Science Buddies: "Ask an Expert"

Hi there,
I am conducting the "Swimming in acid" experiment from this website for a regional science fair that is coming up soon. I love all the information that has been posted but there are a couple of things that don't make sense to me and I just wanted to clear them up so I know what I'm talking about and don't make a mistake in front of the judges!
Firstly, I'm not sure why you would use white vinegar in place of carbon dioxide, how does it give the same results?
Secondly, I'm a little confused as to why this experiment works with just the shell, not the oyster, dosen't it need to be alive to build it's shell?
I really appreciate any help with these questions
Thank you!

Vinegar is a better choice than carbon dioxide. Carbon dioxide must dissolve first to form acid called carbonic acid. Carbonic acid is less acidic than vinegar. In addition, it's easier, cheaper, and more obtainable than carbonic acid.

Thank you for replying,
Do you have any insight on whether the shells are slightly dissolved in the water or whether they become lighter due to the carbonate being taken out of the water (explained on the project page on this website)?

Think about mixing baking soda with vinegar. When you do, you get an impressive reaction, but you lose some baking soda, and some vinegar. So that being said, what do you think causes the shells to lose mass?

Hi OceanBlue275,

You have already gotten some good information from the other experts. I just want to add a little bit to what has already been said.

First, let's talk about what ocean acidification - or, really, acidification - means. When we talk about acidity, we are talking about pH, which is a measure of the concentration of hydrogen ions in solution. More H+ ions makes the solution more acidic (lowers the pH). It doesn't matter where the H+ ions originated; if there are more of them, the solution is more acidic. This is why you can use a different acid (vinegar, which is acetic acid) for your experiment, for all the reasons Mickeyhng mentioned.

To understand what is happening to the shells, you need to understand the underlying chemical reaction. First, let's start with how CO2 makes the solution acidic:

CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

Carbon dioxide combines with water to form carbonic acid. Carbonic acid is a weak acid, which means some of the molecules stay together and some of them split into H+ (acid) and HCO3- (bicarbonate ion).

To understand how this affects shells, you have to understand that most mollusc shells are made of calcium carbonate: CaCO3. Having a lot of H+ ions in solution encourages the carbonate ion, CO3(-2), to combine with some of the H+ ions to form HCO3-. It's all about chemical equilibrium and Le Chatilier's prinicple, which are good terms to look up when doing your background research.The point is that the calcium carbonate shells are breaking down chemically, and this chemical breakdown is accelerated by acidity.

I hope this helps. Please post again if you have more questions.

Heather

Hi Heather,
Thank you very much for your advice, it was exactly what I needed.
I feel I understand my project well now and can't wait for the Science Fair (tommorrow!!)

Thank you to everyone who has posted, your help was greatly appreciated.

Hi OceanBlue275,

Glad to help! Good luck at the fair, and please let us know how it goes!

Heather