Edexcel IGCSE Physics: Solids, Liquids and Gases — Revision Notes

Condensed for the final weeks. For the full explanation, use the Solids, Liquids and Gases study guide.

Density and pressure

density   rho = m / V
pressure  p = F / A
fluid     p = rho g h

Pressure in a liquid depends only on depth and density — not on the container’s shape or the total volume of liquid. This is why a dam is built thicker at the base.

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 = h rho g = 5.0 x 1000 x 9.8 = 49 000 Pa

Particle model

State	Arrangement	Movement
Solid	Regular, close-packed	Vibrate about fixed positions
Liquid	Close, irregular	Slide past one another
Gas	Far apart, random	Fast, random, in all directions

Gas pressure arises from molecules colliding with the walls and exerting a force on them. Every gas-law explanation traces back to that mechanism. In a liquid or gas at rest, pressure at a given point acts equally in all directions — it has no preferred direction, unlike a force.

The gas laws

p1 V1 = p2 V2                (constant temperature, fixed mass)
p1 / T1 = p2 / T2            (constant volume, fixed mass)

4PH1 lists only these two two-variable relations, each requiring a fixed mass of gas — the general combined gas law pV/T = constant (which drops the fixed-mass condition) is beyond the specification and not supplied.

Temperature must be in kelvin: T(K) = θ(°C) + 273. Using Celsius is the single most common cause of a wrong answer here.

Explaining the laws using the particle model is where the marks are:

• Reducing volume at constant temperature → molecules hit the walls more frequently → pressure rises.
• Raising temperature at constant volume → molecules move faster, so they hit the walls more frequently and harder → pressure rises.

Note that a temperature increase changes both frequency and force of collisions, whereas a volume decrease changes only frequency. Making that distinction is what separates a full answer.

Absolute zero (0 K, −273 °C) is the temperature at which molecules have minimum kinetic energy — not zero energy. The Kelvin temperature of a gas is directly proportional to the average kinetic energy of its molecules — doubling the Kelvin temperature doubles the average kinetic energy.

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

p1V1 = p2V2
(2.0x10^5)(0.30) = p2 (0.10)
p2 = 6.0x10^5 Pa

Thermal physics

E = m c delta-theta        specific heat capacity

Specific heat capacity — the energy needed to raise 1 kg by 1 °C.

Specific latent heat (E = mL) is the energy needed to change the state of 1 kg with no temperature change — but the equation itself is beyond the 4PH1 specification; a separate calculation beyond this specification is needed for latent heat during a state change, and only ΔQ = mcΔθ is examinable.

Temperature stays constant during a change of state because the energy supplied goes into breaking the forces between particles rather than increasing their kinetic energy. That is the standard explanation question.

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?

E = m c dt = 2.0 x 4200 x 80 = 672 000 J

Thermal expansion occurs because particles vibrate more vigorously and take up more space — the particles themselves do not expand.

Exam traps

• Using °C in the gas laws.
• Saying pressure depends on the volume of liquid rather than the depth.
• Explaining a gas law without referring to collisions with the walls.
• Saying molecules stop at absolute zero.
• Using E = mcΔθ during a change of state.
• Saying particles expand when a solid is heated.
• Forgetting that Kelvin temperature is proportional to average kinetic energy, not simply “hotter means faster” without the proportionality.

Self-test

1. What does pressure in a liquid depend on?
2. Explain, using particles, why compressing a gas raises its pressure.
3. Explain why heating a gas at constant volume raises its pressure — and give both effects.
4. Why does temperature stay constant while ice melts?
5. What is happening at absolute zero?
6. Find the pressure difference at a depth of 5.0 m in water (ρ = 1000 kg/m³, g = 9.8 N/kg).
7. Calculate the energy needed to heat 2.0 kg of water from 20 °C to 100 °C (c = 4200 J/kg°C).
8. A gas at 2.0 × 10⁵ Pa and 0.30 m³ is compressed at constant temperature to 0.10 m³. Find the new pressure.

Answers: 1. Depth, density of the liquid and gravitational field strength — not the container’s shape or total volume. 2. The molecules have less space, so they collide with the walls more frequently, and more frequent collisions mean a greater force per unit area. 3. The molecules move faster, so they hit the walls both more frequently and with greater force, raising the pressure. 4. The energy supplied is used to overcome the forces holding the particles together rather than to increase their kinetic energy. 5. Particles have the minimum possible kinetic energy — not zero energy. 6. p = hρg = 5.0 × 1000 × 9.8 = 49 000 Pa. 7. E = mcΔθ = 2.0 × 4200 × 80 = 672 000 J. 8. p₁V₁ = p₂V₂, so p₂ = (2.0×10⁵ × 0.30) ÷ 0.10 = 6.0×10⁵ Pa.