5.3.6Chemical Energetics
Boyle’s Law — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 12 of 12 Unit quiz
Section 1 of 5Pressure and Molecular Collisions
Gas pressure comes from molecules striking the walls of their container, so changing the volume changes how often those collisions happen.
Pressure
- Force per unit area exerted by gas molecules colliding with the container walls, measured in pascals (Pa).
Volume
- The space the gas occupies, equal to the volume of its container, usually measured in cubic metres (m³).
Fixed mass
- The amount of gas stays the same during compression or expansion, so the number of molecules does not change.
Constant temperature
- The temperature must not change, because a higher temperature would increase molecular speed and alter the pressure.
Section 2 of 5Why Pressure Changes with Volume
Compressing a gas at constant temperature follows the same sequence every time.
1
Reducing the volume of a fixed mass of gas at constant temperature makes the molecules travel a shorter distance between the walls.
2
The molecules still move at the same average speed, so they strike the walls more frequently.
3
More frequent collisions produce a larger net force on each area of wall, which means the pressure rises.
4
Doubling the volume halves the pressure because collisions with the walls become less frequent.
5
A graph of pressure against 1/volume is a straight line through the origin, showing that pressure is inversely proportional to volume.
| Change in volume | Effect on pressure |
|---|---|
| Volume halved | Pressure doubles |
| Volume doubled | Pressure halves |
| Volume reduced to one quarter | Pressure becomes four times larger |
Section 3 of 5Using the Law
The equation applies to a single sample of gas whose temperature stays fixed.
Using the Law
Equation
- p₁V₁ = p₂V₂, where the subscripts 1 and 2 refer to the initial and final states of the same sample of gas.
Units
- Pressure and volume can use any units, provided the same unit is used for the same quantity on both sides.
Temperature
- Kelvin temperature is not needed for Boyle's law, but the temperature must stay constant throughout the change.
Graphs
- A pressure against volume graph is a curve, whereas a pressure against 1/volume graph is a straight line.
Section 4 of 5Worked Example
Question
- A fixed mass of gas has a pressure of 1.5 × 10⁵ Pa and a volume of 2.0 × 10⁻³ m³ at constant temperature.
- Calculate the new pressure when the volume is reduced to 0.5 × 10⁻³ m³.
- Write the lawp₁V₁ = p₂V₂.
- Rearrange to make the new pressure the subject: p₂ = p₁V₁ / V₂.
- Substitutep₂ = (1.5 × 10⁵ × 2.0 × 10⁻³) / (0.5 × 10⁻³).
- Calculatep₂ = 6.0 × 10⁵ Pa.
Answer
- 6.0 × 10⁵ Pa.
- The volume fell to one quarter, so the pressure became four times larger.
Section 5 of 5Exam Checklist & Tips
5.3.6 – Key Points for the Exam
- Boyle's law At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume.
- Pressure Force per unit area, measured in pascals (Pa), where 1 Pa = 1 N/m².
- Kinetic theory The model that gas molecules are in constant random motion and exert pressure by colliding with surfaces.
- Ideal gas A model gas whose molecules have negligible volume and negligible forces between them.
- Inverse proportion Two quantities where one doubles as the other halves, so their product stays constant.
Common Mistakes
- Saying gas molecules become larger or smaller when a gas is compressed; only the space between them changes.
- Stating that pressure is directly proportional to volume;
- the relationship is inverse, so doubling the volume halves the pressure.
- Changing the temperature during a Boyle's law calculation, which means the law no longer applies.
- Forgetting that the mass of gas must stay fixed, so the number of molecules is unchanged.
Last updated:
\n