M. Murat TEKİN
11 FB-B
06.04.2012
Biology Lab Report 002
Aim

To investigate the relation between rate of diffusion and the surface area to volume ratio of
a block of agar jelly.
Research Question

How could the correlation between the surface area to volume ratio of differently sized
blocks of agar jelly and the rate of diffusion be observed by placing the blocks with acidity
indicators into a 0.1M HCl solution?
Variables



Independent Variables
o Size of the blocks
Dependent Variables
o Time taken for acidity indicator to change color totally
Controlled Variables
o The content of the agar jelly blocks
o The concentration of HCl solution
o Temperature of the environment and the HCl solution
Procedure:

o
o
Apparatus
o Ruler
o Beaker(50ml)
o Filter paper
o Flask(500ml)
o Funnel
o Ruler
o Measuring cylinder(50ml)
o Paper towels
o Scalpel

1
Knives
Stopwatch
Chemicals
o Agar jelly
o Water
o Hydrochloric acid solution
(0.1M)
o Phenol red
M. Murat TEKİN
11 FB-B
06.04.2012
Method
Firstly, five different sizes of agar jelly blocks with phenol red were to be cut out of a petri dish of
readily made agar jelly of height 5mm.
The dimensions of agar blocks to be used for the experiment were determined to give a wide range
of different surface area to volume ratios in the limitations of measuring equipment. The dimensions
and the shapes of the blocks are given in the table below, with the surface area and volume values
for each calculated.
Table 1: General information about tested agar jelly blocks grouped by size
Block
Group
Number
Wide Surface
Shape
Length of
Perpendicular
sides/mm (±1)
Height
/ mm
(±1)
Total Surface
Area
/mm2
Total
Volume
/ mm3
Surface Area to
Volume Ratio
/mm-1
1
2
3
Square
Square
Triangle-Isosceles
Perpendicular
Square
Square
5
10
15
5
5
5
150
400
481
130
500
560
12.0
8.0
8.6
15
20
5
5
750
1200
1130
2000
6.7
6
4
5
To prepare the agar jelly blocks following steps were applied;






A Petri dish of agar jelly was placed on a dry filter paper for an easy cutting.
The height of the agar jelly was measured at different points of it with a ruler and their
average value for the height was recorded.
The circular agar jelly block was divided into 5 long pieces of desired widths for experiment
with the help of a scalpel.
Each long piece was then cut down into the agar blocks of desired sizes, in accordance with
their widths.
Three samples for each size were prepared for the repetition of the experiment.
Remaining agar jelly was preserved to be used in case it could be required to cut more blocks
out of it.
After the blocks were prepared, the HCl solution of concentration 0.1 M was gathered. As a safety
measure, gloves were worn before this stage to prevent direct contact of skin with acid solution. It
was already prepared for use in the laboratory. The flask was washed cleanly with water and (500 ml)
was filled with it up to its 350ml level- nearly 50ml more than needed for the experiment to ensure
excess was at hand for use if needed.
When the HCl solution was gathered, all of the equipment was washed cleanly-except for electronicsto start the tests. For each test the following steps were taken;



The measuring cylinder was filled with 20ml of 0.1 M HCl solution.
The 20ml HCl solution was poured into the small beaker (50 ml).
The stopwatch was prepared.
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M. Murat TEKİN




11 FB-B
06.04.2012
One block of desired size was taken with the help of knives and held ready to be dropped
into the HCl solution.
Once the block was made fall into the acid solution, the stopwatch was started.
When the phenol red inside the block totally turned yellow the stopwatch was stopped and
the time taken was recorded.
After each test the small beaker and measuring cylinder were washed cleanly for the next
test.
Throughout the experiment, to keep the controlled variables constant,



The solutions were made wait in the laboratory environment for over 10 minutes. In that
way their temperatures would have been equal to the temperature of the laboratory which
is maintained at around 22 degrees Celsius. By using this way, the effects of temperature
changes on the results of the experiment were tried to be minimized.
The HCl solution used was kept in a flask with a narrow brim and was never changed. This let
the evaporation from the surface to be reduced and a systematic error caused by changing
the concentration of the HCl solution by changing the solution used, to be avoided. In that
way the concentration of the HCl was tried to be maintained constant.
The agar jelly blocks were never placed anywhere different than the filter paper and their
contact with other materials were limited. In order to keep them safely pure, they were not
touched with hand but only knives were used to hold them. Also the agar jelly used all
through the experiment was never changed, which ensures no other agar block with a
different content was used. In that way the content of the agar jelly, which would affect the
rate of diffusion was tried to be kept constant.
The data recorded throughout the experiment is displayed in the following table.
Table 2: Experiment Results
Block Group
Number
1
2
3
4
5
Time Taken for Phenol Red to Turn Yellow(T)/seconds (±1)
Trial 1
Trial 2
Trial 3
57
56
60
60
60
60
59
61
65
65
63
65
75
74
75
Average
57,7
60,0
61,7
64,3
74,3
In the aftermath of the experiment, the data collected was to be analyzed. To achieve this, a
computer program- Microsoft Office Excel 2010- was used. The data was graphed on a suitable grid.
The graph is given below.
Graph 1: The relation between surface area to volume ratio and the time taken for Phenol Red inside
agar jelly blocks to turn yellow
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M. Murat TEKİN
11 FB-B
06.04.2012
Time Taken for Phenol red to Turn Yellow
Totally(T)/sec
80,00
75,00
70,00
65,00
60,00
55,00
y = 0,3191x2 - 7,8307x + 105
50,00
4,00
6,00
8,00
10,00
12,00
14,00
16,00
Surface Area to Volume Ratio/mm-1
As phenol red is the indicator of acidity, it turning yellow from red means that an acid exists in the
environment. In the experiment, the existence of acid in the blocks of agar jelly has just one reason
which is the diffusion of hydrochloric acid into the blocks. Thus the earlier the phenol red totally
turned yellow, the earlier the acid diffused into the whole of the agar block. In that sense the data
revealed from the experiment does actually tell the time by which the diffusion ended.
The following graph has been sketched to demonstrate the relation between the rates of diffusion
compared to the surface area to volume ratios of the agar jelly blocks. The diffusion rate is expressed
as T-1.
Arbitrary Diffusion Rate (T-1)/sec-1
0,019
0,018
0,017
0,016
0,015
0,014
0,013
0,012
4,00
6,00
8,00
10,00
Surface Area to
12,00
Volume Ratio/mm-1
4
14,00
16,00
M. Murat TEKİN
11 FB-B
06.04.2012
As seen on the graph the experiment has succeeded in reaching to the conclusion that there exists a
relation. The graphed data reveals that as surface area to volume ratio increases, the rate of diffusion
increases. Even though the uncertainty is very high for the surface area to volume ratio, the
correlation is observable. It is not possible to give numerical values using the data from this
experiment but the nature of the relation could be described as parabolic or directly proportional.
Conclusion and Evaluation
The experiment was conducted with great care but since there are many strong factors that would
lead to errors, the results reached were not precise, nor were they greatly accurate. But still the
experiment has brought up successful results that help understand the relation between the surface
area to volume ratio and rate of diffusion of HCl into the agar blocks.
The main systematic errors in the experiment and possible solutions for them are discussed in the
following;



The cutting marks and possible crack formations on the surface of the agar jelly blocks that
formed while cutting. These would increase the surface area, resulting in much shorter time
for diffusion to be completed, thus failing the experiment. The formation of the cutting
marks could be avoided by using a thinner knife or by using forceps which would have been
more helpful in avoiding cutting marks in special.
The water remaining inside the measuring cylinder and the beaker may have also played a
role in the experiment by decreasing the concentration of HCl solution. This problem could
have been solved by simply drying the two equipments with paper towels when needed.
The water loss of the agar jelly blocks was also an important source of error. Loss of water
could have made the agar jelly stiffer that the rate of diffusion would have fallen
dramatically. This could be avoided by simply placing the agar blocks into a small and well
insulated container and preserving them better there, for that they wouldn’t have lost water
by evaporation.
The main random errors in the experiment and possible solutions for them are discussed in the
following;


The difficulties with observation were the greatest source of error for the experiment. The
color change was so slow that the human eye couldn’t really understand when to stop the
time. The errors caused by this confusing weakness of human observer could have been
avoided by using a colorimeter if possible by recording the samples’ color change and using
computer programs to detect the total color change.
Another source of random error was the too simple way of maintaining constant
temperature all through the experiment. Given that the experiment took around 100
minutes to complete, some temperature change is normal to occur and affect the results.
Thus using a water bath would have been the safest way of maintaining constant
temperature.
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