10 6 A student determined the latent heat of vaporisation of a liquid using an electrical heater to boil the liquid in a Pyrex beaker. The apparatus used is shown below. Pyrex beaker heater liquid balance (a) She connected the heater into a circuit and took measurements of the potential difference V and the current I for the heater. Complete the circuit diagram to show a suitable circuit. (2) heater 11 Turn over (b) The student monitored the mass of the beaker and the liquid m over the time t for which the liquid was boiling. Her results are plotted on the graph. 150 160 170 180 190 200 210 220 0 100 200 300 400 500 600 m / g t / s The student used her graph to determine a value for the latent heat of the liquid in the beaker. She concluded that the liquid was pure water. Liquid Latent heat of vaporisation / MJ kg−1 Pure water 2.26 Weak salt water solution 2.10 Strong salt water solution 2.00 Comment on the validity of the student’s conclusion. V = 20.5 V I = 10.5 A (7) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. 12 (c) Explain how this method might be modified to improve the accuracy of the student’s conclusion. (2) .................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... . ................................................................................................................................................................................................................................................... . ................................................................................................................................................................................................................................................... . .................................................................................................................................................................................................................................................... (Total for Question 6 = 11 marks) 13 Turn over BLANK PAGE