# 3.1.1 Solids, Liquids & Gases

### Solids, Liquids & Gases

• The three states of matter are solid, liquid and gas
• The kinetic theory of matter is a model that attempts to explain the properties of the three states of matter
• In this model, particles are assumed to be small solid spheres

#### Solids

• Particles in solids:
• Are held together by strong intermolecular forces
• Are closely packed
• Are arranged in a fixed pattern (lattice structure)
• Can only vibrate about their fixed positions
• Have low energies compared to particles in liquids and gases

• As a result of the arrangement and behaviour of their particles, solids:
• Have a fixed shape (although some solids can be deformed when forces are applied)
• Have a fixed volume
• Are very difficult to compress
• Have higher densities than liquids and gases

#### Liquids

• Particles in liquids:
• Are held together by weaker intermolecular forces compared to the forces between particles in solids
• Are closely packed
• Are randomly arranged (i.e. there is no fixed pattern)
• Can flow past each other
• Have higher energies than particles in solids, but lower energies than gas particles

• As a result of the arrangement and behaviour of their particles, liquids:
• do not have a fixed shape and take the shape of the container they are held in
• have a fixed volume
• are difficult to compress
• have lower densities than solids, but higher densities than gases

#### Gases

• Particles in gases:
• Have negligible intermolecular forces between them
• Are far apart (the average distance between the particles is ∼10 times greater than the distance between the particles in solids and liquids)
• Are randomly arranged
• Move around in all directions at a variety of speeds, occasionally colliding with each other and with the walls of the container they are in
• Are negligible in size compared to the volume occupied by the gas
• Have higher energies than particles in solids and liquids

In a gas, particles can move around freely in all directions (shown by the arrows).

• As a result of the arrangement and behaviour of their particles, gases:
• Do not have a fixed shape and take the shape of the container they are held in
• Do not have a fixed volume and expand to completely fill the available volume
• Can be compressed
• Have the lowest densities (∼1000 times smaller than the densities of solids and liquids)

#### Worked Example

Liquids are about 1000 times denser than gases. Let d be the diameter of a molecule. Estimate the average intermolecular distance in a gas. Give your answer in terms of d

Step 1: Recall the equation for density

ρ = m / V

Step 2: Write down the relationship between the density of a gas and the density of a liquid

ρliquid = 1000 ρgas

Step 3: Write down the relationship between the volume of a liquid and the volume of a gas

• since the mass stays the same, the relationship between the densities translates into a relationship between volumes

Vliquid = 1000 Vgas

Step 4: Relate the volume to the average distance between the molecules, x

• the average distance x between the molecules is related to the cube root of the volume

x = ∛1000 Vliquid = 10d

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