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

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Introduction

Romin Parekh X - B Biology Lab Experiment Aim To investigate the effect of a differing Surface Area: Volume ratio on the rate of diffusion. Theory Agar is a polysaccharide which can be obtained by seaweed or red algae. It is a base i.e. it has a pH higher than 7 where 7 on the pH scale is the pH of pure water. When the agar block is placed in the test tube containing the acid, Hydrochloric Acid, diffusion occurs. Diffusion is the movement of particles from a high concentration to a low concentration. Here the acid molecules are in high concentration in the test tube and so diffuse into the agar block for to maintain an equal concentration throughout the acid and the agar block. When diffusion takes place, the agar block loses its property of being a base into a substance of neutral pH which 7. Due to the presence of an indicator Phenolphthalein, the agar block turns pink. As time passes by it fades turning colorless which means that there is diffusion taking place and the pH is decreasing i.e. becoming more neutral. The rate of diffusion of acid molecules can be monitored by the surface area: volume ratio. A large surface area: volume ratio would result in a faster rate of diffusion whilst a small surface area: volume ratio would result in a slower rate of diffusion. The surface area and volume ratio can be altered by changing the dimensions of the agar block. In this experiment, the volume of the agar block is however kept constant as the surface area is being tested. A Scalpel Agar Block Hypothesis The agar block with the largest surface area: volume ratio will take the least time to diffuse and hence have the fastest rate of diffusion. The main reason why I have chosen this hypothesis as larger the surface area of the block more is the area of contact with the acid and hence a larger area for the acid to diffuse into the agar block which thus reduces the time. ...read more.

Middle

2 x 0.5 x 1 8 1 8 : 1 122.30 A2 2 x 0.5 x 1 8 1 8 : 1 124.73 7. 2.5 x 2 x 0.2 17.2 1 17.2 : 1 61.51 8. 2.5 x 2 x 0.2 17.2 1 17.2 : 1 62.30 9. 2.5 x 2 x 0.2 17.2 1 17.2 : 1 61.02 A3 2.5 x 2 x 0.2 17.2 1 17.2 : 1 61.61 Testing Volume by keeping Surface Area constant 10. 1 x 4.5 x 1 20 4.5 4 : 0.9 54.69 11. 2 x 1.5 x 2 20 6 10 : 3 67.42 12. 1.6 x 1 x 5.25 20 8.4 5 : 2.1 75.59 A4 1.53 x 2.33 x 2.75 20 6.3 3.17 : 1 65.9 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. Symbol Stands for Which means A1 Average 1 Average of timings of Agar Blocks 1 - 3. A2 Average 2 Average of timings of Agar Blocks 4 - 6. A3 Average 3 Average of timings of Agar Blocks 7 - 9. A4 Average 4 Average of timings of Agar Blocks 10 - 12. Simplifying the Ratios Sr. No. Surface Area: Volume ratio Volume: Surface Area ratio Volume of HCl acid used (cm3) Time (s) + 0.01 s Testing Surface Area by keeping Volume constant 1. 6 : 1 1 : 0.167 20 cm3 185.50 2. 6 : 1 1 : 0.167 20 cm3 187.09 3. 6 : 1 1 : 0.167 20 cm3 188.11 4. 8 : 1 1 : 0.125 20 cm3 126.78 5. 8 : 1 1 : 0.125 20 cm3 125.11 6. 8 : 1 1 : 0.125 20 cm3 122.30 7. 17.2 : 1 1 : 0.058 20 cm3 61.51 8. 17.2 : 1 1 : 0.058 20 cm3 62.30 9. ...read more.

Conclusion

b. Do not leave the agar block subjected to the air for a very long time as it will react with it giving inaccurate results. c. Do not round off the lengths of the agar blocks. d. Do not swallow any of the chemicals including the agar as it can be dangerous for your health. e. 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. ...read more.

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