Understanding Solids, liquids and gases: Edexcel IGCSE Physics Explained

From ice melting in your drink to the air pressure holding up an aeroplane, the behaviour of matter in its three states is something you experience every day. Here is how the Edexcel IGCSE Physics specification breaks it all down.

The solids, liquids and gases edexcel igcse section is one of the most heavily examined parts of the course, particularly the density and pressure topic. It covers four areas: units, density and pressure, change of state, and ideal gas molecules. The calculations are generally approachable, but the concepts behind them require careful understanding. Getting the physics right here sets you up for strong marks across both papers.

These edexcel igcse physics revision notes take you through each topic, with worked examples, flagged mistakes, and self-check questions. If you have ever wondered why a heavy ship floats while a small coin sinks, or why a pressure cooker cooks food faster, you are about to find out.

Units for solids, liquids and gases

This section uses a wide range of units. Make sure you are comfortable with all of them:

Temperature: degree Celsius (°C) and Kelvin (K)
Energy: joule (J)
Mass: kilogram (kg)
Density: kilogram per metre cubed (kg/m³)
Length: metre (m)
Area: metre squared (m²)
Volume: metre cubed (m³)
Speed: metre per second (m/s)
Force: newton (N)
Pressure: pascal (Pa)
Specific heat capacity: joules per kilogram per degree Celsius (J/(kg °C))

Density and pressure

Density

Density is mass per unit volume: density = mass / volume. The unit is kg/m³. A material with high density packs more mass into each cubic metre. Steel has a higher density than wood, which is why a steel block sinks in water while a wooden block of the same size floats.

Worked example: A block has a mass of 540 g and dimensions 10 cm x 6 cm x 3 cm. Calculate its density in kg/m³.
Volume = 0.10 x 0.06 x 0.03 = 0.00018 m³
Mass = 0.54 kg
Density = 0.54 / 0.00018 = 3000 kg/m³

To measure density experimentally, find the mass using a balance and the volume using a ruler (for regular shapes) or a displacement can (for irregular shapes).

Pressure

Pressure = force / area, P = F / A. The unit is the pascal (Pa), where 1 Pa = 1 N/m². Pressure tells you how concentrated a force is. A drawing pin has a sharp point with a tiny area, so even a small force produces high pressure, allowing it to push into a surface.

Worked example: A box weighing 600 N rests on the floor. Its base has an area of 0.3 m². What pressure does it exert?
P = F / A = 600 / 0.3 = 2000 Pa

In a gas or liquid at rest, pressure at any point acts equally in all directions. This is a key concept that explains how hydraulic systems work and why a diver feels pressure from all sides.

Pressure in liquids

The pressure difference in a liquid is given by: p = h x ρ x g, where h is the height of the liquid column (m), ρ is the density of the liquid (kg/m³), and g is gravitational field strength (N/kg). Pressure increases with depth because there is more liquid above pressing down.

Worked example: Calculate the pressure at a depth of 5 m in water (density = 1000 kg/m³, g = 10 N/kg).
p = h x ρ x g = 5 x 1000 x 10 = 50,000 Pa = 50 kPa

Change of state

Particle model

The arrangement and motion of particles differs between the three states of matter:

Property	Solid	Liquid	Gas
Arrangement	Regular pattern, closely packed	Irregular, close together but free to move	Random, widely spaced
Motion	Vibrate about fixed positions	Move around each other	Move rapidly in random directions
Forces between particles	Strong	Moderate	Very weak

When a solid is heated, it melts to form a liquid. When a liquid is heated further, it evaporates or boils to form a gas. During a change of state, the temperature remains constant even though energy is still being supplied. The energy goes into breaking the bonds between particles rather than increasing their kinetic energy. A temperature-time graph for a substance being heated shows flat sections at the melting point and boiling point.

Specific heat capacity

Specific heat capacity (c) is the energy required to raise the temperature of 1 kg of a material by 1 °C. The equation is:

change in thermal energy = mass x specific heat capacity x change in temperature

ΔE = m x c x Δθ

Worked example: How much energy is needed to heat 2 kg of water from 20 °C to 100 °C? (c for water = 4200 J/(kg °C))
ΔE = m x c x Δθ = 2 x 4200 x (100 - 20) = 2 x 4200 x 80 = 672,000 J = 672 kJ

Water has one of the highest specific heat capacities of any common material, which is why it is used in central heating systems and why coastal climates are more moderate than inland ones.

Ideal gas molecules

Gas pressure and particle motion

Gas molecules move in random directions at various speeds. They collide with the walls of their container, and each collision exerts a small force on the wall. The combined effect of billions of collisions produces a measurable pressure. Increasing the temperature increases the average speed of the molecules, so they hit the walls harder and more often, increasing the pressure.

Absolute zero and the Kelvin scale

Absolute zero is -273 °C. At this temperature, particles have the minimum possible kinetic energy. The Kelvin scale starts at absolute zero: T(K) = T(°C) + 273. At 0 K, there is no molecular motion (in the classical model). The Kelvin temperature of a gas is proportional to the average kinetic energy of its molecules.

Gas laws

For a fixed mass of gas:

At constant temperature: pressure x volume = constant. This is Boyle's law: p₁V₁ = p₂V₂. If you halve the volume, the pressure doubles.
At constant volume: pressure is proportional to Kelvin temperature. p₁/T₁ = p₂/T₂. If you double the Kelvin temperature, the pressure doubles.

Worked example (Boyle's law): A gas has a volume of 0.6 m³ at a pressure of 100,000 Pa. The gas is compressed to a volume of 0.2 m³ at constant temperature. What is the new pressure?
p₁V₁ = p₂V₂
100,000 x 0.6 = p₂ x 0.2
p₂ = 60,000 / 0.2 = 300,000 Pa

Worked example (pressure-temperature law): A sealed container of gas is at 27 °C and 200,000 Pa. The temperature rises to 127 °C. What is the new pressure?
Convert to Kelvin: T₁ = 27 + 273 = 300 K, T₂ = 127 + 273 = 400 K
p₁/T₁ = p₂/T₂
200,000 / 300 = p₂ / 400
p₂ = (200,000 / 300) x 400 = 266,667 Pa (approximately 267 kPa)

Common mistakes in solids, liquids and gases

Using Celsius in gas law calculations. The pressure-temperature law requires Kelvin, not Celsius. Forgetting to convert is one of the most common errors in the edexcel igcse physics solids, liquids and gases questions. Always add 273.
Confusing density and mass. A large object can have a lower density than a small object. Density depends on how tightly packed the mass is, not on how much mass there is.
Saying temperature increases during a change of state. During melting or boiling, temperature stays constant. The energy supplied breaks intermolecular bonds rather than increasing kinetic energy.
Forgetting unit conversions. Volume must be in m³ (not cm³) and mass in kg (not g) for the standard equations. 1 m³ = 1,000,000 cm³. 1 kg = 1000 g.
Confusing evaporation and boiling. Evaporation happens at the surface at any temperature. Boiling happens throughout the liquid at a specific temperature (the boiling point). Both produce gas from liquid, but the conditions differ.

Self-check: edexcel igcse physics practice questions

A metal cube has sides of 0.04 m and a mass of 0.384 kg. Calculate its density.
A person weighing 700 N stands on one foot with an area of 0.014 m². Calculate the pressure under their foot.
How much energy is needed to heat 0.5 kg of aluminium (c = 900 J/(kg °C)) from 25 °C to 75 °C?
A gas at 100,000 Pa occupies 0.004 m³. It expands to 0.008 m³ at constant temperature. What is the new pressure?
Convert 57 °C to Kelvin.

Answers: (1) Volume = 0.04³ = 0.000064 m³, density = 0.384 / 0.000064 = 6000 kg/m³; (2) P = 700 / 0.014 = 50,000 Pa; (3) ΔE = 0.5 x 900 x 50 = 22,500 J; (4) p₂ = (100,000 x 0.004) / 0.008 = 50,000 Pa; (5) 57 + 273 = 330 K.

These edexcel igcse physics notes give you the complete solids, liquids and gases section of the igcse 4ph1 solids, liquids and gases specification. The edexcel igcse physics explained approach here turns each concept into something you can use directly in an exam answer. Practise the calculations until the steps feel automatic, and this section of the exam will reward you well.