O. As a solid, it is known as ice and as a gas, it is known as steam.

#### Flashcards in States of Matter 38

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What are states of matter?

States of matter are one the distinct physical forms in which matter can exist. There are three main states of matter: solid, liquid and gas. However, plasma is another common state of matter.

What are examples for describing states of matter?

The most common example you'll come across for describing states of matter is water. At temperatures below 0°C, it forms a solid we call ice. At temperatures above 100°C, it boils to form a gas we call steam. At temperatures in between, it is found in its liquid state.

What are solid and liquid states of matter?

Solid and liquid are two distinct states of matter. The particles in a solid are arranged very closely together and vibrate on the spot with a low energy. Solids also have a fixed volume and shape. The particles in a liquid, on the other hand, are packed less closely together and move about randomly with more energy. Although liquids still have a definite volume, they change shape to fill their container.

What is gas state of matter?

Gas is a type of state of matter, in which the particles are spaced far apart and move about quickly and randomly, with a lot of energy. Gases don't have a fixed volume or shape. Instead, they expand to fill their container.

What causes states of matter to change?

Heating or cooling a substance causes it to change its state of matter. The thermal energy is used to increase the kinetic energy of the particles and overcome some of the intermolecular forces between them, causing a change of state.

## Test your knowledge with multiple choice flashcards

What are the three main states of matter?

In which state of matter do the particles have the most energy?

In which state of matter are the particles held most closely together?

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