|
IB Standard |
Comments |
Text of the Lab |
|
Defining the problem or research question Identifies
a focused problem or research question Formulating the hypothesis or prediction Relates
the hypothesis or prediction directly to the research question and explains
it quantitatively where appropriate Selecting variables Selects
the relevant independent and controlled variables |
The report includes a research
question. The relevant
background information could be expanded and explained in more detail A hypothesis is stated,
however it is not clear what the student means by the last sentence The relevant independent variable and the controlled
variables are implied but not explicitly stated |
Investigation
of the Effect of Heat on Vitamin C Background: Vitamin C, or ascorbic acid, is a water-soluble vitamin of
molar mass 176 g/mol. It is known
that cooking foods destroys the vitamin C.
Ascorbic acid also reacts with free iodine in a 3 to 1 ratio. This free iodine is formed by mixing KIO3
with acidified KI. Research Question: How is vitamin C affected by short and
extended exposure to high temperatures? Hypothesis: Because cooking food
destroys vitamin C, extended exposure to high temperatures will destroy
ascorbic acid, and the longer the duration the less vitamin C that will
survive. At
lower temperatures, the vitamin C should be destroyed to a degree
proportionate to the temperature. |
|
Selecting appropriate materials or apparatus Designing a method for the control of variables Designing a method for the collection of sufficient
relevant data |
The procedure at the left indicates most of the equipment
used. It fails to
discuss the reasons for the time and temperature intervals chosen. There is no real definitive list of controls Hence the discussion of experimental design needs to be
improved |
Procedure: Approximately 15 cm3 of
0.1966 M ascorbic acid was placed in a 50-cm3 flask. Then the flask was placed on a hot plate,
heated up to 50oC, and kept there for two minutes. Then about
5 cm3 of the solution (accurately massed) was placed in a 25-cm3
beaker. Then 15 drops of 0.10 M KI,
25 drops of 1.0 M HCI, and 10 drops of 5% soluble starch was added, and the
solution was stirred. Then a dropper
was filled with .01 M KIO3 and was added to the solution until a
dark blue/black color appears (starch-iodine indicating there is no more
ascorbic acid to react) and remains even after stirring. Then the KIO3 dropper was
weighed again and the difference recorded.
Then another 5-cm3 of the solution was titrated in the same
manner as the first. These two trials
were repeated a flask heated to 75oC for two minutes, a flask at
room temperature; and three flasks heated to 100oC for two
minutes, five minutes, and ten minutes, respectively. |
|
Collecting and recording raw data Organizing and presenting raw data Records
appropriate raw data (qualitative and/or quantitative, including units and
uncertainties where necessary Presents
the raw data clearly allowing for easy interpretation |
There is some inconsistency in the degree of precision
recorded for the data. Why is the
time measured only tom the nearest minute No error propagation attempted |
Data
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Processing raw data Processes the raw data
correctly Presenting processed data Presents processed data
appropriately, helping interpretation and, where relevant takes into account
errors and uncertainties |
The raw data is calculated appropriately or so it appears The student needs to provide more explanation. Some attempt was made to indicate error, but that the idea
of percent variance in this context
needs to be explained Indicate units and explain the percent variance |
Calculations: For the
following table this equation is used to calculate the values in the second
column, shown here with the example of the first row of data:
Sample
calculation
= 0.0162 M vitamin C Temp (oC): Time (m) Molarity of Vitamin C Solution 19.5 2 .0162 19.5 2 .0162 50 2 .0178 50 2 .0165 75 2 .0181 75 2 .0207 100 2 .0153 100 2 .0178 100 5 .0279 100 5 .0295 100 10 .0358 100 10 .0402 Percent
Variance: Experiment: % Variance 2 min @ 19.50C 0 = (.0162-.0162)/.0162 2 min @ 50oC 7.9 = (.0178-.0165)/.0165 2 min @ 75oC 14.4 = (.0207-.0181)/.0181 2 min @ 100oC 16.3 = (.0178-.0153)/.0153 5 min @ 100oC 5.7 = (.0295-.0279)/.0279 10 min @ 100oC 12.3 = (.0402-.0358)/.0358 |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Drawing conclusions Gives a valid conclusion
based on the correct interpretation of results with an explanation of
results, with an explanation and where appropriate compares results with
literature values Evaluating procedures and
results Evaluates procedure(s) and
results including limitations, weaknesses or errors Improving the investigation Identifies weaknesses and
states realistic suggestions to improve the investigation |
A graph would have been helpful here to illustrate the
point An attempt was made to relate the data to the original
purpose and hypothesis Some attempt was made to evaluate the procedure Does not know how to handle the aberrant data Identifies a weakness but does not make an effort to
suggest a way to remedy the flaw |
Conclusion: The length
of exposure to a high temperature has a significant effect on the
concentration of vitamin C. It is
clear from the data that the longer
the exposure to high temperatures, the greater the concentration. It appears that there is little effect on
Vitamin C at temperatures below about 100 degrees Celsius. For the three lowest temperatures, all are
very similar. There does appear to be
a slight upward trend as the temperature increases, but this is small and is
not conclusive, especially considering the relatively high % variance of the
two trials when compared with the difference between concentrations. However when compared to the rest of the
data, this contradicts the prevalent pattern. At 100 degrees, the data becomes more varied. There looks to be a slight to mild drop in
concentration between the 75o test and the 100o
test. However the percent of
variances of these two samples are the two largest of all the experiment, and
considering that his doesn’t seem to match the overall pattern of the results,
it is possible this is merely an anomaly, though maybe not. The conclusion that the longer the
exposure to heat, the higher the concentration of vitamin C is the complete
opposite of my hypothesis. This does
not make any sense, however. How
could vitamin C be created by high temperatures, especially when ascorbic
acid is supposed to be destroyed at this point? There has to be something wrong with the procedure. The procedural method was very
solid. It was fairly simple and
straightforward, with pretty good precision.
There were, however a couple possible sources of error. While we tried to add the extra
ingredients consistently and according to the directions, using droppers is
not very accurate and there being different quantities of these could affect
the outcome. Also, it is difficult to
find the exact point where the black becomes apparent, and is easy to add a
little bit too much, undermining the accuracy of the results. However all these are fairly minor, and
were carefully controlled whenever possible.
So the method cannot be to blame for the unbelievable results, there
must be a fundamental flaw in the titration procedure. The issue must be with the heated ascorbic
acid because only the ascorbic acid and water and heated, and it’s inconceivable
that hot water would significantly affect such a reaction. The differences cannot be caused merely by
the hotter environment of titration, because all the trials at 100 degrees
should cool at the same rate, yet have very different results. So there must be something about the
heated vitamin C that causes it to react more with the free iodine. A possible explanation is that the altered
state of the heated vitamin C causes it to oxidize with multiple iodine
molecules. Another possible cause is that
denatured vitamin C hinder the formation of free iodine in some way. This experiment had major
problems. The results were completely
counterintuitive. Before this
experiment could be done again, the source of this problem would have to be
identified and remedied, or another method of measuring vitamin C would need
to be found. Unfortunately, I’m not
aware of a method for either and thus cannot suggest concrete changes to the
experiment. |