@dalehileman,
Im suggesting they use vinegar, that's why the smashing. If it fizzes with an HCL it could be calcite or Dolomite. Calcite will be barely noticeable in household vinegar but ground up, it fizzes nicely.
I don't want kids messing with heavy duty acids till they understand Normality and how to make tst solutions so they don't burn they damn hands. HCl can leave nasty blisters, and if the bottle spills in your pack, well bye bye lunch and stuff.