Home Home Astronomy Chemistry Electronics Mathematics Physics Field Trips Home  

Experiment 4: Lime Water Test For CO2

Date

10 November 2009 by Quest 2, to link in with Chapter 4 of our book.

Aim

To use lime water to detect CO2.

Method

Make lime water. Add a small amount of calcium hydroxide to a test tube and add water. Add bung and shake. Allow to settle. Decant the clear solution into a beaker: lime water.

Turn the lime water cloudy. Add about 2 cm lime water to a test tube. Blow through a drinking straw to bubble CO2 through the lime water solution until it goes cloudy (20-30 seconds).

Turn the lime water clear again. Continue to bubble CO2 through the solution until the precipitate just disappears.

Materials & Equipment

  • Calcium hydroxide, Ca(OH)2.
  • Water.
  • Test tubes, bung.
  • Pipette.
  • Beaker.
  • Drinking straws (not for drinking through).

Results/Observations

The lime water turned cloudy quite easily but didn't want to turn completely clear again, even after much blowing.

Conclusion

  • Our result was consistent with the claim that lime water can be used to detect CO2.
  • More time should have been taken to make the lime water.
  • It can be quite hard to blow just air, not saliva as well.

Discussion

Basically, the carbon dioxide in our breath reacts with the lime water to make a white solid, calcium carbonate (which is what limestone is made of). Blowing more carbon dioxide into the lime water turns the water acidic, which dissolves the calcium carbonate again.

Action/Explanation Formula
When carbon dioxide dissolves in water, it forms carbonic acid. CO2(g) + H2O(l) → H2CO3(aq)
Lime water neutralizes the carbonic acid and carbonate ion is formed. H2CO3(aq) + 2 OH-(aq) → CO32-(aq) + 4 H2O(l)
Calcium carbonate is insoluble and precipitates. Ca2+(aq) + CO32- → CaCO3(s)
As still more CO2 is bubbled into solution, all the OH- is used up, and the solution becomes acidic. CO2(g) + H2O(l) → H2CO3(aq)
Carbonate ion is converted into bicarbonate, which has a very soluble calcium salt. CaCO3(s) + H2CO3(aq) → Ca2+(aq) + 2 HCO3-(aq)

In nature, calcium is a very common ion in igneous rocks. As these rocks weather and the calcium goes into solution, CO2 in the atmosphere precipitates some of it as CaCO3. Found in massive sedimentary deposits, this material is called limestone. Cracks in limestone deposits allow water (containing more CO2) to percolate through, dissolving part of the material and forming limestone caves. Re-precipitation of the CaCO3 results in the formation of stalactites and stalagmites.

Tap water that is "hard" because it comes from wells and has been in contact with limestone for a long time contains relatively large amounts of calcium and bicarbonate ions. When this water is heated in coffee pots or water heaters and allowed to cool, limy deposits called "boiler scale" build up.

Experiment Refinements

If repeating the experiment, we would make the following refinements.

  1. Prepare a more carefully controlled sample of limewater that definitely didn't have any extra solid calcium hydroxide floating around in suspension.
    1. Leave the solution longer to settle.
    2. Use the pipette to remove the clear solution instead of pouring it out.
    3. After decanting, make a slightly less than saturated solution by adding a little fresh water to dissolve the last remnants of calcium hydroxide.
    This will mean all the cloudiness in the lime water is caused by CO2.
     
  2. Use a control (set aside a sample) at each stage that we can compare with so we can definitely see the change in cloudiness.
     
  3. If bubbling CO2 through the lime water doesn't completely turn it clear again, simply leave a sample exposed to the air for a few days. It may absorb enough CO2 from the air to dissolve the suspended limestone. Shake occasionally to ensure it's not just settling.