Solid, liquid and gas

Three states

Matter around us is found in three states: solid, liquid and gas. These states come from the way that the particles are arranged, and from the strength of the force between them.

Solids

A solid, such as a stone, a coin or a pencil, has these properties.

• It has a fixed shape and a fixed volume.
• It is hard to compress. Its particles are very close together, and the force between them is strong.
• Its particles do not move from place to place. They only vibrate about fixed positions.
• A solid is rigid. It keeps its shape when we push it a little. A rubber band and a sponge change shape, but they go back, and a sponge can be compressed because it has holes with air in them.

Liquids

A liquid, such as water, oil or milk, has these properties.

• It has a fixed volume, but no fixed shape. It takes the shape of the container.
• It flows. The particles can slide past each other.
• It is hard to compress, but a little less hard than a solid.
• It diffuses into other liquids more easily than a solid does.

Gases

A gas, such as air, oxygen or the gas in a cooking gas cylinder, has these properties.

• It has no fixed shape and no fixed volume. It fills the whole container.
• It is easy to compress. The particles are far apart, with a lot of space between them.
• Its particles move very fast, in all directions, and they hit the walls of the container. This makes the pressure of the gas.

Density

How much mass is in a given volume of matter? This is the density.

Example. A brass weight has a mass of $0.34\ \text{kg}$ and a volume of $4.0 \times 10^{-5}\ \text{m}^3$. Its density is $\dfrac{0.34\ \text{kg}}{4.0 \times 10^{-5}\ \text{m}^3} = 8500\ \text{kg/m}^3$.

In the laboratory we often weigh in grams and measure in cubic centimetres. Then $1\ \text{g/cm}^3 = 1000\ \text{kg/m}^3$. Water has a density of $1\ \text{g/cm}^3$, which is $1000\ \text{kg/m}^3$.

Solids usually have a higher density than liquids, and liquids a higher density than gases, because their particles are closer together. Some solids, such as ice, are less dense than the liquid (water), and so ice floats on water.

You should find that a block floats when its density is below $1000\ \text{kg/m}^3$, and that a block of $500\ \text{kg/m}^3$ floats half under water.

Beyond the three states

There are two more states of matter. Plasma is a gas of charged particles, found in the Sun, in stars and in a fluorescent tube. A Bose-Einstein condensate is a very special state that forms at a temperature very close to the lowest possible. We study the three common states in this chapter.

Practise with new numbers

Try it in Python

This program finds the density of one cubic metre of four materials.

Remember these

• A solid has a fixed shape and a fixed volume. A liquid has a fixed volume and no fixed shape. A gas has neither.
• Solids and liquids are hard to compress. A gas is easy to compress.
• Density is mass divided by volume.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A gas spreads to fill the whole of any jar. Why?

2. A piece of metal has a mass of $0.45\ \text{kg}$ and a volume of $5.0 \times 10^{-5}\ \text{m}^3$. What is its density? Will it float on water?

3. Why can a liquid be poured, but a solid keeps its shape?

4. A wooden block floats with $0.6$ of it under water. What is its density?