Science Buddies: "Ask an Expert"

Hello,

I am a Grade 8 student. I did this experiment with mussel and clam shells at both room (22 deg. C) and fridge (2 deg. C) temperatures. I had three jars for each sample - a total of 24 jars. My regular seawater had a pH of 8.1 and my acidified water had a pH of 7.5. The shells were in the jars for one month and I did not open the jars for the entire month. I dried all the shells overnight before weighing them. My results were as follows:

1. At room temperature, both mussel and clam shells had a greater loss of weight in the regular seawater than the acidified seawater. Unexpected result!

2. At the cold temperature, this effect was reversed. At the cold temperature, both the mussel and clam shells had a greater loss of weight in the acidified water than the regular seawater.

3. At room temperature, the mussel shells had a greater loss of weight than the clam shells in both types of water.

4. At the cold temperature, this result was also reversed. At the cold temperature, the clam shells lost more weight than the
mussels in both types of water.

5. Overall, room temperature water had a greater loss of weight than the cold water for both types of shells and both types of water.

Could you please offer any advice or guidance on how to interpret my data? I think an error was maybe to not retest the pH at the end of the experiment. Is it possible that the pH of the water changed over the month? Does temperature affect pH?

I am thinking of rerunning my experiment, but opening the jars every week to test if the pH is staying constant or not? I would appreciate any input or suggestions you could offer to me.

Thank you!

Moderator note: You will see that I have removed your duplicate post on this topic. I have however cross-posted your question to the Grade 6-8 Physical Sciences forum, hoping that the experts in that forum will have some good suggestions. In the meantime, please be patient - the experts here at Science Buddies are all volunteers, so please be patient if you don't get an immediate response. Thank you.

Hello kimpey12!

Very interesting experiment! Ocean acidification is a relatively new area of study and an important topic to look into when considering the well-being of ocean ecosystems!

Firstly, you should only use one type of shell for your experiment because by using two types of shells that have different chemical compositions, this means that they will dissolve at different rates. I suggest you choose one of the shells and use the results you measured for this type of shell only, or you can re-run the experiment with one type of shell. It would also be best to only test the effect of temperature OR pH on this one type of shell, not both, as testing both factors will give you too many variables which seems to be your problem.

To answer your questions about all of the data you collected, yes, the temperature does have a small effect on pH, in that increasing temperature causes a slight decrease in pH. For example, at 0 degrees Celcius, the pH of pure water is 7.47 and the pH of pure water at 100 degrees Celcius is 6.14, which is more acidic. For your first question, I suspect that a slight error was made, possibly a mix up between acidic and regular seawater? So I do suggest that you repeat that step.

However, cold water also dissolves more carbon dioxide easier than warm water does, which is what breaks down the shells. So, it makes sense that both shells lost the most weight in the cold, more acidic (more carbon dioxide-containing) water.

I suggest re-running your experiment with fewer variables since the results should be more accurate and there will be less room for error. Also, when re-running your experiment, you may want to make the acidified sample even more acidic than a pH of around 7, as this is normally the pH of pure water, which is neither acidic or basic.

Let me know how it goes or if you have any more questions! Good luck!
-cnoonan180

Hello Kimpey12,

In addition to the excellent suggestions from the expert cnoonan, I suggest that you NOT open the jars until the end of the experiment, since you wouldn't want to mess with the amount of carbon dioxide. But your idea to test the pH in each jar at the end of the experiment is a good idea.

Even though you do re-run the experiments, hang on to your previous results. It will be interesting to compare those results with the results of your new experiments.

Let us know if this helps!