| Date | May 2017 | Marks available | 1 | Reference code | 17M.3.SL.TZ1.1 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | State | Question number | 1 | Adapted from | N/A |
Question
Urease is an enzyme that breaks down urea into ammonia and carbon dioxide. The ammonia produced causes the pH of the solution to rise. This reaction can be followed using a pH indicator or a pH probe.
In an experiment conducted by a student the time taken for the pH indicator, thymol blue, to change from yellow to blue was recorded at different temperatures.
Outline what the standard deviations reveal about the data from this experiment.
One result in this experiment can be classified as an outlier as its value is very distant from those of the other values.
Explain an appropriate procedure for dealing with outliers.
Outline the effect of temperature on the activity of urease enzyme.
State one factor that would need to be controlled in this experiment.
Markscheme
a. standard deviation indicates the degree of variation in the data
OR
the spread of data around the mean
b. high standard deviation at 30°C «in proportion to the average» indicates less reliable data
c. low standard deviation for results from 40 to 70°C indicates more reliable data
a. outliers should not be deleted from the data set
OR
trend lines may ignore outliers>
OR
use a z-score/test
b. could be incorrectly entered or measured data
c. repeat experiment/reading/trial «for that point»
a. the reaction / activity increases with temperature
b. rate slows from 60°C to 70°C
c. fastest increase in activity is between 30°C - 40°C
d. optimum temperature is 60°C
a. concentration / volume of urea «solution»
b. concentration / volume of urease «solution»
c. concentration / volume of indicator
d. lighting conditions / background «to test tubes»
