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 or model assumes that all matter is made up of particles and that 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 amount of 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.

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.

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.

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.

Physical changes

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 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 any given temperature carry out the following quick steps:

• 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.

• 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.

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.


• The particle theory suggests that the collisions between particles will be elastic; that is collisions with no loss of kinetic energy; however it is highly likely that kinetic energy is not conserved and the collisions are in fact inelastic.

Key points

• All matter is made of constantly moving particles that have kinetic energy. The particles in gases have the most kinetic energy, solids the least. Adding or removing kinetic energy can cause matter to change its state (solid, liquid, gas) through physical changes like melting, freezing, and boiling. No new substances are formed during a physical change



• In solids the particles are tightly packed in fixed positions and vibrate. This gives solids a fixed shape and volume, and makes them incompressible. In liquids the particles are close but can move and slide past each other randomly. Liquids have a fixed volume but take the shape of their container and are also incompressible. In gases the particles move very fast and are far apart with weak attractions between the particles. Gases fill their container and have no fixed shape or volume, and they are easily compressed.


• The strength of the forces between particles affects the energy needed to change state. Stronger forces (like in giant structures with ionic or covalent bonds) lead to higher melting and boiling points. Weaker intermolecular forces require less energy to overcome.


• You can predict the state of a substance at a given temperature by comparing that temperature to its melting and boiling points (a quick thermometer sketch can help!). The particle model has limitations. It simplifies particles as solid spheres with no forces, but in reality, they can be ions or molecules with attractive forces. It also assumes perfectly elastic collisions, which isn't entirely accurate.


• The particle model is used to explain many of the properties of the three states of matter.

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