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Kinetic Theory — All matter is made up of particles which are in constant motion.
Solids
- Fixed shape and volume
- Particles are held together by relatively strong forces, and are incompressible
- Particles do not have free movement, but can vibrate about fixed positions
- An increase in temperature will give the particles more energy, the amplitude of vibration increases, therefore the solid expands
Liquids
- No fixed shape, but a fixed volume
- Particles are further apart (due to weaker forces), so there is slight compressibility
- Forces are present between particles, but weaker than in solid so particles can move throughout the bulk of the liquid
Gases
- No fixed shape or volume
- Particles are very far apart, so are easy to compress
- Intermolecular forces are negligible, but do exist. Particles move in a random fashion.
Diffusion
Diffusion is the process by which a gas fills all the space available to it. Gases will diffuse (mix with each other) because the particle are moving randomly and quickly in all directions. Lighter particles diffuse quicker than heavier particles.
Diagram showing changes of state needed:
evaporation
melting boiling
SOLID LIQUID GAS
freezing condensation
sublimation
- Changes from solid to liquid to gas are endothermic processes
- Changes from gas to liquid to solid are exothermic processes
If energy is supplied to a solid, its particles will vibrate more. If they vibrate enough, they may separate from each other and become free to move. This is called melting. At the melting point, energy goes into breaking forces between the particles, and so there is no temperature increase. The temperature at which the solid melts is called the melting point.
Heating a liquid makes the particles move faster. At evaporation, some molecules move faster than others and have more energy, so they overcome the forces of attraction. If the temperature is increased further, the kinetic energy of the particles increases until the boiling point, when the forces between the particles are almost completely broken. At the boiling point energy goes into breaking the forces between the particle, and so there is no temperature increase.
For an impure sample the melting point is lowered and the boiling point is raised.
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