C1aL5 Thermal Decomposition of Limestone

Key Words

Calcium carbonate - the main mineral in limestone;

Formula - the chemical make up of a compound expressed in symbols.

Limewater - a solution of calcium hydroxide.

Quicklime - calcium oxide;

Suspension - very tiny insoluble particles are carried in water, but not dissolved.

Thermal Decomposition - compound breaks down to simpler compounds due to heat.

Test Yourself

Homework
Chemistry GCSE
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Grade E

Limestone is a rock whose main mineral is calcium carbonate, CaCO3.  This formula tells us that there are:

  • 1 calcium atom;

  • 1 carbon atom;

  • 3 oxygen atoms.

Notice that the 3 is subscript, meaning it is written below the line.  It also comes after the symbol for the element.

Please, be a good chap and don't write CO3.  The superscript 3 means it's cubed.  You cannot have "oxygen cubed".

If we heat calcium carbonate carbonate, it breaks down by thermal decomposition to calcium oxide and carbon dioxide.  Calcium oxide is often called quicklime.  The word equation is:

calcium carbonate ® calcium oxide + carbon dioxide

Calcium oxide is nasty stuff.  It readily reacts with water to form slaked lime, calcium hydroxide.  This reaction is strongly exothermic, meaning it gives out lots of heat.

calcium oxide + water ® calcium hydroxide

Slaked lime is used on acid soils to increase the pH.

Grade C

When a chemical reaction occurs, atoms are never destroyed, nor are they made.  New substances are formed.  We can show this by using symbols (formulae) in the equations:

calcium carbonate ® calcium oxide + carbon dioxide

CaCO3 ® CaO + CO2

This is a balanced equation; the number of calcium atoms on the left hand side balances the number on the right hand side.  And the same is true for oxygen and carbon.

calcium oxide + water ® calcium hydroxide

CaO + H2O ® Ca(OH)2

Again the number of atoms of each element is the same on the left hand side as the right hand side.  The calcium forms Ca2+ ions.  Hydroxide is OH-. So we need two OH- ions for each calcium ion.  We put the OH in brackets, followed by a subscript 2.  (Spoken - see ay o aitch twice).

Do not write CaO2H2 or Ca2(OH) or CaOH2 or Ca(OH)2.  Please.  These kinds of drivel litter many exam papers.  They will cost you marks.

 

Grade A

If lots of water is added to calcium oxide, the calcium hydroxide dissolves to make a solution called limewater, the classical test for carbon dioxide.  As the gas is bubbled through, it reacts with the calcium hydroxide to form calcium carbonate.  This is insoluble and makes a milky suspension.  The equation is:

CO2 + Ca(OH)2 ® CaCO3 + H2O