You'll need a couple of shells (large or small, any variety, but for the sake of comparison, it's good to have two of the same), two cups or beakers, vinegar and water.
Place each shell in a cup/beaker.
Cover the other shell with vinegar.
Vinegar, while a weak acid, is quite a bit stronger than acidified ocean water. The stronger acid speeds up the process, making it visible within a short period of time - perfect for students to grasp an understanding of the process. Shells in an acid ocean environment would be subject to the same chemistry, it would just take place at a slower rate.
You can learn more about the chemistry taking place, as well as ocean acidification here.
You may wish to continue to leave the shell in the vinegar for an extended time to observe further. If so, it may be necessary to replace or replenish the vinegar. (I used a small enough amount of vinegar that the chemical reaction came to a halt within a day or so).
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