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This lesson covers how the particle model is used to explain the properties of the three states of matter, but also why the model has important limitations, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You also need to be able to predict the state of a substance from melting and boiling point data, and understand how simple and giant structures affect melting points.
The particle model represents particles as small, solid spheres. Despite its simplicity, it is remarkably useful for explaining many observations.
Gas particles are far apart with large gaps between them. When pressure is applied, the particles can be forced closer together, so the volume of the gas decreases. In contrast, solid and liquid particles are already close together, so they cannot be compressed significantly.
In a liquid, the forces between particles are weaker than in a solid, so particles can slide past each other. This allows the liquid to flow and take the shape of its container.
Solid particles are held in fixed positions by strong forces of attraction. They can vibrate but cannot change their position relative to their neighbours, so the solid retains its shape.
When a gas is heated, the particles gain kinetic energy and move faster. They collide with the walls of the container more frequently and with greater force, increasing the pressure.
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. It happens because particles in liquids and gases are in constant random motion. Gas particles move very fast, so diffusion in gases is rapid. Liquid particles move more slowly, so diffusion in liquids is slower.
Exam Tip: When explaining any physical property using the particle model, always mention (1) the arrangement or spacing of the particles, (2) the movement of the particles, and (3) the forces between the particles. This three-part approach ensures you gain full marks.
While the particle model is very useful, it is a simplified representation and has several important limitations:
Real atoms and molecules are not hard, solid balls. They are composed of nuclei and electron clouds and have complex shapes. The model does not represent the actual shape, size or internal structure of particles.
The model shows particles as separate objects but does not represent the forces of attraction between them. In reality, it is these intermolecular forces that determine the melting and boiling points of a substance.
In particle diagrams, the relative sizes of particles and the spaces between them are not drawn to scale. For example, in a real gas the particles are typically hundreds of times further apart than their own diameter, but diagrams do not show this.
The model uses identical spheres for all particles. It does not show whether the particles are atoms, molecules, or ions, and it does not represent the different sizes of different atoms or molecules.
The particle model alone does not explain why diamond has a much higher melting point than ice. To understand this, you need to consider the type of bonding and structure (covered below).
| Feature | What the Model Shows | What the Model Misses |
|---|---|---|
| Particle shape | Simple spheres | Real atoms/molecules have complex shapes |
| Forces | Not shown | Different substances have different intermolecular forces |
| Scale | Not to scale | Gaps in gases are proportionally much larger |
| Types of particle | All the same | Atoms, molecules and ions are different |
| Bonding | Not represented | Covalent, ionic and metallic bonds behave differently |
Exam Tip: If asked about the limitations of the particle model, give at least two specific limitations. Simply saying "it is too simple" is not enough. You must explain what specific features of real substances the model fails to represent.
You will frequently be given data tables showing the melting and boiling points of substances and asked to predict their state at a given temperature.
Rule:
| Substance | Melting Point (\u00b0C) | Boiling Point (\u00b0C) |
|---|---|---|
| Oxygen | −219 | −183 |
| Water | 0 | 100 |
| Iron | 1538 | 2862 |
| Ethanol | −114 | 78 |
At 25 \u00b0C (room temperature):
At 2000 \u00b0C:
Exam Tip: Read the data carefully. A common mistake is to confuse melting and boiling points. The melting point is always lower than the boiling point for the same substance. If the given temperature is exactly equal to the melting or boiling point, the substance is in the process of changing state.
The type of structure a substance has dramatically affects its melting point.
Giant structures have very high melting and boiling points because many strong bonds must be broken:
Giant covalent structures (e.g. diamond, silicon dioxide):
Giant ionic structures (e.g. sodium chloride, magnesium oxide):
Giant metallic structures (e.g. iron, copper):
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