Chemguide: Core Chemistry 14 - 16

Limestone, quicklime and slaked lime

This page looks at the origin and uses of limestone, and its conversion into quicklime, CaO, and slaked lime, Ca(OH)2.

Limestone and marble

Chemically, limestone is calcium carbonate. It is a sedimentary rock formed from the shells and skeletons of marine creatures which fell to the bottom of ancient seas. These were turned into rock by the pressure of more sediments forming on top of them.

Geologically there are a wide range of limestones - from soft, crumbly white rocks like chalk to some very hard dark grey ones.

Marble is a metamorphic rock formed when limestone is subjected to heat and pressure under the ground. It is still calcium carbonate, but has been rearranged into a more crystalline form.

Note:  Sedimentary rocks are exactly what they say. They are formed by material which forms sediments on the sea bed and is then compressed into rock by further sediments on top of it. The sediments might be mud or sand or the remains of sea creatures.

A metamorphic rock is one that has been changed by heat and pressure deep under the ground.

Just for the record, an igneous rock is one formed by the welling up of molten material from very deep in the earth. Granite is a common example.

Uses of limestone

Limestone is mined in vast amounts. An estimate in The Essential Chemical Industry suggests it could be 15 billion tonnes a year worldwide.

It is used in the construction industry:

  • as a building material;

  • to make cement, which in turn is used to make mortar and (in huge quantities) concrete;

  • in road building.

It is used in the iron industry:

  • as one of the raw materials which go into the blast furnace in the extraction of iron from its ore.

In neutralising acids:

  • Limestone has been used to treat lakes acidified because of acid rain;

  • Ground limestone is commonly used to raise the pH of acidic soils. Where a farmer talks about "liming" a field, they are most likely to be using calcium carbonate.

Quicklime, CaO, and slaked lime, Ca(OH)2

When calcium carbonate is heated strongly, it decomposes to give calcium oxide and carbon dioxide

CaCO3(s)    CaO(s) + CO2(g)

Calcium oxide is traditionally known as quicklime.

If you add water to calcium oxide, you get calcium hydroxide (slaked lime).

CaO(s) + H2O(l)    Ca(OH)2(s)

There is a useful bit of video which shows the conversion of calcium carbonate into calcium oxide and then calcium hydroxide.

It is actually quite long (about 8 minutes) and frankly could do with editing down to half that, but it is all good chemistry.

You might have noticed the bright white glow of the limestone while it was being heated to make quicklime. This is the origin of the term "limelight".

Turning limestone into quicklime industrially

The next video shows a modern industrial plant for producing quicklime, calcium oxide.

It carries far more information than you need, and you really only need to concentrate on what happens in the kiln itself (starting just after 2 minutes). This is where the limestone is heated and quicklime is formed.

It isn't made clear in the video (it often isn't!) that you need a flow of air up through the kiln to remove the carbon dioxide produced.

Although we almost always write the equation for this reaction as one-way, it is in fact reversible.

CaCO3(s)    CaO(s) + CO2(g)

When you heat limestone in the open air, of course, the carbon dioxide just gets lost and the back reaction can't happen. In an enclosed kiln, though, it can. The net effect of this is to stop the limestone decomposing.

You have to sweep the carbon dioxide away while the limestone is being heated.

Uses of quicklime and slaked lime

There are a large number of uses for these compounds, but trying to find specific ones is a real problem because the word "lime" is used to apply to limestone, quicklime and slaked lime.

If this is on your syllabus, the safest thing to do would be to search past papers and mark schemes to find out exactly what your examiners will allow, and then learn that.

There are, however, two uses that I am confident about.

  • Quicklime (calcium oxide) is used in the steel making process. You will find out a bit more about this in a later page in this section.

  • Slaked lime (calcium hydroxide) is used to neutralise acid soils. It is more expensive than using limestone, but is faster acting.

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