3 states of matter

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:

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:


particle arrangement in  solid In a solid:


particle arrangement in  a liquid In a liquid:


particle arrangement in a gas In a gas the particles:

Changing state

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

thermometer scale 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.

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: