Solids, Liquids and Gases

This guide covers Topic 5, Solids, liquids and gases, in full — sub-topics (a) Units, (b) Density and pressure, (c) Change of state and (d) Ideal gas molecules — from the Pearson Edexcel International GCSE in Physics (4PH1), Issue 4 specification. Statements marked “P” are Physics-only content.

Before studying this

This resource assumes energy transfer concepts from Energy Resources and Energy Transfers.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE PHYSICS (4PH1) — Topic 5

(a) Units — using degree Celsius (°C), Kelvin (K), joule (J), kilogram (kg), kilogram/metre³ (kg/m³), metre (m), metre² (m²), metre³ (m³), metre/second (m/s), metre/second² (m/s²), newton (N) and pascal (Pa); [P] joules/kilogram degree Celsius (J/kg °C).

(b) Density and pressure — using density = mass/volume, investigated practically; using pressure = force/area; understanding pressure in a liquid or gas at rest acts equally in all directions; using pressure difference = height × density × gravitational field strength.

(c) Change of state — [P] explaining how heating changes stored energy, raising temperature or changing state; [P] describing melting and evaporation/boiling; [P] describing particle arrangement and motion in solids, liquids and gases; [P] obtaining a temperature-time graph during a change of state; [P] knowing specific heat capacity as energy per degree Celsius per kilogram; [P] using ΔQ = mcΔT; [P] investigating specific heat capacity practically.

(d) Ideal gas molecules — explaining gas pressure from random molecular motion and force on container walls; understanding absolute zero (-273 °C); describing the Kelvin scale and converting between Kelvin and Celsius; understanding why higher temperature means higher average molecular speed; knowing Kelvin temperature is proportional to average kinetic energy; explaining pressure-volume and pressure-temperature qualitative relationships for a fixed gas amount; using p₁/T₁ = p₂/T₂ at constant volume; using p₁V₁ = p₂V₂ at constant temperature.

Density and pressure

Density relates mass to volume:

density = mass / volume
ρ = m/V

Pressure is force distributed over an area:

pressure = force / area
p = F/A

In a liquid or gas at rest, pressure at a given point acts equally in all directions. Pressure difference with depth in a fluid is given by:

pressure difference = height × density × gravitational field strength
p = h × ρ × g

Worked example. Find the pressure difference between the surface and a depth of 5.0 m in water (ρ = 1000 kg/m³, g = 9.8 N/kg):

p = hρg = 5.0 × 1000 × 9.8 = 49,000 Pa

Change of state (Physics only)

Heating a system transfers energy into it, which can either raise its temperature or produce a change of state — but not usually both at once. During a change of state (e.g. melting or boiling), a temperature-time graph shows a flat, constant-temperature region while the energy supplied goes into changing the arrangement of particles rather than their kinetic energy. Solids have particles in fixed positions close together; liquids have particles close together but able to move past each other; gases have particles far apart moving freely and randomly.

Specific heat capacity c is the energy required to raise unit mass by one degree Celsius:

change in thermal energy = mass × specific heat capacity × change in temperature
ΔQ = m × c × ΔT

Worked example. How much energy is needed to heat 2.0 kg of water (c = 4200 J/kg°C) from 20°C to 100°C?

ΔQ = mcΔT = 2.0 × 4200 × 80 = 672,000 J

The Kelvin scale and absolute zero

Absolute zero, -273°C, is the lowest possible temperature, at which particles have minimum kinetic energy. The Kelvin scale is built on this: 0 K = -273°C, and one kelvin is the same size as one degree Celsius, so:

T(K) = T(°C) + 273

The kinetic theory of gases

Gas pressure arises because gas molecules move randomly and collide with the walls of their container, exerting a force on them. As temperature increases, the average speed (and hence average kinetic energy) of gas molecules increases — the Kelvin temperature of a gas is directly proportional to the average kinetic energy of its molecules.

For a fixed mass of gas, two relationships hold: at constant volume, pressure is proportional to Kelvin temperature:

p₁/T₁ = p₂/T₂

and at constant temperature, pressure and volume are inversely related:

p₁V₁ = p₂V₂

Worked example. A gas at 2.0 × 10⁵ Pa and volume 0.30 m³ is compressed at constant temperature to 0.10 m³. Its new pressure:

p₁V₁ = p₂V₂
(2.0 × 10⁵)(0.30) = p₂(0.10)
p₂ = 6.0 × 10⁵ Pa

Common mistakes

• Forgetting T(K) = T(°C) + 273 must be used before applying the gas laws — both gas equations require Kelvin temperature, not Celsius.
• Assuming temperature keeps rising during a change of state — it stays constant while the energy goes into changing particle arrangement rather than kinetic energy.
• Confusing p₁/T₁ = p₂/T₂ (constant volume) with p₁V₁ = p₂V₂ (constant temperature) — check which quantity is being held fixed in the problem.
• Using ΔQ = mcΔT across a change of state — this equation applies to temperature changes without a state change; a separate calculation (beyond this specification) is needed for latent heat during a state change.

Quick revision checklist

• ρ = m/V; p = F/A; p = hρg
• Particle arrangement in solids, liquids and gases; flat regions on a temperature-time graph during state change (Physics only)
• ΔQ = mcΔT (Physics only)
• T(K) = T(°C) + 273; p₁/T₁ = p₂/T₂; p₁V₁ = p₂V₂

Related resources

• Energy Resources and Energy Transfers — the previous topic
• Magnetism and Electromagnetism — the next topic
• Pearson Edexcel International GCSE Physics hub

Written against the Pearson Edexcel International GCSE in Physics (4PH1) specification, Issue 4. Always check the current specification for your examination year.