EK 2F practice test - Physical Sciences #21

[Gauss44]

Question: Limestone has low solubility in neutral water. However, water containing carbon dioxide is slightly acidic and dissolves limestone as per the following reaction:

CaCO3(s) + H2O(l) + CO2(g) ⇌ Ca2+(aq) + 2HCO3-(aq)

Based on this reaction, which of the following most likely occurs during the formation of stalactites and stalagmites?

A. Carbon dioxide gas is released by water as it drips from the cave ceiling.
B. Carbon dioxide gas is absorbed by water as it drips from the cave ceiling.
C. Calcium bicarbonate precipitates as water drips from the cave ceiling.
D. Solid calcium precipitates as water drips from the cave ceiling.

Highlight for correct answer: A is correct.

Now can anyone explain why any of the the other answers are NOT the correct answer?

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[Gauss44]

Anyone?

(I figure I'd give this question one more chance on SDN then turn it over to a tutor.)

 

[Balantidiumcoli]

Le Chatelier's principle.

You'll note that the the beginning equilibrium stipulates that the reagents are in equilibrium with each other but as water leaves (dripping) CO2 gas is expelled due to the conversion of Ca2+(aq) + 2HCO3-(aq) into CaCO3(s) + H2O(l) + CO2(g) to maintain the equilibrium. The formation of CaCO3 is what makes the stalactites and stalagmites.