Ozanalp Eryılmaz
11/B
EXPERIMENT: AGAR - PHENOL RED DIFFUSION
DESIGN
Aim
To investigate the relationship between surface area to volume ratio and the rate of diffusion.
Theory
A block of agar containing an acid-base indicator called phenol red is prepared. This indicator is red in
basic conditions but turns yellow in acidic conditions.
Research Question
How does the rate of diffusion vary by different volumes of agar blocks?
Hypothesis
The rate of diffusion increases as the volume of agar blocks decreases due to the exposion of
greater surface area to acid particles.
Variables
Independent Variable: Volume of agar blocks
Dependent Variable: Rate of diffusion
Controlled Variables: Volume of hydrochloric acid, mass of hydrochloric acid and
temperature
Ozanalp Eryılmaz
11/B
Apparatus

One large block of agar

Tweezers

Knife

Chopping board

100 cm3 hydrochloric acid

Test tubes

100 cm3 beakers

Stopwatch
Safety Precautions
Hydrochloric acid is a highly corrosive acidic hence rinse immediately with water if you
splash any acid on your skin. Laboratory gloves are strongly suggested to be used while
carrying out the experiment.
Method
1. Cut cubes of agar to the following dimensions (use a knife and tweezers to make each as accurate
as possible): 5, 10, 15, 20, 25 mm3.
2. Add enough dilute hydrochloric acid to a test tube or small beaker so that when one of the agar
blocks is placed in it, it will be totally submerged.
3. Place one of the agar blocks into the test tube or beaker containing the hydrochloric
acid, and immediately start timing.
4. Record in a table, how long it takes for all the red colour in the block to disappear (indicating the
time it takes the acid to reach the centre of the block).
5. Repeat for all the other block sizes.
6. If time allows, repeat the experiment as many times as possible.
Ozanalp Eryılmaz
11/B
DATA COLLECTION AND PROCESSING
Beaker
Height / mm
Length / mm
Width / mm
1
2
3
4
5
7
7
7
7
7
6
10
16
24
30
8
15
27
34
40
Volume of
HCl / cm3
20
20
20
20
20
The surface area of each agar block can be calculated by the formula below:
Surface Area = Length x Width
Beaker
Length / mm
Width / mm
1
2
3
4
5
6
10
16
24
30
8
15
27
34
40
Surface area of agar
blocks / mm2
48
150
432
816
1200
The volume of each agar block can be calculated by another formula below:
Volume = Height x Length x Width
Beaker
Height / mm
Length / mm
Width / mm
1
2
3
4
5
7
7
7
7
7
6
10
16
24
30
8
15
27
34
40
Volume of agar
blocks / mm3
336
1050
3024
5712
8400
Ozanalp Eryılmaz
11/B
Time taken for diffusion of agar blocks is recorded in the table below:
Beaker
1
2
3
4
5
Surface area of
agar blocks /
mm2
48
150
432
816
1200
Volume of agar
blocks / mm3
Volume of
HCl / cm3
Time taken for
diffusion / min
336
1050
3024
5712
8400
20
20
20
20
20
3.58
4.26
4.47
5.05
5.29
CONCLUSION AND EVALUATION
Blocks of agar, a gelatinous substance, were used in the experiment investigating the
rate of reaction. When the blocks of different dimensions were placed in a solution of
hydrochloric acid, their original colour(red) began turning yellow as time passed. Time for
diffusion to take place was recorded after the surface areas and volumes of five agar blocks
were calculated. Then, it was certainly observed that the one having both the least surface area
and volume diffused completely first. Time taken for the reactions to occur increased as the
surface area and volume of agar blocks increased as well. Therefore, it can be concluded that
the rate of diffusion is inversely proportional to surface area and volume.
In the experiment, there could be inaccuracy caused by a number of experimental
errors. First of all, a knife was used randomly to divide agar into rectangular pieces and a
ruler(with an uncertainty of 1 mm) to measure the height, length and width of pieces. Five
molds with different volumes could be made use of instead. Then, a graduated cylinder(with
an uncertainty of 1 cm³) was used to measure the volume of hydrochloric acid. A burette(with
an uncertainty of 0.1 cm³) should be preferred during an experiment. The alternative
apparatus stated decreases the percentage of random error and increases both accuracy and
reliability as much as possible. Besides, a stopwatch was used to record time for diffusion to
occur and it has an uncertainty of about 0.2 s due to human reaction time at the start and stop
moments.