Limestone caverns

The types of aqueous equilibria described in this section have been given special names, and it is essential that you be able to recognize them. Keep in mind, however, that the principles described in the previous sections apply to all chemical equilibria. Chemists categorize equilibria for convenience, but they treat all equilibria the same way. Our Chemistry and the Environment Box explores the roles of these equilibria in a spectacular natural process, the formation of limestone caverns. 

Limestone caverns are among nature s most spectacular displays. These caves occur in many parts of the world. Examples are Carlsbad Caverns In New Mexico, Jeita Caves in Lebanon, the Blue Grotto in Italy, and the Jenolan Caves In Australia. Wherever they occur, the chemistry of their formation involves the aqueous equilibria of limestone, which Is calcium carbonate. Three such equilibria, linked to one another by Le Chatelier s principle, play essential roles In cave dynamics. 

C18-0131. Limestone caverns are formed by the reaction of H2 O and CO2 with natural deposits of calcium... 

As I started to go down the slope, I was astonished to see the pond water begin to churn and to see the logs stand up on end in an impossible defiance of gravity and several other rules of physical mechanics Of course, the most obvious explanation of this experience—that I was in an altered state of consciousness—never occurred to me. Instead, and this is the resolution I spoke of above, the earth opened up and the pond water and the logs slid through the aperture into the psychedelic cavern that opened my story. It s a limestone cavern, I exclaimed to no one in particular as I ran down to examine its vast and exotic volume. 

Dissolution, precipitation, and deposition processes (e.g., those present during the formation of limestone caverns or in the formation of brines or high-salinity waters). 

Assuming that water containing Ca(HC03)2 deposits 1 mg of CaC03 per minute on the ceiling of a limestone cavern, how long will it take to produce a stalactite with a mass of 75 metric tons (1 metric ton = 1 X 10 g) ... 

Bretz, J H., 1942. Vadose and phreatic features of limestone caverns, J. Geol. 50 675-811 Brucker, R.W. and Bums, D.P., 1964, The Flint Ridge Cave System, Mammoth Cave National Park, Kentucky, Cave Research Foundation, Washington, DC, Folio, 3pp text plus 31 maps. 

Limestone caverns, shown in Figure 6, form over millions of years. They are made as rainwater, slightly acidified by HsO", gradually dissolves rocks made of calcium carbonate. This reaction is still going today and is slowly enlarging caverns. The reverse reaction also takes place, and solid calcium carbonate is deposited as beautiful stalactites and stalagmites. 

Stalactites and stalagmites in limestone caverns form because of a slight displacement of a reaction from equilibrium. 

When ants sense danger to the ant colony, they emit a substance called formic acid that alerts the entire colony. Acids in rainwater hollow out enormous limestone caverns and destroy valuable buildings and statues. Acids flavor many of the beverages and foods you like, and it s an acid in your stomach that helps digest what you eat. Bases also play a role in your life. The soap you use and the antacid tablet you may take for an upset stomach are bases. Perhaps you have already concluded that the household products you used in the DISCOVERY LAB are acids and bases. 

Davis, W.M. (1930) Origin of limestone caverns. Geological Society of America Bulletin 41, 475-628. 

The walls of limestone caverns are composed of solid calcium carbonate. The ground water that makes its way down from the surface into these caverns is often acidic. The calcium carbonate and the H+ ions ftom the acidic water react to dissolve the limestone. If this happens to the ceiling of the cavern, the ceiling can collapse, leading to what is called a sinkhole. Write the net ionic equation for the reaction between the solid calcium carbonate and the aqueous H ions. 

Following the severe bomb damage sustained by BSA at its Small Heath works in November, 1940, ui ent discussions were held with the MAP at which a plan was formulated for the removal of 20% of the company s Browning gun barrel capacity into the long-disused limestone caverns at Dudley. 400 men would work underground, and the company required... 

Stalactites are icicle-like deposits of calcium carbonate, which hang from the roofs of limestone caverns. They are formed by crystallisation from ground-water. 

Stalagmites are conical deposits of calcium carbonate on the floors of limestone caverns, formed by crystallisation from groundwater. 

For example, consider the following heterogeneous equilibrium that is important in the formation of limestone caverns ... 

It is only under water that tarras mortar acquires its proper hardness for if suffered to dry by exposure to the air, it never sets into a substance so firm as if the same liitie had been mixed with good clean common sand, but is very friable and crumbly. Ash mortar is reckoned to be superior for works that are sometimes wet and sometimes dry, but tarras has the advantage when constantly under water Tarras mortar when kept always wet, and consequently in a state most favorable to its cementing principle throws out a substance something like the concretions in limestone caverns called stalactites, which substance acquires a considerable hardness, and in time becomes so exuberant fts to deform the face of the wails. 

The dissolution and precipitation of limestone (CaCOs) underlie a variety of natural phenomena, such as the formation of limestone caverns. Whether a solution containing Ca and COs ions undergoes precipitation depends on the concentrations of these ions. In turn, the CC>3 ion concentration depends on the pH of the solution. To develop a better understanding of the conditions imder which CaCOs dissolves or precipitates, we must consider equilibrium relationships between Ca and COs , and between COs , HaO, and HCOs . This requirement suggests a need to combine ideas about acid-base equilibria from Chapters 16 and 17 with ideas about the new types of equilibria to be introduced in this chapter. 

---