States of Matter

What is Matter ?

In science, all physical objects are called matter. All matter in the Universe is made up of atoms and molecules. The diagram below shows the structure of an atom:-

Image result for atom diagram


  1. Protons - Found in the nucleus and have a positive charge.

  2. Neutrons - Found in the nucleus and have no charge.

  3. Electrons - Orbit the nucleus and have a negative charge.

The number of protons and electrons in an atom will always be equal. This means that an atom has no overall charge.

The number of protons inside the atom tells us what type of matter it will be, for example:-

  1. 6 Protons - Carbon

  2. 8 Protons - Oxygen

  3. 92 Protons - Uranium

If matter is made from two or more atoms joined chemically together, it is called a molecule.

The diagram below shows several examples of atoms and molecules:-

The Three States of Matter

A state of matter is used to describe the structure of a material. In BGE Science, matter is divided into three states of matter:-

  1. Solids

  2. Liquids

  3. Gases

The following table gives descriptions of the three states of matter:-

Particle Spacing and Structure

By looking closely how the atoms or molecules are arranged in a solid, liquid or a gas can tell us a lot about how the matter acts:-

Looking At Solids

The diagram below shows a close up diagram of the atoms or molecules within a solid:-

In a solid the atoms or molecules are:-

  1. In a Fixed Position.

  2. Vibrating back and forth.

  3. Very close together.

  4. Have strong bonds between them.

Heating Solids

When a solid (or a liquid or gas) is heated it expands (gets bigger). This is because the hotter you make a solid, the more the atoms or molecules vibrate, making the object expand. The diagram below shows how this works:-

There are several experiments that can be done in the lab to show that solids expand when heated, or contract (get smaller) when cooled:-

The Bimetallic Strip

A Bimetallic Strip can be used to demonstrate thermal expansion. A bimetallic strip consists of two different metals bonded together, attached to a handle.

The strip is made so that when the strip is at room temperature, both metals have the same length. If the strip is heated, because the two metals are different, they will expand by different amounts. If they are attached together, the expansion of one more than the other causes the strip to bend, in whatever direction is away from the metal of biggest expansion.

The diagram below shows a visual summary of this process:-

T- Bar Expansion

The diagram below shows a Bar and Gauge apparatus:-

Image result for bar and gauge

At room temperature, the bar and the gap in the gauge have exactly the same length. If the bar is heated within a Bunsen flame, the metal of the bar expands, and will no longer fit in the gap.

Ball and Hoop Expansion

The diagram below shows a Ball and Hoop apparatus:-

At room temperature, the ball will fit through the Hoop. If the ball is heated within a Bunsen flame, the metal of the ball expands, and will no longer fit through the hoop. If the hoop is then heated, the hoop will expand and the ball will once again fit.

Thermal Expansion Case Study : Expansion Joints

Thermal expansion is a very important process to think about in Engineering. If the materials used to build something change size when changing Temperature, this could cause a lot of problems.

An example of this is railroad tracks. If railroad tracks are lain when it is cold, they will expand on a warm day. If the tracks are lain touching each other, when the Temperature rises the tracks buckle, as they can't expand along their length. The image below shows the effect of this happening on a set of railroad tracks:-

Image result for thermal expansion  train tracks

In order to stop this happening, the tracks are laid with gaps between them called expansion joints. When it gets warm, the track can expand to fill these gaps, without buckling the track. It is the sound of the train wheels passing over these expansion joints which gives the train its characteristic "clickity-clack" sound. The image below shows an expansion joint in a set of railroad tracks:-

Related image

These expansion joints are used in all construction that contains materials which expand, below can be seen an image of the expansion joints on the road surface of the Forth road bridge:-

Image result for expansion joints

The video below shows a video demonstration of thermal expansion:-

Looking at Liquids

The diagram below shows a close up diagram of the atoms or molecules within a liquid:-

In a liquid the atoms or molecules:-

  1. Are close together.

  2. Can move past each other.

  3. Have weak bonds between them.

The key difference between solids and liquids is that liquids can flow. Liquids can flow because the bonds between the atoms are weak, so they slide past each other. As the atoms or molecules are only held loosely together this causes the liquid to take the shape of the bottom of its container.

Brownian Motion

It was in liquids that the idea of matter being made of small invisible particles was formed. This was because of an observation of how pollen grains move when floating on water.

A Scottish scientist called Robert Brown used a microscope to look at pollen grains floating on water. He saw that the pollen grains moved about in random directions, changing direction all the time. He decided that the pollen was being hit over and over by invisible particles, which caused it to move. He called this Brownian Motion.

We now know that it is the water molecules themselves colliding with the pollen grain that causes it to move. The animation below shows a representation of this process:-


The random movement of molecules through Brownian motion allows another effect that can be seen in both liquids and gases - Diffusion.

Diffusion is the process of atoms or molecules moving from a place where there are lots of them to a place where there are few of them.

This can be seen in a liquid if a drop of dye or food colouring is added to a beaker of water. Over several minutes, the colour spreads out throughout the beaker, as Brownian motion causes dye molecules to spread out through the water.

The diagram below shows the process of diffusion in a liquid and a gas:-

Hiding a Liquid inside a Liquid

In the above process, when two chemicals were mixed together, the end point had both liquids spread evenly through each other.

This can have an unusual effect, depending on the chemicals used. The diagram below shows an experiment to show how a liquid can "hide" inside another:-

In the above experiment, 4 cm3 of water and 4 cm3 of methylated spirit are mixed together. The resulting mixture has a volume of only 7.5 cm3 , which means that 0.5 cm3 is missing!

But where has this 0.5 cm3 gone ?

In order to explain this effect, a second experiment can be done:-

In this experiment, 4 cm3 of peas and 4 cm3 of barley are mixed together. The resulting mixture has a volume of only 7.5 cm3 , which means that again 0.5 cm3 is missing!

This time we can see what is happening, however. Before the two are mixed, it can be seen that there are gaps between the peas. When the peas and barley are mixed together, the barley fills the gaps between the peas, giving a lower volume.

This is exactly what happens with the water and methylated spirits, the meths fills the gaps between the water molecules, giving a lower volume.

Looking at Gases

The diagram below shows a close up diagram of the atoms or molecules within a gas:-

Image result for gas particles

In a gas the atoms or molecules:-

  1. Are far apart.

  2. Can move past each other.

  3. Have almost no bonds between them.

Like liquids, gases can flow because the bonds between the atoms are so weak, so they can move far apart. As the atoms or molecules are not held together, this causes the gas to take the shape of the whole of its container.

As the gas atoms or molecules are so far apart, a gas can be compressed (squeezed into a smaller volume). The diagram below shows this process:-

In the above diagram, the gas is described as either a high or low Pressure.

Gas Pressure is a measure of how many particles of a gas strike a surface every second.

The greater the number of particles striking the surface every second, the higher the Pressure will be.

Examples of Gas Pressure

There are many examples of uses of gas Pressure, a few are listed below:-

  1. Compressed gas cylinders.

  2. Car Tires.

  3. Magdeburg Hemispheres.

Compressed Gas Cylinders

A compressed gas cylinder is an economical way to store gas. If the gas was stored at a low Pressure, it would take up a lot of room which would be expensive. By compressing the gas, its Volume gets smaller and is cheaper to store. When the gas is released from the cylinder, the gas expands again.

The diagram below shows how the atoms or molecules of the gas are compressed within a cylinder:-

Vehicle Tires

When both cars and bicycles were invented, they were built with solid wheels and tires. The image below shows a Boneshaker Bicycle built with wooden wheels covered in Iron tires:-

Image result for boneshaker bicycle

Having solid tires made the ride very uncomfortable (hence the name "Boneshaker") as the solid tires did not provide any cushioning to the bumps in the road.

By using tires containing air, the ride can be made much smoother, as when the tire hits a bump, the air in the tire is compressed, reducing the impact on the rider.

Magdeburg Hemispheres

A set of Magdeburg Hemispheres are shown in the image below:-

Image result for Magdeburg Hemispheres

Magdeburg Hemispheres are used to show the strength of air Pressure. When the hemispheres have air inside them, the air Pressure on the inside surface is equal to the air Pressure on the outside. This means there is no overall Force on the hemispheres, and they can be easily pulled apart.

If all the air is removed from the hemispheres (creating a vacuum inside with ~zero Pressure), then the air pressure outside is much bigger than the pressure inside, and the hemispheres are very very hard to pull apart!

There are competitions run in the town of Magdeburg every year to try and pull apart Magdeburg Hemispheres:-

Changing the State of Matter

So far, we have looked at each state of matter separately. We can, by heating or cooling, change matter from one state into another, for example:-

Heating Graphs

When a solid is heated until it becomes a gas, the following shaped graph will be recorded:-

In the graph above, when the matter changes state, the graph becomes flat.

This is because the heat Energy that goes into the material is used to break the bonds between the atoms or molecules, and so at that point, the temperature does not increase.

Cooling Curves

When a material is cooled, the drop in temperature is not the same every second, it does not follow a straight line. This is because Temperature changes quicker if there is a bigger Temperature difference.

This means that a cooling graph will be a curve:-

The graph above shows the Temperature of a beaker of freshly boiled water over a time of 6 hours. As can be seen, the Temperature reduces quickly when there is a large Temperature difference, then more slowly when there is a small Temperature difference. Eventually, the graph becomes flat when the beaker reaches room Temperature.