Density and States of Matter Notes

Q: What is density?

Density is mass per unit volume: \rho = \dfrac{m}{V}. A change of state keeps mass the same but usually changes volume, so it changes density. Density is the mass of a substance in each unit of volume, in kg/m^3 with mass in kg and volume in m^3.

AQA GCSE Physics: Revision notes

Key facts

Density is mass per unit volume: $\rho = \dfrac{m}{V}$ , in kg/m $^3$ .
In a solid particles are closely packed in a regular pattern and vibrate in place.
In a liquid particles are closely packed but random, and move past each other.
In a gas particles are far apart and move quickly in all directions.
Changes of state conserve mass and are physical changes.

Density

Density is mass per unit volume: $\rho = \dfrac{m}{V}$ . A change of state keeps mass the same but usually changes volume, so it changes density.

Density is the mass of a substance in each unit of volume, in kg/m $^3$ with mass in kg and volume in m $^3$ .

When a substance changes state its mass is conserved, but its volume usually changes, so its density changes too.

Mass against volume for aluminium: a straight line through the origin whose gradient is the density, 2.7 g/cm³.

Density $\rho = \dfrac{m}{V}$

Worked example

A 2 kg block has a volume of 0.001 m³. Calculate its density.

1
Use $\rho = m/V$ .
2
Substitute: $\rho = 2 / 0.001$ .

A 500 kg object has a volume of 2 m³. What is its density?

The particle model

Solids, liquids and gases differ in how closely packed their particles are and how they move.

The particle model explains states of matter by how atoms or molecules are arranged and move. You should be able to draw simple particle diagrams of a solid, a liquid and a gas.

A gas has the lowest density because its particles are far apart, so there is little mass in each unit of volume.

Solid

Liquid

Gas

Particle diagrams of a solid, a liquid and a gas.

Solid Liquid Gas Arrangement Closely packed, regular pattern Closely packed, random Far apart, random Movement Vibrate in place Move around each other Move quickly in all directions Density Highest (usually) Middle Lowest

	Solid	Liquid	Gas
Arrangement	Closely packed, regular pattern	Closely packed, random	Far apart, random
Movement	Vibrate in place	Move around each other	Move quickly in all directions
Density	Highest (usually)	Middle	Lowest

Solid

Arrangement:: Closely packed, regular pattern
Movement:: Vibrate in place
Density:: Highest (usually)

Liquid

Arrangement:: Closely packed, random
Movement:: Move around each other
Density:: Middle

Gas

Arrangement:: Far apart, random
Movement:: Move quickly in all directions
Density:: Lowest

Which state of matter has the lowest density?

Changes of state

Mass is conserved in a change of state, and the change is physical, so the original substance can be recovered.

The changes of state are melting, freezing, boiling, evaporating, condensing and subliming. Mass is conserved: no matter is created or destroyed.

They are physical changes: no new substance forms, so if the change is reversed the substance recovers its original properties.

1
Solid
particles vibrate in place
2
Melting
solid becomes liquid
3
Liquid
particles move around each other
4
Evaporating or boiling
liquid becomes gas
5
Gas
particles move quickly, far apart

Changes of state as energy is added (reverse them by removing energy)

Melted ice is refrozen. What is true about the new ice?

Try an exam question

A block has a mass of 2 kg and a volume of 0.001 m³. Calculate its density. Describe how the arrangement and movement of particles in a gas differ from those in a liquid.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.