Required practical 8: Investigation of Boyle's law (constant temperature)…

The idea

This practical tests two gas relationships. For Boyle's law, a fixed mass of trapped gas is compressed or expanded slowly at room temperature; pressure p is measured with a Bourdon gauge or pressure sensor and volume V from a syringe scale or oil column length. Plotting p against 1/V should give a straight line through the origin, since pV is constant. For Charles's law, a fixed mass of gas is heated in a water bath while pressure is kept at atmospheric pressure by allowing the gas to expand; volume is read from a capillary tube or syringe and temperature from a thermometer. Plotting V against absolute temperature T in kelvin should give a straight line through the origin, since V/T is constant. Allow thermal equilibrium before each reading, repeat readings, and use absolute temperatures.

What earns the marks

• Boyle's law: for a fixed mass of gas at constant temperature, pV = constant, so p is inversely proportional to V and a graph of p against 1/V is a straight line through the origin.
• Charles's law: for a fixed mass of gas at constant pressure, V/T = constant, so V is directly proportional to absolute temperature T, and a graph of V against T in kelvin is a straight line through the origin.
• Apparatus and method for Boyle's law: trap a fixed mass of gas, vary volume slowly, measure pressure with a pressure gauge or sensor and volume from a syringe or column length, allowing time for thermal equilibrium.
• Apparatus and method for Charles's law: heat a fixed mass of gas in a water bath, keep pressure atmospheric by allowing expansion, measure volume and temperature, and convert temperatures to kelvin by adding 273.
• Graph analysis: plot p against 1/V for Boyle's law and V against T for Charles's law; a straight line through the origin supports the proportionality, and gradients relate to the constant pV or V/T.
• Control of variables: keep temperature constant in the Boyle's law experiment and pressure constant in the Charles's law experiment; use a fixed mass of gas and avoid leaks.
• Uncertainties and improvements: repeat readings, use a wide range of values, allow thermal equilibrium, avoid parallax when reading volume, and consider the dead volume in tubing or syringe.

Common mistakes and how to put them right

• Using Celsius temperature in Charles's law calculations or graphs.

  Put it right: Convert to kelvin by adding 273 before plotting or calculating.

• Plotting p against V for Boyle's law and expecting a straight line.

  Put it right: Plot p against 1/V, which should be a straight line through the origin.

• Changing the mass of gas during the experiment, for example by leaking gas when adjusting a syringe.

  Put it right: Seal the apparatus and use a fixed mass throughout.

Exam tips

• State the controlled variable explicitly: constant temperature for Boyle's law and constant pressure for Charles's law.
• When describing graphs, name the axes and the expected shape, and explain how the shape supports the proportionality.
• Include one practical improvement, such as allowing thermal equilibrium or repeating readings and taking a mean, and explain how it reduces uncertainty.