Geologists love testing rocks and minerals and, admittedly, one of the most fun tests is the FIZZ TEST. The mineral calcite is made of calcium carbonate (CaCO3).
Hydrochloric acid is the most common acid used among geologists to test whether a rock has any calcite content. In this reaction, the calcium carbonate reacts with the acid and produces carbon dioxide gas, water, and calcium chloride. The carbon dioxide produces the bubbles that you see on the surface of the rock.
Whoa! Hydrochloric Is Too Strong for Kids to Use
We agree with you that hydrochloric acid is not always the best choice to use with kids. For one thing, when kids are learning to identify rocks and minerals you want them to be able to test and explore without worrying if they are going to burn their fingers with acid.
Weak acids like lemon juice (citric acid) and vinegar (acetic acid) are the perfect solution for performing safe lab experiments with younger kids. The only problem with weak acids like lemon juice and vinegar is that sometimes it is harder to see the reaction (the bubbles).
The best way to fix this problem is to create a fresh surface for the kids to test. Since using a rock hammer in class isn’t always advisable, I suggest giving each child a paper clip that they can use to scratch the surface of the sample. When you scratch the surface you are removing some of the older weathered exterior or areas that were already tested and giving the children a fresh surface to test. Once they scratch off a small area to test, they can use a dropper bottle or a straw to place a few drops of lemon juice or vinegar on the surface. If it bubbles, you know that there is the mineral calcite in your rock. This is a great test for limestones and marbles which are made completely of calcite.
If you need help teaching kids how to identify rocks and minerals check out this book which will give you all of the details you need.