Changing of states of matter- Solid, Liquid and Gases

The kinetic theory model can be used to explain how a substance changes from one state to another. If a solid is heated the particles vibrate faster as they gain energy. This makes them ‘push’ their neighboring particles further away from themselves. This causes an increase in the volume of the solid, and the solid expands. The expansion has taken place.

Eventually, the heat energy causes the forces of attraction to weaken. The regular pattern of the structure breaks down. The particles can now move around each other. The solid has melted. The temperature at which this takes place is called the melting point of the substance. The temperature of a pure melting solid will not rise until it has all melted. When the substance has become a liquid there are still very significant forces of attraction between the particles, which is why it is a liquid and not a gas.

Solids that have high melting points have stronger forces of attraction between their particles than those which have low melting points. A list of some substances with their corresponding melting and boiling points is shown in Table.


Substance  Melting point/°C Boiling point/°C
Aluminum  6612467 
Ethanol−117  79
Magnesium oxide  8273627
Mercury −30 357

If the liquid is heated the particles will move around even faster as their average energy increases. Some particles at the surface of the liquid have enough energy to overcome the forces of attraction between themselves and the other particles in the liquid and they escape to form a gas. The liquid begins to evaporate as gas is formed.

Eventually, a temperature is reached at which the particles are trying to escape from the liquid so quickly that bubbles of gas actually start to form inside the bulk of the liquid. This temperature is called the boiling point of the substance. At the boiling point, the pressure of the gas created above the liquid equals that in the air – atmospheric pressure.
Liquids with high boiling points have stronger forces between their particles than liquids with low
boiling points.

When a gas is cooled the average energy of the particles decreases and the particles move closer together. The forces of attraction between the particles now become significant and cause the gas to condense into a liquid. When a liquid is cooled it freezes to form a solid. In each of these changes energy is given out.


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