Limestone cave formation

Acids and bases are found throughout nature. In fact, acids and bases are used as a defense mechanism by insects and can produce beautiful limestone cave formations. If not monitored carefully, however, acids and bases in the environment can cause a lot of harm. 

The story does not end with cavern formation. Many limestone caves, such as Carlsbad Caverns, contain spectacuiar formations that inciude staiagmites, stalactites, and limestone columns. We need to examine the equiiibria more cioseiy to understand how these structures form. 

Figure 7.1 Cave formations are caused when rainwater and carbon dioxide mix and form a weak carbonic acid, which then dissolves the calcium carbonate of limestone beneath the earth, allowing for cave formation. The photo shows stalactites and stalagmites and other formations at Luray Caverns in Virginia.

The formations inside limestone caves are made up of limestone, too. Water that drains through the cracks in rock and into a limestone cave often contains dissolved limestone. As the drop of water hangs from the ceiling, some of the carbon dioxide trapped... 

Stalagmites are cave formations that start on the floor and grow upward. These formations are caused when water that contains dissolved limestone drips from the ceiling of the cave and lands on the same spot. Sometimes, water dripping off of a stalactite will actually form a stalagmite directly below it. Eventually, a column forms when the stalactite and stalagmite meet. This feature can take thousands or even millions of years to form. 

The effect of pH on the solubility of CaC03 has important environmental consequences. For instance, the formation of limestone caves, such as Mammoth Cave in Kentucky, is due to the slow dissolution of limestone (CaC03) in the slightly acidic natural water of underground streams. Marble, another form of CaC03, also dissolves in acid, which accounts for the deterioration of marble monuments on exposure to acid rain (Interlude, pages 650-651). 

This process and its reverse account for the formation of limestone caves and the stalactites and stalagmites found there. The acidic water (containing carbon dioxide) dissolves the underground limestone deposits, thereby forming a cavern. As the water drips from the ceiling of the cave, the carbon dioxide if ->st... 

Predict the effect of increasing acidity of rain on the rate of formation of limestone caves. 

Consider just a few cases of aqueous equilibria. The magnificent formations i n limestone caves and the vast expanses of oceanic coral reefs result from subtle shifts in carbonate solubility equilibria. Carbonates also influence soil pH and prevent acidification of lakes by acid rain. Equilibria involving carbon dioxide and phosphates help organisms maintain cellular pH within narrow limits. Equilibria involving clays in soils control the availability of ionic nutrients for plants. The principles of ionic equilibrium also govern how water is softened, how substances are purified by precipitation of unwanted ions, and even how the weak acids in wine and vinegar influence the delicate taste of a fine French sauce. In this chapter, we explore three aqueous ionic equilibrium systems acid-base buffers, slightly soluble salts, and complex ions. 

Calcium carbonate, which contains the Ca and ions, is very common in nature, occurring in eggshells, limestone, marble, seashells, and coral. The spectacular formations seen in limestone caves are also composed of calcium carbonate. 

Dissolution and precipitation occur both within us and around us. Tooth enamel dissolves in acidic solutions, for example, causing tooth decay, and the precipitation of certain salts in our kidneys produces kidney stones. The waters of Earth contain salts dissolved as water passes over and through the ground. Precipitation of CaC03 from groundwater is responsible for the formation of stalactites and stalagmites within limestone caves. 

The calcium ion and the bicarbonate ion, HC03, remain dissolved in water the solution of CO2 in water dissolves the limestone, leaving a cave or hole in the limestone formation. In some regions, such as parts of southern Missouri, this has occurred to such an extent that the whole area is underlain by limestone caves and potholes these are called karst regions. 

What indirect role is played by bacteria in the formation of limestone caves ... 

Limestone (CaCOs) is also responsible for the beautiful natural formations found in limestone caves. Natural groundwater is slightly acidic because of dissolved C02(g) and is essentially a solution of carbonic acid, H2CO3. 

Stalactites and stalagmites are limestone (CaC03) formations in limestone caves produced by the slow decomposition of Ca(HC03)2(aq). 

A karst structure . e. a limestone formation which has been eroded resulting in a large scale, open system comparable to a cave... 

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. 

Carbonic acid (HjCO ) is produced by dissolving carbon dioxide in water. When formed under pressure, it is the gas used in carbonated drinks. In nature, it dissolves the limestone in caves, resulting in the formation of stalactites and stalagmites. It is corrosive as are other acids, although it is considered a rather weak acid. 

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