A student is investigating how the boiling point of a salt solution varies with pressure. It is suggested that the relationship between the Celsius temperature θ at which the water of the solution starts to boil and the air pressure P is θ = kσP^q where σ is the density of the solution and k and q are constants. Design a laboratory experiment to test the relationship between θ and P. Explain how your results could be used to determine values for k and q. You should draw a diagram, on page 3, showing the arrangement of your equipment. In your account you should pay particular attention to • the procedure to be followed, • the measurements to be taken, • the control of variables, • the analysis of the data, • any safety precautions to be taken.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

1. [1] P is the independent variable and θ is the dependent variable, or vary P and measure θ. (Allow σ is the independent variable and P is the dependent variable.)
2. [1] keep density of salt solution constant or keep σ constant
3. [1] labelled diagram of workable experiment including: sealed container e.g. bell jar, sealed conical flask; tube connected to pump (for changing pressure) or other workable method; salt solution, labelled
4. [1] workable method to heat salt solution within the sealed container, e.g. hot plate/(electrical) heater
5. [1] description of pressure gauge or manometer to measure P
6. [1] use of a thermometer to measure θ or labelled thermometer (in salt solution) in diagram
7. [1] plot a graph of lg θ against lg P (or In θ against In P)
8. [1] q = gradient
9. [1] k = 10^(y-intercept) / σ (for In θ against In P: k = e^(y-intercept) / σ)
10. [1] D1 safety precaution relating to pressure, e.g. safety screen
11. [1] D2 use of (protective) gloves to handle hot salt solution/beaker/flask
12. [1] D3 density of salt solution or ρ given by m/V
13. [1] D4 measuring cylinder to measure volume and difference in top pan balance/scales readings to measure mass of salt solution
14. [1] D5 slowly/gradually increase/decrease the temperature/pressure
15. [1] D6 measure P and θ when salt solution starts to boil
16. [1] D7 lg θ = q lg P + lg kσ
17. [1] D8 relationship valid if a straight line
18. [1] D9 identification of when (salt solution) starts to boil, e.g. wait until (vapour) bubbles are on the surface or surface moves or temperature remains constant (for heating methods)
19. [1] D10 recheck that density of salt solution is constant/add water to keep density constant/take multiple solutions from a large volume