Question

Why does the sedimentary rock limestone react with `HCl` acid?

Answer 1

Limestone is basically calcium carbonate, and the carbonate ion within it is a base.

Explanation:

Here is the reaction:

`color(blue)("Carbonate ion from limestone") + color(red)("hydrogen ion from acid") \rarr color(magenta)("carbon dioxide + water")`

`"Ca"color(blue)("CO"_3)+2 color(red)("H")"Cl" \rarr color(magenta)("CO"_2+"H"_2"O")+"CaCl"_2`

The calcium and chloride ions that do not directly react form calcium chloride which is soluble in water. So the acid makes the limestone dissolve away.

There is actually more to this story. The combination of carbon dioxide and water, either formed above or already existing from air and natural waters, can act as a weak acid and form water-soluble bicarbonate ion:

`color(blue)("Carbonate ion")+color(magenta)("carbon dioxide + water")\rarr color(purple)("easily soluble bicarbonate ion")`

`"Ca"color(blue)("CO"_3)+color(magenta)("CO"_2+"H"_2"O")\rarr "Ca" color(purple)("(HCO"_3")"_2)`

With this second reaction making the soluble bicarbonate, a strong acid is not really needed to corrode limestone. Over time, just water that has dissolved carbon dioxide from the air, or from respiration of aquatic organisms, can do it.