Basics of Solids, Liquids and Gases

Back to Physics Grade 9/10

Solids, Liquids and Gases

With some exceptions, you can generally classify everyday things as either a solid, liquid or gas. You probably already know what a solid, liquid and gas are, but let's have a quick review.

A solid is a substance that has a very close and organised molecular structure. The intermolecular forces (the forces between the molecules) are very strong. Consequently, the molecules don't move over one another, however they are able to vibrate in a fixed place. Because the molecules don't move over one another, solids don't move or flow - they maintain a fixed shape. The very close proximity of the molecules in a solid means there's little to no space between them - consequently, a solid cannot be compressed. A good example of a solid is a block of wood or ice.

A liquid is a substance whose particles are randomly arranged, and slightly further away than those in a solid (i.e. they're still VERY close to each other). The intermolecular forces are strong enough to keep the molecules together, but weak enough to allow movement over one another. This means liquid molecules can move about randomly. It also means that a liquid substance will flow with relative ease (especially compared to solids), and so a liquid can change shape. This means that it will take on the shape of whatever container it is put in. Because there's such little space between the molecules in a liquid, liquids generally don't compress. A good example of a liquid is water at room temperature.

A gas is a substance whose particles are far apart. There are no forces between gas molecules and they are arranged in a completely random manner. The particles vibrate and move freely in all directions, including up. This means that a gas will take on the shape of the entire container it is in (i.e. unlike a liquid which will take on the shape of just the lower part of the container (unless filled to the brim)). The extra space between the particles means that a gas can be compressed. A good example of a gas is steam.