Higher and foundation tier

## The three states of matter

The particle model as its name suggests is a model or theory used by scientists to explain many of the physical properties of matter (solids, liquid and gases). The particle theory assumes that all matter is made up of particles. These particles:

• are constantly moving in a random way.
• All the particles in solids, liquids and gases have energy. The particles in solids have the least energy and the particles in a gas have the most energy.
• if the particles gain or lose enough energy they can change from one state to another.
• when collisions happen between the particles there is no loss of kinetic energy - that is the collisions are elastic.
• are assumed as being solid spheres with no forces between them.
The particle model is used to descibe how the particles in solids, liquids and gases are arranged. I am sure you will have seen these particle pictures of the three states of matter from previous science lessons. The particle pictures are shown below:

## Solids

In a solid:

• The particles are close together in fixed positions. They vibrate about these fixed positions.
• There are strong forces of attraction between the particles in a solid which keeps them in place and prevents them from moving, but they can vibrate about their fixed position.
• Since the particles cannot move freely from one point to another. Solids have a fixed shape and a fixed volume.
• Solids cannot be compressed. The particles are already touching and so cannot be forced closer together.

## Liquids

In a liquid:

• The particles are in constant random motion.
• the particles are touching, this means like solids they cannot be compressed. The particles in a liquid cannot be forced any closer together.
• The particles can slide and flow over each other so liquids they have no fixed shape, they take the shape of their container. They may take the shape of their container but liquid have a fixed volume.

## Gases

In a gas the particles:

• move very fast, in all directions in a totally random way.
• spread out quickly to fill any container they are in. They have no fixed shape but take the shape of the container they are in
• have weak forces of attraction between them.
• have large gaps between them so gases are mostly empty space, so they are easily compressed.

## Changing state

During a chemical change (a chemical reaction) new substances are made, the reactants turn into products. During a physical change no new substances are made. The particles simply rearrange themselves by losing or gaining energy e.g. melting, freezing and boiling are all physical changes.

To change the state of a substance from a solid to liquid to gas, at each step the particles need to gain kinetic energy, that is they need to move faster. The particles will also start to separate and move apart as they gain kinetic energy (move faster). You need to think about what type of structure the substance being heated has. Does it have a small molecular structure with weak intermolecular bonds or does it have a giant structure, either ionic or a giant covalent structure? Remember that intermolecular bonds are around 3-10% the strength of a typical covalent bond so will require considerably less energy to break.

The stronger the forces of attraction between the particles the more heat energy will have to be provided in order to overcome and break the bonds/intermolecular forces holding the particles together. If the substance being heated has a giant structure with lots of strong ionic or covalent bonds then lots of energy will have to be supplied to break these bonds and the substance will have a very high melting and boiling point.

## Predicting states

To predict the state of a substance at a certain temperature is not always as straight forward as it seems, especially when the melting and boiling points are very very low e.g. nitrogen gas has a melting point of -2100C and a boiling point of -1960C . To help you predict the state of a substance at a given temperature quickly sketch out a small thermometer at the side of your page (as shown opposite). Mark on it room temperature, 250C . Also as a guide mark on 00C and 1000C , the melting and boiling points of water. Do not try and mark your scale accurately, just space the numbers out approximately. Next mark on your thermometer the melting and boiling points of the substance you have been given, in this case the m.p. is -2100C and the b.p. is -1960C . Just mark these numbers roughly where they might come on the scale, don't try to be accurate, it's not necessary. Mark or colour above and below these numbers as shown in the diagram opposite.

• The blue section, below its melting point at -2100C, means that the substance has not yet melted, so it's a solid here.
• The greenish section is below the boiling point (b.p.) so it has not yet boiled, the substance is still a liquid.
• The yellow portion is above the boiling point, so the substance, nitrogen in this case is a gas.
Drawing these thermometer diagram as a quick sketch only takes a few seconds and helps avoid you getting the wrong answer when asked to predict the state of a substance at a given temperature.

## Limitations of the particle model

Like most scientific model, the particle model is not perfect, it has its good and bad points. However some of the assumptions that it makes are not always valid:

• One of the assumptions of the particle model is that it assumes that all the particles are solid spheres with no forces between them. In reality the particles are likely to be ions or molecules which will have attractive forces between them. The particles could be molecules with fairly large intermolecular bonding/forces of attraction between them. These intermolecular forces can have large effects on properties such as viscosity and melting and boiling points. If the particles are charged, such as ions, then the forces between them will be large.
• Particle theory suggests that the collisions between particles will be elastic (no loss of kinetic energy). However it is highly likely that kinetic energy is not conserved and the collsions are in fact inelastic.