9.01.1 Particle Model

Syllabus

The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.
The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres. Particle theory can help to explain melting, boiling, freezing and condensing.
The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance. The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles the higher the melting point and boiling point of the substance.

What does this mean?

Particle arrangement.

Scientists believe that all matter is made of tiny pieces that they call particles.

Matter can exist in three states: solid, liquid and gas.

The particles in each state are arranged differently:

In a solid the particles are arranged in an orderly way, all touching, very close together. Solids are dense because of this close-packing
In a liquid all the particles are still touching but are a little further apart. There are spaces between some particles because the arrangement is not orderly but liquids are still dense because the particles are still close.
In a gas, none of the particles are touching. They are all very far apart. So, gases are not dense. And, unlike liquids and solids, they can be squashed in to smaller spaces by pushing their particles closer - they are compressible.

Movement and Energy.

We call the particle model Kinetic Theory because all particles can move.

The move differently in each state depending on how much energy they have in that state.

And they always move faster when they have more energy

Solids

Particles in a solid have the least energy.
They can't change place because they are held firmly to each other by very strong forces.
But they can vibrate up and down and from side by side.
If you heat them they vibrate faster and faster until eventually they can slip past each other - they have become liquid.
So they have a fixed shape and volume - in other words, if you pick up a single piece of a solid and move it, it doesn't get any larger or smaller or change in any way (unless you break it).

Liquids

In a liquid the particles have more energy than in a solid but less than in a gas.

And they are further apart so they can still vibrate but they can also slowly slip past each other.

But there are still strong forces so they can't move away from each other totally and still touch

But, when you pour them out of a container the particles slip past each other to take up the shape of the bottom of its new container.

However, liquids have a fixed volume but no fixed shape. In other words, if you pour a litre of liquid from one container to another it will still occupy one litre.

Gases

Particles in a gas have much more energy even than in a liquid. So they can completely break the strong forces.

And they are no longer touching each other at all.

So, and move around quickly but keep changing direction because they keep colliding with each other.

Gases have no fixed shape and no fixed volume. In other words, if you burst a Helium balloon the Helium particles drift apart until they are evenly spread around the room however large it is and whatever its shape.

This is diffusion.

What are the changes of state?

It is possible to change from one state to another without difficulty - changing state is a physical change - it is reversible.

In other words, if you melt ice it is easy to freeze it again.

You'll often be asked to name the changes of state in exams.

You'll already know most of these although you may never have seen a solid sublime - turn to a gas without melting into a liquid first.

In which case, you're unlikely to have seen a gas being deposited - turning straight to solid without condensing into a liquid first.

Liquids like water evaporate from their surface at all temperatures but they are only boiling when the whole body of the liquid is turning into a gas at the same time - that's what is in the bubbles you see when water is boiling.

In an exam you can call the change from liquid to gas either.

How do particles change state?

Forces of attraction between neighbouring particles in solids keep them from splitting apart.

The closer the particles the stronger these forces.

When you heat a solid its particles vibrate more - this pushes its neighbours a little further apart, weaken weaken the forces and the solid expands.

When particles are far enough apart the forces are weak enough for particles to slip past each other - the solid has melted.

Particles in a liquid don't all have exactly the same amount of energy.

The fastest moving ones sometimes have enough energy to break the surface and escape as a gas - this is evaporation.

When all the particles have enough energy to overcome the forces the liquid is boiling - all turning to gas at the same time.

If you cool down a gas the particles lose energy and slowly come together to condense back into a liquid.

Further cooling takes away enough energy so that the particles can't move about any more and so they become a solid again - it freezes.

Different substances melt at different temperatures because forces of attraction between particles are different for each substance.

Substances with very strong forces will have high melting and boiling points - eg diamonds, titanium, or salt.

Substances with very weak forces will have very low melting and boiling points - eg Hydrogen, Nitrogen, helium etc

A few substances can turn straight from a solid to a gas without melting into a liquid. We call this sublimation.

The substances then turn straight back to a gas when cooled - we call this deposition.

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