# 3.1.2 Solids, Liquids & Gases

### The Particle Model

• All matter is made up of very small particles, or atoms
• The particle model is a model that describes the arrangement and movement of particles in a substance
• The particle model can be used to explain
• The different states of matter e.g. solids, liquids and gases
• Physical properties e.g. differences in density

### Solids, Liquids & Gases

• Matter can exist in one of three different states: solid, liquid, or gas

#### Solids

• In a solid:
• The particles are closely packed
• The particles vibrate about fixed positions
• Solids have:
• A definite shape (they are rigid)
• A definite volume

#### Liquids

• In a liquid:
• The particles are closely packed
• The particles can flow over one another
• Liquids have:
• No definite shape – they are able to flow and will take the shape of a container
• A definite volume

#### Gases

• In a gas:
• The particles are far apart
• The particles move randomly
• Gases have:
• No definite shape – they will take the shape of their container
• No fixed volume – if placed in an evacuated container they will expand to fill the container
• Gases are highly compressible, this is because:
• There are large gaps between the particles
• It is easier to push the particles closer together than in solids or liquids

Diagram showing the three states of matter in terms of shape and volume

Solid, Liquid, Gas Summary Table

### Differences in Density

#### Solids & Liquids

• In solids and liquids, the molecules are tightly packed together
• The difference is, in a liquid, the molecules have enough energy to push past each other
• As a result of this, the density of solids and liquids are roughly the same

The molecules in solids and liquids are tightly packed, giving them a high density

#### Gases

• In a gas, the molecules are widely separated
• As a result of this, gases have significantly lower densities than solids or liquids
• At room temperature, the distance between molecules in a gas is roughly ten times (in each direction) the distance between molecules in a solid or liquid
• As a result, the density of a gas is typically around one-thousandth (1/1000) of the density of a solid or liquid, for example:
• The density of water is 1000 kg/m3
• The density of air at sea level and room temperature is 1.3 kg/m3

The molecules in a gas are widely spaced, giving it a much lower density

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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