Core practical 14: pressure and volume of a gas

Method

• Trap a column of air in a sealed glass tube above some oil. A foot pump pushes oil into the tube, compressing the air; a Bourdon gauge reads the pressure.
• The volume is proportional to the length of the air column (V = length x cross-sectional area).
• Increase the pressure in steps. After each change, wait for the air to return to room temperature, then read the length and the pressure.
• Read the scale at eye level to avoid parallax.

Analysis

pV = NkT, so for fixed N and T, p = NkT / V. A graph of p against V is a curve; a graph of p against 1/V is a straight line through the origin with gradient NkT. You can find N from the gradient if T is known.

Errors

• Compressing a gas quickly warms it: waiting keeps T constant.
• A leak would change N, so seals must be tight.
• Using length instead of volume is fine for checking proportionality, because the tube has a uniform cross-section.

Worked example

The air column is 30 cm long at 1.0 x 10⁵ Pa. What length do you expect at 1.5 x 10⁵ Pa?

1. pV is constant and V is proportional to length.
2. l_2 = 30 x 1.0 / 1.5

Answer: l_2 = 20 cm

Check yourself

1. An air column is 30 cm long at 1.0 x 10⁵ Pa. The pressure is raised to 1.5 x 10⁵ Pa at constant temperature. Find the new length in cm.
   Show answer

   20 cm. pl is constant: l = 30 x 1.0 / 1.5 = 20 cm.

2. The air column is 26.0 cm long in a tube of internal diameter 1.0 cm. Find its volume, in units of 10^-5 m³, to 3 s.f.
   Show answer

   2.04 x 10^-5 m³. V = πr^2l = π x (0.0050)² x 0.260 = 2.04 x 10^-5 m³.

3. A graph of p against 1/V has a gradient of 4.2 Pa m³ at 293 K. Find the number of molecules, in units of 10²¹, to 3 s.f.
   Show answer

   1.04 x 10²¹. Gradient = NkT, so N = 4.2 / (1.38 x 10^-23 x 293) = 1.04 x 10²¹.

4. Why should you wait before taking a reading after increasing the pressure?
   • Compressing the gas warms it, so you wait for it to return to room temperature
   • The oil needs time to change colour
   • The gauge needs time to reset to zero
   • To let some gas escape
   Show answer

   Compressing the gas warms it, so you wait for it to return to room temperature. The experiment needs constant temperature.

5. Which graph gives a straight line through the origin in this experiment?
   • p against V
   • V against T
   • p against 1/V
   • p against V²
   Show answer

   p against 1/V. p = NkT x (1/V), in the form y = mx.

6. Why must the apparatus have no leaks?
   • So the number of molecules stays constant
   • So the temperature stays constant
   • So the oil does not spill
   • So the pressure stays constant
   Show answer

   So the number of molecules stays constant. Boyle's law applies to a fixed mass of gas.

7. Why should the length scale be read at eye level?
   • To reduce the temperature
   • To keep the pressure steady
   • To avoid parallax error
   • To make the reading larger
   Show answer

   To avoid parallax error. Reading at an angle shifts the apparent position of the oil surface.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)
• Cognito: Edexcel International A level Physics specification and core practicals

Last updated 7 November 2026. Spotted a mistake? Tell us.