Background: Solutional caves almost always form in areas where the rocks are made of calcium carbonate (CaCO3), because that mineral reacts most easily with acidic water. The uneven distribution of that mineral in the world explains why some areas have very few or no caves, and why other areas have many caves. The largest and greatest number of caves are found in limestone (the other rocks formed from calcite are marble, dolomite, and gypsum). Cavers hunt caves by looking for areas made of those rocks. In fact, the Gunung Buda area in which our expedition takes place was explored when someone spotted limestone mountains from an airplane, then went back on foot to specifically hunt for caves. Carbonic acid forms when carbon dioxide from the air reacts with water (CO2 + H2O ---> H2CO3), which then dissolves the limestone away as it moves through it. This occurs most actively just below the water table, where fresh, acidic water hasn't picked up enough calcite to become saturated. And while we're at it, cave passages formed under the water table (phreatic) tend to be round (due to being equally dissolved in all directions), while those formed above the water table (vadose) tend to be roughly rectangular in shape due to the dissolution occurring mostly on the floor. And its worth mentioning here that the rock beds of calcite were laid down by ocean organisms when the area was covered by shallow seas about 450 million years ago. You know, shells, bones, and corals. This CaCO3 is the same stuff your bones and teeth are made of, which is why the weak acids in you mouth (from the bacteria nestled between your gums and teeth) attack your pearly whites so successfully.

Procedure: Have the students write up the lab paper from the overhead using your template. Pass out sets of rocks to each group of students. The set should contain 2 non-calcite type rocks ('pert near anything but limestone and marble will work), and 1 calcite sample. As mentioned on the unit lesson plan page, call the local monument company if you don't live near limestone or marble. Chalk works, but is a tad lame next to your rocks. Have the students use a nail to scratch an area of each rock, enough to form a small area with some powdered rock in it. After that, have them apply 1 or 2 drops of white vinegar or weak hydrochloric acid to the area and observe. Bubbles will appear on the calcite-based rocks, which is the acid reacting with the calcite to form hydrogen gas. No bubbles will appear on the other rocks. No reaction, no dissolution, no caves. Have them finish the lab write-up, and discuss the implications.

Alternate labs: With older students you can put 2 rock samples (1 being calcite-based) into 20% solutions of hydrochloric acid after weighing them. Replace the acid when the reaction stops, and re-weigh after an hour. Use appropriate safety measures, of course.