A student is investigating how the rate at which water evaporates varies with temperature. It is suggested that the relationship between the volume of water evaporated per unit time Y and the Celsius temperature θ of the water is Y = kθˢ where k and s are constants. Design a laboratory experiment to test the relationship between Y and θ. Explain how your results could be used to determine values for k and s. 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] temperature (of the water)/θ is the independent variable and volume per unit time/Y is the dependent variable or vary θ and determine Y
2. [1] keep temperature of room/surroundings constant
3. [1] labelled diagram of workable experiment including: beaker open to air water, labelled workable method to heat water
4. [1] method to determine (change in) volume, e.g. measuring cylinder or method to determine (change in) mass, e.g. top-pan balance
5. [1] measure time with a stop-watch/timer (for change in volume/mass or evaporation)
6. [1] use a thermometer to measure θ or labelled thermometer in water in diagram
7. [1] plot a graph of lg Y against lg θ (or In Y against In θ)
8. [1] s = gradient
9. [1] k = 10^(y-intercept) (for In Y against In θ, k = e^(y-intercept))
10. [1] D1 use of (protective) gloves to handle hot beaker/water
11. [1] D2 keep surface area constant (by using the same cylindrical container)
12. [1] D3 keep water temperature constant (while water is evaporating)
13. [1] D4 method to keep temperature constant while water is evaporating e.g. adjust heater/gently heat water to maintain temperature/use a water bath
14. [1] D5 use a large surface area to increase the rate of evaporation
15. [1] D6 Y = (initial volume – final volume)/time or ΔV/time
16. [1] D7 lg Y = s lg θ + lg k
17. [1] D8 relationship valid if a straight line
18. [1] D9 insulation/lagging around (sides) of beaker (not a lid)
19. [1] D10 switch off fans or close windows to avoid draughts