How Sinkholes Develop?... With Chemical Equation
•Naturally occurring sinkholes are most commonly found in a type of terrain known as karst topography, which consists of bedrock (rock beneath the soil) filled with nooks and crannies. The underlying bedrock in karst landscapes is usually made of limestone. A great portion of the state of Florida is, in essence, sitting atop one continuous slab of limestone, making it vulnerable to sinkholes. Limestone is composed of calcium carbonate (CaCO3), which primarily comes from the remnants of corals and other types of marine organisms, whose shells are made of calcium carbonate.
•Sinkholes often form when acidic groundwater or acid rain dissolves limestone, a porous 9-rock present in the soil, creating voids and cavities. The soil resting on top of the limestone then sinks or collapses, causing a sinkhole.
•Limestone builds up slowly after these animals die and their shells are deposited and accumulate over time. Other substances composed of calcium carbonate include marble, chalk, Tums antacid tablets, and eggshells. To understand how limestone bedrock contributes to sinkholes, consider what happens when you place an egg in a glass of vinegar, which contains 5% acetic acid (CH3COOH). You will notice that little bubbles of carbon dioxide gas form almost immediately and, within a day or two, the eggshell will have completely disappeared, leaving you with the egg’s translucent membrane to protect the egg. The eggshell, which is composed of calcium carbonate, does not normally dissolve in water, but in the presence of acetic acid, calcium carbonate and acetic acid react with each other, causing the eggshell to dissolve according to the following chemical reaction:
•➞ [Ca2+ (aq) + 2CH3COO– (aq)] + H2O (l)
•➞ Calcium acetate + Water + CO2 (g) + Carbon dioxide
•Any substance made of calcium carbonate will react with an acid. Limestone, being made of calcium carbonate, will react with an acid and will be slowly worn away. But are there acids underground?
•To answer this question, consider what happens to rainfall (which eventually 9-becomes groundwater) as it passes through the atmosphere. While falling through the air, the rain comes into contact with carbon dioxide. Although carbon dioxide comprises only about 0.04% of the atmosphere, that is enough to make rainfall acidic, lowering its Ph to about 5.6. So, by the time rainfall reaches the ground, it has turned into acid. The reaction is as follows:
•H2O (l) + CO2 (g) ➞ H2CO3 (aq)
•Water + Carbon dioxide ➞ Carbonic acid
•Carbonic acid then dissociates to give a hydrogen ion (H+) and a bicarbonate ion (HCO3 –):
•H2CO3 (aq) ➞ H+ (aq) + HCO3 – (aq)
•The ability of carbonic acid to dissociate by producing hydrogen ions is what makes this molecule an acid. Over time, acidic rainwater seeps into the ground and comes into contact with limestone bedrock. Water makes its way into cracks or pockets in the rock, reacting with the limestone and eventually making holes and fissures in the rock. Sinkholes occur when acidic rainwater has eaten away so much of the underlying limestone bedrock beneath the soil that the ground collapses.
•The more it rains, the greater the amount of carbonic acid leaching into the soil below.
•The acidity of rainwater is not the only reason water in the ground is acidic. Decaying organic materials and root respiration also produce carbon dioxide, which dissolves in soil water to form carbonic acid.
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