Diffusion in Agar Block

Independent Variables:

• Surface Area of Agar block
• Volume of Agar Block

Only 1 particular independent variable is tested at 1 time i.e. when Surface Area is being tested the Volume is kept constant.

Dependant Variable:

• Time Taken for decolourization
• Colour change observed

Constant Variable:

• The Agar being used
• Concentration of Hydrochloric Acid
• Volume of Hydrochloric Acid
• Surface Area of Agar Block (in the part where volume of block is being tested)
• Volume of Agar Block(in the part where the surface area of block is being tested)
• Equipment Used (scalpel, beakers, Petri dish)
• Temperature and Atmospheric Conditions

Method: The method has been split into PARTS so that it can be understood by another user.

Part A

1. Use the marker to mark each beaker from with numbers from 1 to 12.
2. Pour a little of the Hydrochloric Acid into the measuring cylinder so that it touches the 25ml mark.
3. Empty out all the liquid in the measuring cylinder into beaker 1.
4. Repeat steps 2 and 3 and fill all the beakers from 1- 12 with 25ml of Hydrochloric Acid.

Part B

1. Cut a large piece of the Agar using the scalpel. Do not forget to place the cover back on to the Petri dish.
2. Quickly place this piece on a graph sheet and measure 3cm of length and 2cm of width and cut the agar using the scalpel so that the cuboid formed will have the following dimensions 3cm(length) * 2cm(width) * 1cm(height). The height will remain constant. Put the remaining piece of agar back in the Petri dish.
3. Put this piece into Beaker A.
4. Start the stopwatch as soon as you completed step 3.
5. Lift the beaker and place a blank white sheet of paper behind it so that it is easier to notice the colour change.
6. Observe the colour changes taking place and stop the stopwatch only when the agar block has completely decolourised. This step may take some time.
7. Use the Pen and Paper provided to note down this time.
8. Remove the agar block from the solution and record any changes in appearance and dimensions if observed.
9. Repeat steps 1 – 7 with two different dimensions- 5cm (length) * 1.2cm (width) * 1cm (height) and 6cm (length) * 1cm (width) * 1cm (height). Remember to use the different beakers provided. In this case use beakers ‘2’ and ‘3’.
10. Repeat steps 1 to 8 twice so that you get 3 readings for each dimension. Remember to use different beakers. In this case use beakers ‘4’, ‘5’, ‘6’, ‘7’, ‘8’ and ‘9’.

Part C

1. Repeat steps 1 – 7 of Part B with different dimensions. The new dimensions to be used are-

1. 1.0cm (length) * 2.0cm (width) * 1.0cm (height)
2. 1.5cm (length) * 1.4cm (width) * 1.0cm (height)
3. 3.0cm (length) * 0.5cm (width) * 1.0cm (height)

Remember to use different beakers. In this case use beakers ‘10’ , ‘11’ and ‘12’.

Results:

Testing Surface Area by keeping Volume constant

Average Readings for Testing of Surface Area

Testing for Volume by keeping Surface Area Constant

Observations Made:

• With time the agar started to decolourize. The pink started to become lighter and eventually the block was transparent again.
• The centre of the block became decolourized at the end, while other parts of the agar block lost colour sooner.
• There were no changes in the dimensions and appearance of the agar block before and after the experiment.
• The solution of Hydrochloric Acid did not change colour.

Graphs:

Graph 1

Graph 2

Graph 3

Graph 4

Conclusion:

The graphs drawn show that the hypothesis was proved correct. It can be seen in graph 3 that as the surface area rises the time taken for the agar block to decolourize decreases this confirms the predictions made in the hypothesis. It can also be seen that the points lie on the line of best fit and there are no anomalies within the results. The line of best fit indicates that it is possible to predict the time taken for decolourization of the agar block.

It can also be seen that the rate of diffusion drops as the experiment proceeds this confirms the point stated in the ‘theory’, and shows us that the rate of diffusion depends upon the steepness of the concentration gradient. As the colour starts diffusing out of the agar the concentration gradient becomes less steep and when diffusion stops this gradient is a flat line.

In graph 4 it can be seen that as the volume of the agar block is increased the time taken for decolourization also increases but it can be noticed that this time difference is much smaller than the time difference between blocks of agar with different surface area. In fact the time differences are so small that they may not have been caused due to increasing volume as could have just been a product of errors and uncertainty. So we cannot entirely know whether changing the volume of the agar block while keeping surface area the same affects the rate of diffusion. This makes sense because diffusion occurs at the areas where the hydrochloric acid is in contact with the agar block and this means that only the surface area affects the rate of diffusion.

Graph 1 and 2 show that Time Taken for decolourization is directly proportional to the Surface Area: Volume ratio BUT is inversely proportional to Volume: Surface Area ratio. Graph 1 shows that the block with the lowest Volume: Surface Area ratio takes the highest time to lose its colour. Graph 2 shows that the block with the lowest Surface Area: Volume ratio has the lowest time to lose its colour.

It can be said that the Surface Area does affect the rate of diffusion because a larger surface area means there is more space for the reaction to occur and this increases the speed of the reaction reducing the time taken for its completion. This also explains why smaller animals have to eat food more often than larger animals, this is because they have a larger surface area as compared to their volume and so they lose heat quickly and need a constant supply of food to maintain internal processes.

Evaluation:

The experiment was a success as the results obtained were hand in hand with the ones predicted and the hypothesis was also accepted. There were no large anomalies as such and as seen in the graph all the points lied around the best fit line. Since nine readings were taken we can say the results are consistent but there were still a few unavoidable errors caused by the equipment used in this experiment. For example a +/-0.05 error while measuring 3cm would give a 1.67% error which is a significant error. The same problem occurs in the measuring cylinder, next time I would use a burette instead of the measuring cylinder as it has a lower error. The stopwatch used also had an error of +/-0.01; though this is a small value a more accurate stopwatch would have provided better results. Because substances in the agar react with the gases in the air some of the agar got partially decolourized, this also could have caused errors in the experiment but on the whole the experimental errors were within the range and so the experiment was a successful one.

Error Analysis:

• Large errors may have been caused because the agar is a soft object and it breaks very easily and so it is difficult to cut out exact dimensions. Therefore an error could have been caused which is in addition to the systematic error of the metre rule provided. This could lead to errors of up to 0.5cm per side. This error will also affect the volume and surface area and calculations suggest that each may have had an error of 1.5cm3 and 1.5 cm2.
• When the agar was being taken out, gases in the air like carbon dioxide may have reacted with the components of the agar causing it to lose its colour gradually. This may have resulted in a lighter piece of agar being used which would cause an error as it would take less time to decolourize.
• There is also a chance of parallax errors occurring while measuring the volume of Hydrochloric Acid and while cutting the agar pieces into shape.
• The stopwatch may not have been started at the exact moment; this error can be large if only one person is conducting the entire experiment. The stopwatch may also not have been stopped at the exact moment and this adds to the error.
• The volume of Hydrochloric Acid added may not have been exactly the same.

Fair Test:

• Keep the blank white paper behind the beaker at all times during the experiment as it is only then that detection of the changes in colour can be made.
• Do not leave the agar out open in the air. Always shut the lid of the Petri dish after removing the agar.
• All the equipment must be clean; the deposition of dust and dirt may affect the rate of diffusion.
• Do not move or shake the beaker while the experiment is taking place.
• The concentration of acid and that of the colouring agent (phenolphthalein) is the same through out the experiment.
• Conduct the experiment in atmospheric conditions and do not change any elements such as temperature etc.

Safe Test:

• Be careful with equipment such as the beaker so that you do not end up dropping it.
• Do not cut yourself with the scalpel.
• Do not touch any of the chemicals with open wounds as open wounds are prone to infection.
• Do not ingest any of the chemicals and make sure you wash your hands after handling the agar etc. as they may cause itching.

Bibliography:

IGCSE BIOLOGY – Text Book

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