AGAR PHENOL RED DIFFUSION
RESEARCH QUESTION
How does surface area to volume ratio affect time taken for 1.0 M HCl solution to turn the
color of solid agar solutions totally from red to orange?
VARIABLES
VARIABLES
INDEPENT VARIABLE
DEPENDENT
VARIABLE
Surface area to
volume ratio
Time taken for
change of color of
solid agar solutions
from red to orange
CONTROLLED VARIABLES
UNITS
Concentration of HCl
M
Temperature of HCl
˚C
pH of HCl
pH
HYPOTHESIS
UNITS
RANGE
cm-1
0.41-0.67
s
106.1-293.7
Possible Effect(s) on Results
Increase in concentration of HCl
would decrease time taken for
color change of solid agar
solutions since HCl molecules
would move faster
Increase in temperature of HCL
would decrease time taken for
color change of solid agar
solutions since activity of HCl
solution increase
Decrease in pH of HCl solution
would decrease time taken for
color change of solid agar
solutions since acid would be
stronger. It would react faster.
Surface area to volume ratio is important for most of the reactions. As surface area to
volume ratio increase, the rate of reactions increase. Thus they are directly proportional. For
my experiment, as surface area to volume ratio increase, rate of reaction between HCl and
solid agar solutions will increase. Therefore, time taken for color change of solid agar
solutions will decrease.
APPARATUS
 A plate containing solid agar solution
 A lancet
 A ruler
 5 beakers (200 ml)
 A stopwatch
 1.0 M of Hydrochloric acid solution
 A measuring cylinder(50 cm3)
 A pair of gloves
 Lab coat
 A Dropper
 A Tong
 A board marker
PROCEDURE

Wear safety equipments; a pair of gloves and a lab coat.

Firstly bring a plate containing solid agar solution.

Cut the solution by the help of a ruler with a lancet to get 5 samples.

Make sure the samples have shapes of rectangular prisms and different size lengts.

With a board marker, name the beakers A,B,C,D and E.

Fill measuring cylinder with 20 cm3 of 1.0 M HCl by the help of a dropper.

Then pour 20 cm3 of 1.0 M HCl to beaker A.

Fill other beakers with 20 cm3 of 1.0 M HCl too.

Hold a sample of solid agar solution with a tong and put it into beaker A.

As soon as sample of solid agar solution contacts HCl solution, start the stopwatch.

Record the time taken for color change of sample of solid agar solution totally from
red to orange.

Repeat the steps for other 4 samples of solid agar solution and record the results.
RECORDING RAW DATA
SAMPLES
SAMPLE A
SAMPLE B
SAMPLE C
SAMPLE D
SAMPLE E
LENGTH OF 1ST
SIDE(cm)
LENGTH OF 2ND
SIDE(cm)
LENGTH OF 3RD
SIDE(cm)
UNCERTAINTY
(±0.05 cm)
5
10
14
16
30
UNCERTAINTY
(±0.05 cm)
5
10
10
13
16
UNCERTAINTY
(±0.05 cm)
5
9
9
9
9
TIME TAKEN
FOR COLOR
CHANGE(s)
UNCERTAINTY
(±0.2 s)
106.1
218.2
241.1
263.9
293.7
PROCESSING RAW DATA

FORMULAE:
o SURFACE AREA OF AGAR SOLUTION SAMPLES:
 2x(LENGTH OF 1ST SIDExLENGTH OF 2ND SIDE+LENGTH OF 1ST
SIDExLENGTH OF 3RD SIDE+LENGTH OF 2ND SIDExLENGTH OF
3RD SIDE)

EXAMPLE;
SURFACE
AREA
OF SAMPLE
A:
2
2x(5cmx5cm+5cmx5cm+5cmx5cm)=150 cm
o VOLUME OF AGAR SOLUTION SAMPLES
 LENGTH OF 1ST SIDExLENGTH OF 2ND SIDExLENGTH OF 3RD
SIDE
 EXAMPLE; VOLUME OF SAMPLE A: 5cmx5cmx5cm=125
cm3
PRESENTING PROCESSED DATA
SAMPLES
SURFACE AREA
OF AGAR
SAMPLES (cm2)
VOLUME OF
AGAR SAMPLES
(cm3)
SURFACE AREA
TO VOLUME
RATIO (cm-1)
TIME TAKEN
FOR COLOR
CHANGE (s)
SAMPLE A
SAMPLE B
SAMPLE C
SAMPLE D
SAMPLE E
150
560
712
938
1788
125
900
1260
1872
4320
1.20
0.62
0.56
0.50
0.41
106.1
218.2
241.1
263.9
293.7
CONCLUSION
In most of the chemical and biological reactions, as surface area to volume ratio increase,
the reaction rate increases. Additionally product of the reaction can be produced faster;
because surface area to volume ratio represents the number of molecules per volume that
need to react with the other reactant. As the number of molecules per volume increase the
rate of reaction with the other reactant increase too.
My experiment satisfied my hypothesis. As I stated before as surface area to volume ratio
increase, the reaction rate also increase. In my experiment, as surface area to volume ratio
increase, the time taken for color change of samples of solid agar solution decrease, which
means that increase in surface are to volume ratio increased the reaction between the
samples and HCl solution. By the help of the results of my experiment I can conclude that
surface area to volume ratio is directly proportional to reaction rate and inversely
proportional to time taken for reaction to be completed.
EVALUATION
While I was performing the experiment, I faced with some challenges. Firstly while cutting
down the agar solution and getting the samples, I could not cut them smootly, because the
lancet was not very sharp and it was hard to get those samples. Therefore the surface area
to volume ratio may be wrongly calculated. However, I guess there is not a big difference
between my measurement and the real value.
Another difficulty for me was about the dropper and the volume of the experiment. Since I
was using dropper while measuring the volume of HCl, I guess I got inaccurate volumes of
HCl which lead the results to be non-precise. It also decreased the reliability of my results.
However it didn’t cause the results to contradict my hypothesis.
To be able to improve my experiment, I need to use a sharper lancet to be able to cut the
samples smoothly. Additionally, I need to use a burette instead of a dropper to be able to
measeure the volume of HCl accurately.