The chemistry of limestone
Limestone is a common rock. You may have seen it in
spectacular cliffs such as Malham Cove or High Tor near Matlock.
Limestone cliffs, Verdon Gorge, France.
It is also a very useful material. It is used for
building and road making, for example, and also as a starting material for
making many other products. This activity illustrates some of the
limestone (calcium carbonate) and other materials made from it.
What you will need
- eye protection
- Bunsen burner, tripod and gauze
- heatproof mat
- 3 test-tubes
- test-tube rack
- dropping pipette
- drinking straw
- a few small lumps of limestone (each about 1cm3)
- deionised / distilled water
- Universal Indicator solution and colour chart
Do not touch the lime that is formed from heating
limestone. It will be hot after heating and lime is an irritant even when
it is cool.
What to do
- Take about half a dozen lumps of limestone.
Examine the stone and describe its colour, texture and any other notable
features briefly. Things to look for include any evidence of fossils.
- Place a couple of lumps on a tripod and gauze and
heat with a roaring Bunsen flame for 15 minutes. Take care; the lumps
will become extremely hot.
- While the lumps are being heated, take two more
lumps of unheated limestone. Add a few drops of water and note any
- Returning to the lumps being heated, note any
changes - particularly in colour. Take care; the lumps will be extremely
- If possible, darken the room and note what happens
when the flame is trained directly on the lumps. It may be possible to
see the lumps glowing – this is the origin of the term ‘limelight’.
Limelight was once used to illuminate theatre performances.
- Remove the lumps from the heat and allow them to
cool to room temperature on the heatproof mat. Then take one of the
heated pieces with your tongs. Gently try to crush it on the heatproof
mat with the tongs. Try the same with a lump that has not been heated.
Record what you find.
- Use tongs to place one of the lumps that has been
heated in a test tube and then gently add a few drops of water with the
dropping pipette. Make sure you are wearing eye protection. Note any
evidence of reaction.
- Now add more water to the test tube until it is
about half full. Shake the test tube and pour off the clear liquid, half
into one test tube and half into another. Add a few drops of Universal
Indicator to one tube and record the pH using a colour chart.
- Place a straw into the clear liquid in the second
tube and blow gently into the liquid through the straw. What do you see?