Science Buddies: "Ask an Expert"

I just finished the Effects of Ocean Acidification experiment and the empty sea shells in my control jars increased in weight after 30 days in seawater. My hypothesis would be that they would have stayed the same weight or decreased slightly. I was surprised that they increased in weight 5-7%. The shells I used were beach shells collected a few years ago. I also did the experiment with mussel shells. I had the same result .. the control jar shells increased in weight about 3%. Considering the shells were thoroughly dried, what would have cause their weight to increase?

I thought only living shells could grow.

Hi seashell,

One possibility is that the shells in the control jar absorbed some of the water into the crystal structure of the shells. Even if the surfaces of the shells were dry, it might take much longer (and perhaps some gentle heating in a warm oven) to drive off the water. I wouldn't have expected a 3 to 7% increase in mass. But, my hypothesis is that even though you dried the surfaces of the shells, there was still water that permeated the crystal structure of the shells.

The good news is that even though the results from the control shells were unexpected, your findings for the rest of your samples are still valid. However, you will need to account for the fact that the shells in the acidic water might have gained weight as water interacted with the shells, in addition to losing weight as the acid corroded them. For example, let's say that you measured that the shells in one particular acidic solution lost 10% of their initial weight. In addition to reporting that, you can report the difference between the mass change of that shell and the control shells. The control shells gained 3% of their weight, which would mean that 13% of the mass of the shells in the acidic solution was actually corroded away.

Let me know if that makes sense. Post back as you have other questions.