Agard Block: To investigate the effect of a differing Surface Area: Volume ratio on the rate of diffusion.

• 1 x 1 x 1: Put it into test tubes 1, 2 and 3 respectively.
• 2 x 0.5 x 1: Put it into test tubes 4, 5 and 6 respectively.
• 2.5 x 2 x 0.2: Put it into test tubes 7, 8 and 9 respectively.

     

9. Hold the scalpel like as it is a pen with the sharp end pointing downwards.
10. Cut the agar block into 3 pieces for each respective dimension.
11. In total there will be 9 agar block pieces with different dimensions.
12. Take the agar block and add it to the test tube containing the acid.
13. Start the stopwatch as soon as the agar block is added to the test tube.
14. Keep checking the color of the agar block by holding the test tube in one hand and placing a white sheet of paper behind it with the other.
15. Stop the stopwatch only when the agar block is completely colorless.
16. Note down your observations and results in the data table.
17. Repeat steps 1 – 16 with the other agar blocks of different dimensions.

Section C

18. Repeat steps 1 – 16 of Section B with the following set of dimensions for the agar block:

• 1cm (length) X 4.5cm (width) X 1cm (height): Test tube 10
• 2cm (length) X 1.5cm (width) X 2cm (height): Test tube 11
• 1.6cm (length) X 1cm (width) x 5.25cm(height): Test tube 12

Variables

We do this experiment to find out the cause and effect of relationships between quantities. There are 3 types of variables in this experiment. The Independent Variable is the one which is constantly changed in the experiment. The Dependent Variable is the one that changes in response to the Independent Variables. The hence “depend” on the Independent Variables. Control Variables are kept constant in the experiment so as only the Independent Variables affect the Dependent Variables and not any other factor.

Independent Variables

• Surface Area
• Volume

Dependent Variables

• Time for Agar block to decolorize
• Observed color changes

Control

• Agar used in the experiment
• Surface Area when Volume was tested
• Volume when Surface Area was tested
• Concentration of Acid
• Measuring apparatus used
• Room Temperature and Atmospheric conditions

Observations

The moment the agar block was added to the test tube containing the acid, the acid started acting on the base (agar block). The proof for this statement is the decolorizing agar block which was pink at first due to the presence of the indicator phenolphthalein and then as time passed by it faded to complete white. The centre of the block was decolorized at the last while the other surrounding parts decolorized much sooner. This is as the acid took time to diffuse into the agar block and reach its centre. The agar block however did not change physically that is in terms of dimensions. It did not alter and were same before and after the experiment. The color of the acid also did not change. A detailed observation for agar blocks of specific dimensions is given in the table below.

Results

Table 1

Note: The results that are bold are the average of the readings taken.

Different symbols have different meanings which are mentioned in the table below.

Simplifying the Ratios

Average Reading of Results 1 – 12

 

Graph 1

                                                               

Graph 2

Graph 3

                                           

Graph 4

Conclusion

The graphs drawn show that my hypothesis was proved right. In graph 3 it can be seen that all the values lie on the line of best fit and there are no erroneous results. As the volume: surface area ratio increases, the time for the agar block to decolorize also increases and so it shows a direct proportionality. The volume amount of mass of the agar block (cm3). As the volume increase, its amount of mass increases too and so the time required for it too completely decolorize will hence increase. The results in Graph 3 support my comment. But it is very difficult to note that only volume also affects the rate of diffusion. As diffusion occurs through the surfaces that in contact with Hydrochloric acid. The line of best fit indicates that it is the best possible results which are seen in Graph 3.

        In Graph 2, though the points lie next to the line of best fit, they show a negative co-relationship. They hence cannot be determined as anomalies. It can also be seen that surface area: volume ratio is inversely proportional to the time required for the agar block to decolorize. This is as the surface area is the major factor that affects rate of diffusion. When the surface area: volume is small i.e. when the surface area is small, the areas of the acid diffusing into the agar block are less and so overall it takes more time to decolorize the agar completely.  

        Graphs 2 and 3 show the type of proportionalities that Surface area: Volume ratio and the Volume: Surface area ratio has with time. In Graph 3, it is seen that the time taken for decolourization of the agar block is directly proportional to Volume: Surface area ratio but however is inversely proportional to Surface area: Volume ratio which is seen in Graph 3. The agar block with the largest surface area has the lowest time for it to decolorize and vice versa. The agar block with the largest volume takes the longest to decolorize completely.

        At the end it can be commented that the concentration of the acid also plays a major role in the rate of diffusion. The diffusion rate decreases as time increases. This is because the concentration inside the agar block is some what the same as that in the acid. So the diffusion rate drops and the diffusion gradient is less steep. When the agar block is white i.e. completely decolorized, diffusion stops and the diffusion gradient is a straight flat line signifying that there are no differences in the concentration inside and outside the agar block.

This experiment can also be looked from a larger perspective i.e. when the agar block can be related to animals. Larger animals have a larger surface area while smaller animals have smaller surface areas. This is the main reason why large hibernating animals like the Grizzly Bear eat a lot of food to store a large content of fat. This is as when fat is broken down into simpler compounds, they provide a lot of energy. But however in comparison with their volumes, large animals have a small Surface Area: Volume ratio while the smaller animals have a larger Surface Area: Volume ratio. So they lose a lot of heat energy faster than the larger animals.

Evaluation

The experiment was successful as they proved my hypothesis to be right. There were no anomalous results. However methods were taken to reduce the error but due to the inaccuracy of the equipment used, there are errors that took place which were unavoidable.

The scale had an error of +0.05 cm which might seem very insignificant but while measuring a length of 2cm it constitutes to 2.5% which is very high. Secondly, a Pipette was used for measuring the volume of acid. This was used instead of pre-decided Measuring Cylinder which had an inaccuracy of +0.05ml. The stopwatch used for measuring the time required for the agar block to completely decolorize had an error of +0.01s. These are unavoidable as they are inaccuracies of the apparatus or the limitations of measuring accurate data. Also with the Systematic errors there are Random errors that can be avoidable. The agar block might have reacted with the atmospheric gases and may have partially decolorized which gives huge errors.

Fair Test

•  Surgical Gloves were used while cutting the agar block into specific dimensions so that the bacterium and moisture from our hands did not affect the agar block and hence decolorize it.
• The same apparatus was used for measuring all the measurements e.g. the same Pipette was used for measuring 20cm3 of Hydrochloric acid. This reduces the systematic errors of using multiple equipments.
• A blank white paper was behind the test tube to check whether the agar block had completely decolorized. This is accurate as any color difference can be easily in the white paper.
• The agar block was not exposed to the air for a long duration. When not needed, it was covered.
• A clean scalpel was used for cutting the agar block. This was done to make sure that there was no moisture left on the scalpel which might decolorize the agar block before adding it to the acid.
• A coloring indicator was used to note the decolourization of the agar block and it was kept through out the experiment.
• All the lengths were not rounded off and precise readings were taken.
• The stopwatch was stopped at the time when the agar block was completely colorless.
• Whenever during the experiment the other agar block in the Petri dish was not needed, it was covered using a glass cover.
• While testing for the Surface area, the Volume was kept constant at 1cm3.
• While testing for the Volume, the Surface area was kept constant at 20cm2.
• Atmospheric conditions were not changed and were kept constant throughout the experiment.
• The agar block was completely submerged in the acid and all of its sides were in contact with the acid.

Precautions

1. Take care, while cutting the agar block with a scalpel as it is sharp.
2. Do not leave the agar block subjected to the air for a very long time as it will react with it giving inaccurate results.
3. Do not round off the lengths of the agar blocks.
4. Do not swallow any of the chemicals including the agar as it can be dangerous for your health.
5. Do not handle equipment with open wounds.

Error Analysis

Inaccuracy = Smallest reading of apparatus / 2

Total Error = Inaccuracy x No. of times used

Systematic Errors : Limitations of Accuracy

1. Plastic Pipette

Inaccuracy of apparatus = 0.05 cm3

Total Errors = 0.05 x 12 = 0.6 cm3

2. Scale

Inaccuracy of apparatus = 0.05 cm

Total Errors = 0.05 x 36 = 1.8 cm

3. Stopwatch

Inaccuracy of apparatus = 0.01 seconds

Total Errors = 0.01 x 12 = 0.12 seconds

Random Error : Limitations of Precision

• It was tough determining when the agar block has completely become colorless and hence stop the stopwatch. This is an error as it is tough to judge for the human eye when to consider the experiment to be complete. So there was a possibility that the stopwatch was not stopped at the correct time; may be it was stopped earlier or later.
• The scale and pipette have parallax errors.
• Agar is a soft substance and it broke into pieces easily. So cutting the agar into accurate dimensions was tough.
• The stopwatch might not have been started at the same time. This further increases the random error.
• While the agar was being cut, there was a chance of the atmospheric gas components to react with it and decolorize it partially.

Improvements

One can never have a complete perfect experiment and there are always improvements that need to be made.

1. Repeat the experiment for a larger range of values of Surface Area and Volume.
2. Different concentrations of the acid can be used make a steep gradient and for extensive judgment that Surface Area affects the rate of diffusion.
3. Different acids can also be used.

Bibliography

http://biology.clc.uc.edu/fankhauser/Labs/Microbiology/Bacterial_Inhibition/Pour_seeded_agar_P7231208md.jpg