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Water — CHEMISTRY STD 9 — Question

ICSE BoardEnglish MediumSTD 9CHEMISTRYWater5 Marks
Question
What are stalgmites and stalactites? How are they formed?
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Answer

In some limestone caves, conical pillar-like objects hang from the roof and some rise from the floor. These are formed by water containing dissolved calcium hydrogen carbonate continuously dropping from the cracks in the rocks. Release of pressure results in the conversion of some hydrogen carbonate to calcium carbonate.
$Ca(HCO_3)_2 → CaCO_3 + CO_2 + H_2O$
This calcium carbonate little by little and slowly deposit on both roof and floor of the cave.
The conical pillar which grows downwards from the roof is called stalactite and the one which grows upward from the floor of the cave is called stalagmite.
These meet after a time. In a year, some grow less than even a centimetre, but some may be as tall as $100 cm.$
$CaCO_3 + CO_2 + H_2O → Ca(HCO_3)_2$
$MgCO_3 + CO_2 + H_2O → Mg(HCO_3)_2$
If the water flows over beds of gypsum ($CaSO_4.2H_2O$), a little bit of gypsum gets dissolved in water and makes it hard.

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