Ah, sorry for posting in the wrong section of the forum. I forgot to mention that I am an undergraduate engineering student (so I posted this in undergrad chemistry because I thought "that's the level of chemistry I'm familiar with"), but the question doesn't have anything to do with my course so Citizen Chemist is probably the right place.
My understanding is that eggshells are about 95% CaCO3, with the other 5% being mainly magnesium, phosphates and proteins. I took a 1mL sample of the water that the off-white precipitate was suspended in, after the CO2 was evaporated out and added some NaOH to it to see if it reacted in any way. I didn't notice any reaction, so I guess the water isn't acidic?
As far as I can tell, I've recreated Cody's experiment to the letter; water + dry ice + eggshell, wait some time, filter the water, boiled off the water, collect precipitate. Is it possible that the rotten egg smell is caused by leftover organic material on the eggshells which rotted over the two weeks? Did I not clean the shells well enough? Or is the off colour and smell caused by Magnesium and/or phosphates? What did Cody do differently to get different results?
I'm not looking for the answers to these questions, just ways that i could answer them myself. Any hints on what sort of tests I could perform to determine the presence of different contaminants? I have tried to research this myself but I'm stuck because the only chemical associated with CaCO3 I can find that smells rotten is hydrogen sulphide, but I don't know where the sulphur could have come from. If the way I'm wording my query is against forum rules, then I could use some tips on how to use this forum more effectively.