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Agar blocks

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Experiment showing rate of diffusion in agar blocks Factors: Size of agar block Concentration of hydrochloric acid (Hcl) Hypothesis: I predict that the bigger the Agar blocks the more time it will take for it to completely diffuse due to the fact that the surface area to volume ratio decreases with size. As for the concentration of the acid I feel that the greater the amount of moles in the acid the quicker the rate of diffusion as the phenolphthalein indicator will diffuse from an area of high concentration to an area of low concentration. Materials and equipment: Beaker Measuring cylinder Knife Agar blocks containing phenolphthalein indicator Ruler Hydrochloric acid Tweezers Stop watch Effect of size on rate of diffusion Procedure #1 1. Cut a pair of agar blocks with a knife to a size 0.5�0.5�0.5 repeat the same for sizes' of 1.0�1.0�1.0 and 1.5�1.5�1.5 2. ...read more.


to volume ratio 0.5 1.5 0.125 :=12:1 1.0� 6.0 1.000 :=6:1 1.5� 13.5 3.375 :=4:1 Agar block of size 0.5�0.5�0.5 Concentration of Hcl (moles) Time (sec) Average time taken (sec) 1st agar block 2nd agar block 0.75 202.2 204.0 203.1 0.50 258.0 269.4 263.7 0.25 273.0 300.0 286.5 Table 3 Graph for table 1 Figure Graph for table 3 Figure 2 Discussion: The results obtained from this experiment are straight forward. When looking at the "size of the agar block" it can be easily seen that the bigger the agar block the longer it took for the phenolphthalein indicator to diffuse completely, no anomalies are present. The same can be said, when looking at the factor: concentration of Hcl, no anomalies are present whatsoever, and it can be easily deduced that the higher the concentration of acid the quicker the phenolphthalein indicator diffuses. Evaluation of results: The results obtained from the above tables are-as previously mentioned-straight forward, in "Table 1" the reason for the increase in ...read more.


I think that in order to improve this hindrance a more accurate instrument should be used instead of a knife and a ruler, as human error is very easy. Another problem when doing the experiment was that the agar block itself contained different shades of colour ranging from very light pink to dark purple, meaning that the concentration of phenolphthalein indicator was different in some areas of the agar block ,and this obviously tampered with the results. A solution to this problem would be to cut most of the light colour off and only use the darkest part of the agar block, however this would waste most of the agar block. So maybe dip the agar block into phenolphthalein indicator the second it's been cut and then put it into the beaker of Hcl. Conclusion: The hypothesis for both factors was correct, Surface area to volume size largely effects the rate of diffusion as does the concentration the acid being used. ...read more.

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A reasonably well conducted experiment but a lack of specific detail and scientific terms in the method together with incorrect graph plotting are major shortfalls.

Marked by teacher Adam Roberts 10/05/2013

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