Changes of State

Depending on the temperature, a substance can be in the form of a solid, a liquid or a gas.

For example, at room temperature, water is a liquid. However, when heated, it turns into water vapour, which is a gas. When water is put in a freezer, decreasing the temperature, it turns into ice, a solid.

Physical changes do not change the particles (the particles are the same in all three states), but their arrangement and movement are different.

Matter is able to change between states and there are 5 different changes of state, which you can see below.

Melting: This is the process that changes a solid to a liquid. When a solid is heated, the particles begin to vibrate as they gain more energy. This then causes the forces between the particles to become weak.

Boiling: Otherwise known as vaporisation, this process changes a liquid to a gas. As mentioned, adding heat to liquid will cause the particles to vibrate and move around even more. This also causes the bonds between the particles to break, forming a gas.

Condensation: This process changes a gas to a liquid. To go from a gas to a liquid, the gas needs to be cooled. This causes bonds to form between the particles, which causes liquid formation.

Freezing: This process changes a liquid to a solid. To freeze a liquid, it will need to be cooled, which causes bond formation.

Sublimation: Under certain conditions, a solid can turn to a gas. An example of this is the sublimation of iodine.

When iodine crystals are heated, they form a purple iodine gas. This is an example of sublimation because iodine changes directly from a solid to a gas.

Melting and Boiling Points

Pure substances, which are substances made out of only one element, have exact melting and boiling points. The table below shows the melting and boiling points for some pure substances.

Element Melting Point (°C) Boiling Point (°C)
Oxygen -219 -183
Sodium 98 883
Nitrogen -210 -196
Fluorine -220 -188
Copper 1085 2560

When looking at melting and boiling point tables online, you will see that there are no freezing and condensing points. This is because the melting point for a substance is the same as the freezing point and the boiling point for a substance is the same as the condensing point.

For example, look at oxygen in the table above. If you cool oxygen to -183°C (the boiling point), oxygen becomes a liquid and if you cool oxygen further to -219°C then oxygen becomes solid.