The Pressure Law (Constant Volume) — Ideal Gases | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable

Low

Stable7%

Gas law calculations (Boyle's Law, pressure-temperature) and kinetic theory explanations appear regularly.

Statement

At constant volume and for a fixed amount of an ideal gas:

P ∝ T (in kelvin)

Equivalently, if subscript 1 marks the initial state and subscript 2 marks a later state:

p₁ / T₁ = p₂ / T₂

where:
- p₁, p₂ = pressures in Pa (any consistent unit, as long as both are the same)
- T₁, T₂ = absolute temperatures in kelvin (always, never °C)

Using the equation

Convert any Celsius readings to kelvin first by adding 273. Forgetting to do this is one of the most common exam mistakes
Rearranging for p₂: p₂ = p₁ × (T₂ / T₁)
Rearranging for T₂: T₂ = T₁ × (p₂ / p₁)

Worked example

Pressure law calculation

A gas in a sealed container has a pressure of 80 kPa at a temperature of 27 °C. The container is heated until the temperature reaches 127 °C. Calculate the new pressure. (Volume stays constant.)

Solution:

Convert both temperatures to kelvin: T₁ = 27 + 273 = 300 K, T₂ = 127 + 273 = 400 K
Substitute into p₁/T₁ = p₂/T₂: 80/300 = p₂/400
Rearrange: p₂ = 80 × 400/300
p₂ = 107 kPa (to 3 s.f.)