Science Buddies: "Ask an Expert"

Yes, they gave me comments and told me that I needed a better explanation for why I used acid( now I now after your explanation), and why soup did best, and vinegars and water did not do as well. I am attching the current discussion and conclusion that I have. I do need to make more clear thorugh since the judges expected more explanationa. Thank you so much for all the help, it is greatly apprecited.

Hi Violin,

Thanks. I've got your attachment now so you can remove it. I'll send another reply with a detailed explanation later today.

Donna

Thank you very much. I really apprecite your help in making everything so clear for me. I do not just want to present my project, I would like the judges to know that I understdand what i did and explian all the whys.
Thank you again.
violin30

Hi Violin,

You have a really good start on the explanation of your results! Lood at the structure of acetic acid (vinegar) in the Wikipedia article. . You can see that acetic acid has 2 carbon atoms with 3 hydrogen atoms, which gives it hydrophobicity, but also contains oxygen and a hydroxyl group (OH). The oxygen atoms make the molecule more polar or hydrophilic. Here is the Wikepedia article on acetic acid; read the section on solvent properties, which describes the molecules as a hydrophilic solvent.

http://en.wikipedia.org/wiki/Acetic_acid

Here is the structure for soap in the Wikipedia article. You can see that soap contains a very long carbon and hydrogen chain, and a polar end containing oxygen atoms. Read the short paragraph in the Wikipedia article on the mechanism of surfactants. Molecules are more soluble in solvents that have a similar structure, so since Captan is very hydrophobic, the longer hydrophobic carbon chain of the soap molecule was more effective in interacting with the pesticide so it could be rinsed from the apple. The acetic acid would work better than plain water, since it has some hydrophobicity.

http://en.wikipedia.org/wiki/Soap


You could put the structure of all of the molecules on your board, so if you are asked about the results, you could explain that the difference in results is due to the chemical structure of the molecules. You could also include the structure of water, HOH, which is completely hydrophilic.

One comment on your last sentence in the first paragraph, “. Silver nitrate solution was used for all samples to keep everything controlled.” The silver nitrate was used to detect the presence of the Captan because the silver ions will precipitate with the Captan, due to the Cl atoms. Silver chloride has a very low solubility in water, so the turbidity in your test samples was due to the insoluble silver-Captan complexes. The concentration of silver nitrate in your samples was one of your controlled parameters. I think it would be better to state the purpose of the silver nitrate (to detect the Captan) first, and then add the explanation of the purpose of the phosphoric acid (to prevent precipitation of silver as silver oxide due to high pH).

http://en.wikipedia.org/wiki/Silver_chloride
http://en.wikipedia.org/wiki/Silver_oxide’’


Let me know if you need more explanation, or if you want me to review any other sections.


Donna