Limestone is produced by the compression of calcium carbonate and sediment. In nature, this soft stone is routinely dissolved by acid rain. When the rain hits a limestone surface, the calcium carbonate breaks down, forming carbon dioxide and calcium chloride. This is how karst landscape features, such as caves, are formed. Bear in mind, this process takes thousands of years. For a speedier reaction, submerge limestone in an acid with a low pH. Acid rain has a pH of 5.2, which is almost neutral. Vinegar, on the other hand, has a pH of 2.4 - the lower the pH, the stronger the acid.
Place a piece of limestone in an ovenproof glass pan.
Pour distilled white vinegar over the limestone. As the reaction begins, the surface of the stone will bubble and fizz, releasing carbon dioxide into the air. Cover the pan with a sheet of plastic cling film to help slow the loss of vinegar through evaporation.
Check on the limestone once a day. Lift the edge of the cling film to release any built-up gas. Add vinegar, as needed, the keep the stone submerged. In three to four weeks the limestone should be completely dissolved.
Replace the vinegar with cola, if desired. Cola contains phosphoric acid and has a pH range of 2.5 - 2.7, depending on the brand you select. This makes it only slightly less acidic than vinegar. In addition, it is generally cheaper and does not produce the off-putting odour that vinegar does. On the downside, the limestone will have to be removed from the container on a daily basis to evaluate the effectiveness of the acid.
Use lemon juice, if you happen to have an abundance of citrus fruits on hand. Lemon juice contains citric acid and has a pH of 2.3. Dilute solutions, such as lemon-aid or lemon-lime soft drinks, can also be used to dissolve limestone, but they will take longer to show definitive results.