Introduction to phases

Compounds are found in different phases at different temperatures. The three phases are solid, liquid and gas.


In a solid the particles that make up a material are packed very close together in a regular structure. A solid keeps its shape and does not become the shape of its container. They have a fixed volume – this means they take up a fixed amount of space. The atoms or molecules in a solid are held together by chemical bonds.

Wood and rocks and metals are examples of solids at room temperature. Granulated substances are also solids such as sand and salt and sugar. These granulated substances can act a bit like liquids but are not liquids. They are just solids broken up into small granules. These granules have no attractive forces between them which is why they behave a little bit like a liquid. But each granule is a group of particles that are chemically bonded together as a solid.


Liquids have weaker intermolecular forces between particles than solids. This allows the particles to flow and slide over each other. Liquids take the shape of their container but still take up a fixed volume (amount of space).

Water and some oils are examples of liquids when at room temperature.


There are no intermolecular forces between particles of a gas. Each gas molecule moves randomly around its container. Gases expand or shrink to entirely fill their container so do not take up a fixed volume (amount of space). They can be squashed.

The air around us is a mixture of gases. It is mostly made up of nitrogen and carbon dioxide and oxygen. Oxygen in the gas we breathe in to live. We breathe out carbon dioxide.

Due to the molecules of gas being so far apart gases are usually transparent. Some gases are coloured such as chlorine gas but these are uncommon in everyday life.


Most compounds can be all three phases. The phase of a compound at room temperature depends on its boiling point and melting point.

The phase change from solid to liquid is called melting. The melting point of a solid is the temperature at which it becomes liquid. The phase change from liquid to gas is called boiling. The boiling point of a liquid is the temperature at which it becomes a gas.

At room temperature water is a liquid. If water is cooled below 0 Celsius it freezes into the solid we call ice. If water is heated above 100 C it becomes the gas we call steam. This means that water has a melting point of 0 C and a boiling point of 100 C.


The three phases a substance can be are solid liquid and gas. The phase taken is due to the temperature of the substance. The temperature at which each phase occurs is due to the melting and boiling points of a substance.



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