The Particle Model and Density of materials

All substances or materials have a known density.
Different solids, such as various woods or metals or plastics, have different densities.
Different liquids, such as water, oil or different alcohols, have different densities.
Different gases, such as hydrogen, oxygen, helium or air (which is a mixture of gases), have different densities.

So, what do we mean by Density?

You will know what we mean by Mass of a material, I hope!
and you will know what we mean by Volume of a material.

The Density of a material is its Mass per unit Volume.
or, Density is "mass over volume".

As an equation, we can write it like:

Ok, its a ratio! But what's so special about it?

Well, unlike the Mass or Volume of a material such as Aluminium which can vary infinitely (it might be 1kg, 2kg, or 3kg etc and it might have a small, medium or large volume), we find that the Density of Aluminium is always a fixed value; it is 2800 Kg/m³.

And this is true for all materials. All named materials have known Density values.
So, of the 3 quantities, Mass, Volume and Density, the only one that is ever fixed which you can look up in a list is Density.

Here is a table of some materials and their densities:

Material	Density, ρ
	(kg/m3)
Aluminium	2800
Copper	8920
Gold	19,300
Ethyl alcohol	806
Fresh Water	1000
Sea Water	1030
Hydrogen	0.09
Helium	0.18
Air	1.29

What can we learn from looking at this table?

Well, compare the Density of Copper to that of Aluminium; it is much larger, isn't it.
This means that a certain size piece of Copper will always be heavier (have a greater mass) than the same size piece of Aluminium.
Look at the density of Gold; this tells us that Gold is even heavier than Copper (it is more than twice as heavy, which is why, in movies, we always see people struggling to lift relatively small bars of Gold).

Let's use the equation to calculate and compare the Masses of a certain Volume of Aluminium and Copper.

What else can we learn from the table?

I'm sure you can recognise that some of the materials are Solids, some are Liquids and some are Gases.
Notice the big difference in the densities of these 3 states of matter.
Solids have large densities compared to Liquids and Gases; Liquids have medium densities; Gases have very small densities.
When scientists first noticed this difference in the densities of the 3 states of matter they gradually developed a theory to explain it.
The theory became known as The Particle Model of Matter.

The theory is simple and states that:
1. All matter, (whether in the state of a solid, a liquid or a gas), is made of tiny particles
2. The arrangement and motion of the particles determines whether a particular piece of matter is in the solid state, liquid state or gas state.

So, in a solid, the particles are very closely spaced such that they form a 3 dimensional pattern, and although they move, they only do so as a vibration about a fixed position. (This suggests that the forces between the particles are significant.) The speed and size of their vibration will increase as the temperature increases. This structure of particles gives rise to the rigid nature that we expect of solids.
Notice in the diagram below how the particles of the solid "hold their own shape" regardless of the shape of the container.

In a liquid, the particles are still quite close but they are free to move over each other. (This suggests that the forces between the particles are present but weak.) Again, their speed of movement will increase if temperature is increased. This structure of particles enables liquids to flow, fitting into different shape containers; all characteristics that we expect of liquids.
Notice in the diagram below how the particles of the liquid do not "hold their own shape" but flow to fit in the shape of the container.

In a gas, the particles are not at all close. They are spaced very far apart (compared to their size) and are completely free to move within any container or to disperse freely outside of a container. The particles are in "constant random motion"; a useful phrase to learn. (This suggests that the forces between the particles are virtually none existant!). The speed of movement of the particles in the gas will increase if temperature increases. This structure explains the characteristics we expect of a gas.
Notice in the diagram below how the particles of the gas freely move within and outside the container.

But how does the Particle Model explain the different densities of the three states of matter?
Its all to do with mass and volume, and the equation for density.

In the three diagrams above, each individual particle is identical and has the same mass, so you can see that the Solid has the largest mass since it has the largest number of particles. Also you can see that the Solid fits into a smaller volume than either the Liquid or the Gas. So, a large mass with a small volume gives rise to a large density ( ie ρ = m/V).
The Gas, with fewer particles, has the smallest mass but a huge volume, since its particles have even spread outside the container. So, the Gas has, by far, the smallest density (ie. small mass and large volume).
The Liquid lies between these two values with a medium density (fewer particles in the container than the solid and a larger volume, but not as extreme as in the gas).

Finally, what do we mean when we use the word Particles?
We either mean Atoms or Molecules (combinations of atoms)
If the material is a pure Element eg helium or copper, then the "particles" are Atoms (eg. of helium or of copper).
But if the material is a piece of wood or water, which are not a pure elements, then the "particles" are Molecules.

Another finally!
Why is the density of water a very neat 1000kg/m³, or altering the units, 1 g/cm³ ?
The answer is very simple, but I will leave it to you to find it out.

Before we move on, have a go at the following questions.

Changing the state of some matter

The particle model tells us that the ONLY difference between, say water, in its solid state compared to its liquid state compared to its gas state (steam) is the arrangement and motion of its particles.

The particles themselves NEVER change in any way at all!

So, in Ice the particles are arranged in a rigid, closely packed 3 D pattern with no space between each particle; the particles will merely vibrate.

However in Water the particles are close to one another but are able to move (or flow) over and around each other.

Whilst in Steam the particles are spaced far apart, moving to the extremes of their enclosure hardly ever encountering each other.

So Change of State is merely a change in how the particles of a substance are arranged and move.

How to cause a Change of State

We hinted at this in our description of the movement of the particles in the previous section. We said that in all three states the "speed (and size) of movement of the particles increased with temperature."

From Solid to Liquid
To alter the arrangement of particles from that of a Solid to that of a Liquid all we do is increase temperature, vibrating the particles of the Solid so much that the large attractive forces are overcome enabling the particles to move with more feedom, and so we have produced a Liquid.

From Liquid to Gas
To change the Liquid to a Gas, we keep increasing the temperature causing the particles to move even faster and further until the forces between them become virtually non-existent, and we have produced a Gas.

So, Change of state is caused by change of Temperature.

A few points to note:
1. That the total number of particles doesn't change when a change of state occurs; there is no reason why it should change. This means that the mass of the substance doesn't change when it goes from solid, to liquid, to gas.

2. When change of state occurs in this way, we say that the solid MELTS from solid to liquid, and that the liquid BOILS from liquid to gas. And in the reverse direction, if the temperature falls, we say that a gas CONDENSES to a liquid, and that a liquid FREEZES to a solid. The fact that these changes of state are reversible is proof that they are physical changes and not chemical changes.

3. When a liquid BOILS like this, all of its particles begin to move rapidly, breaking free of the attractive forces and moving apart, as shown, assuming that the high temperature is maintained.
But there is another way that a liquid can change into a gas and that is by Evaporation.

Evaporation
If you hang a wet cloth on a clothes line on a warm, dry (and slightly windy) day, the cloth will gradually dry. Also, if you were to measure the mass of the cloth whilst it was wet and then again when it was dry, you would find that the mass would decrease.
Another example would be, if you watched a puddle of water over a period of a few hours on a warm dry and slightly windy day, you would notice that the level of the water would fall and fall until eventually the water would be gone.
So what is happening in these two examples?

The simple answer is: the water is evaporating, changing state from liquid to gas, but unlike in the case of Boiling, the evaporation is a much slower process and occurs at any temperature (as long as it is above the freezing point of the substance, eg the water).

Have a look at the following diagram of the puddle example.

From the diagram you should be able to realise that evaporation can occur at any temperature which makes it much different to boiling which only occurs at a specific temperature for each liquid.
But the rate of evaporation will increase if:
1. There is a wind.
2. There is an increase in temperature eg. caused by sunlight or a heater indoors.
3. There is a large surface area (eg a wide but shallow hole in the road or a wet cloth spread out rather than bundled up).

Sublimation

Sublimation is an unusual process where a Solid can change to a Gas without going through the Liquid state!
A good example of a material that sublimates is so-called "Dry Ice".
Dry Ice is used in theatres and concerts to create a smoke or cloud effect where it can be thrown onto a stage (as a solid) and it quickly turns to a thick white Gas. Dry Ice is actually very cold, solid, carbon dioxide and it sublimates (changes from solid to gas) at room temperature.

Like with all the changes of state mentioned so far, when it occurs due to sublimation, there is no change to the overall mass of the substance because the total number of particles never changes.