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biology investigation on diffusion with agar jelly

Asked by miss_adorable | Mar 4, 2004 | AS Level > Biology > Revision
miss_adorable asks:

hi can you help me with my biology assignemnt?

we done an experiment at school about diffusion. at school they gave us an block of agar jelly which is pink in colour due to the presence of sodium hydroxide (a base) and phenolphthalein (an indicator). Phenolphthalein is colourless in acid. there fore when the agar jelly is placed in an acid solution (sulphuric acid) , the acid will diffuse into the jelly causing it to change from pink to colourless. the time taken for the jelly to go clear is a mesure of the amount of acid diffusion into the agar jelly.
basically we cut the jelly into three measuremnt 1*1*1* and 2*2*2 and 3*3*3 we placed all three jelly into a beaker of acid (100mls)and waited until they all went clear. the smallest block of jelly went clear first(in 12 mins) and then the block of jelly which was the one with 2 times 2times times 2 measurements went clear second(25 mins) and the last one with the measurements 3 times 3 times 3 measuremnt(50mins) went clear the last.
*and now i am suppose to write a experimental report and i am having troubles with that? can you help me write a report which consists of aim hypothesis, method, results, risk assesment, conclusion.
* can you find me some back ground information on diffusion.
* why does the smaller cube of jelly diffuse the fastest and the largest one diffusues slow??
* does this experiment require a control and why??
* how can i fix up my human errors
* does this investigation demonastrate the effect of surface area to volume ratio on the rate of diffusion

thank you

etutor answers:
Your first question is a bit vague - I can help you, but only by answering specific questions, like the other ones you ask. You can, however, submit your coursework to this site when it is written up, and a tutor will comment on it.
Diffusion is when particles move from an area of high concentration to an area of low concentration. Diffusion can occur in liquids and gases. A useful explanation of it can be found on the following site:
The experiment does demonstrate the effect of surface area to volume ratio on diffusion. If you work out the sa:vol ration for each cube, you will find that the higher the ratio, the quicker diffusion is. The small cube works fastest because it has the highest sa:vol ratio When you are evaluating your method, you do not need to consider human errors (i.e. mistakes you might have made) - you need to consider weaknesses in the method itself (although some of these may make human errors more likely).

2 student responses

i'm supposed to do this experiment for biology myself, but i have no idea how to plan it! could yo please help me!!
responded Mar 24, 2004 9:57:44 PM GMT
Fantastic! I have to do this as well. I was gonna use cork borers to get a good size, and then cut to different lengths. If anybody has something good, help!!! Thanx
responded Mar 30, 2004 11:36:44 AM BST
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