| Chemical substances exist in different states depending on their structure and the temperature. Changes of state may be brought about by input, or removal, of energy from a system |
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The states of matter
There are three states of matter, solids, liquids and gases. These states are interchangeable on input of heat energy.
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solid
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liquid
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gas
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Solids have strong forces holding the particles together. The particles are not able to move, apart from vibrations.
Liquids have forces holding the particles together, however the energy of the particles is sufficient to allow some translational and rotational motion of the particles with respect to one another.
Gases have particles with relatively large energies compared to the forces that would act between them. However, due to the motion of the gas particles the distances between them are too great to allow forces of interparticular attraction to act. At least, this is an assumption of the ideal gas model.
The different terms used to describe changes of state are shown in the table below.
| Change of state | from | to |
| melting or fusing | solid | liquid |
| solidification | liquid | solid |
| boiling or vaporisation | liquid | gas |
| condensation or liquefaction | gas | liquid |
| sublimation | solid | gas |
| condensation | gas | solid |
Enthalpy of fusion
This is the energy required to overcome the forces holding the particles together in a solid. The solid melts. Throughout the process the temperature of the solid/liquid mixture remains constant.
solid  liquid |
Enthalpy of fusion is always endothermic. Energy is needed for the process to occur.
Enthalpy of vaporisation
This is the energy required to completely separate the particles in a liquid. The liquid state becomes gaseous. Throughout the process the energy input is used to overcome the forces of attraction between particles and the temperature remains constant.
| liquid gas |
Enthalpy of vaporisation is always endothermic. Energy is needed for the process to occur.
Cooling and heating curves
Cooling, or heating, curves demonstrate how the temperature remains constant throughout the changes of state.
A low melting point substance, such as naphthalene, may be heated up steadily from room temperature in a water bath, showing clearly how the temperature remains constant during the solid to liquid phase transition.
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