MadSci Network: Chemistry

Re: Is sack lime that you buy at the store the same thing as limestone?

Date: Mon Apr 24 19:17:10 2006
Posted By: Alex Barron, Graduate Student, Ecology(Biogeochemistry)
Area of science: Chemistry
ID: 1139933581.Ch

Ashley - The two are related but not quite the same thing. Lime is calcium oxide (CaO) while limestone is calcium carbonate (CaCO3) - produced by marine organisms for their shells. Lime is manufactured by heating CaCO3 (coral, chalk or limestone) at very high temp:

CaCO3(solid) --> CaO(solid) + CO2(gas)

The resulting oxide is much more reactive than the carbonate. People have been performing this reaction for thousands of years. The resulting lime is handy for making mortar, which basically runs the previous reaction in reverse. First, you mix lime with water:

CaO + H2O --> Ca(OH)2 (slaked lime)

if you mix in some sand then the slaked lime will react with CO2 in the air:

Ca(OH)2 + CO2 --> CaCO3 + H20

When the water evaporates, you have solid calcium carbonate holding your sand in place in between your bricks.

You can also use lime to make glass, welding gas, and clean SO2 from air. Handy stuff.

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