2(aq),
first results in the precipitation of CaCO3.
Additional CO2, however, redissolves
the precipitate. The reaction can be reversed to give the CaCO3
precipitate once again either by boiling the solution or bubbling air through
it.

Add equal parts of lime water and distilled water to the tall container. Add
water and about an ounce of dry ice to the gas generator. Bubble CO

2
through the lime water solution until the precipitate is well established (20-30
seconds), and then remove the tube and discuss the precipitate. Continue to
bubble CO2 through the solution
until the precipitate just disappears. Fill the test tube about 1/5 of the way
full with the solution (or if a hot plate is being used, fill the beaker about
1/3 full and start it stirring on the hot plate). Begin bubbling air through the
remainder of the solution, using the hose and glass tube from the generator.
Discuss the significance of the re-dissolution and how the reaction can be
reversed. Boil the solution for a couple of minutes with the Bunsen burner. The
precipitate with reappear almost at once, but it takes about 10-20 minutes for
the solution through which air is being bubbled to turn cloudy.

DISCUSSION

When carbon dioxide dissolves in water, it forms carbonic acid. Lime water
neutralizes the carbonic acid and carbonate ion is formed. Calcium carbonate is
insoluble

CO

2(g) + H2O(l)
H2CO3(aq)

H

2CO3(aq)
+ 2OH-(aq) CO32-(aq)
+ 4H2O(l)

Ca

2+(aq) +
CO32-
CaCO3(s)

and precipitates. As still more CO

2
is bubbled into solution, all the OH- is used up, and the solution becomes
acidic. Carbonate ion is converted into bicarbonate, which has a very

CaCO

3(s) +
H2CO3(aq)
Ca2+(aq)
+ 2HCO3-(aq)

soluble calcium salt. In Nature, calcium is a very common ion in igneous
rocks. As these rocks weather and the calcium goes into solution, CO

2
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.

HAZARDS

None. Although Ca(OH)

2 is a
strong base, it is so insoluble that a saturated solution of it does not even
give the bitter taste typical of basic substances.

REFERENCES

B. Z. Shakhashiri, "Chemical Demonstrations", Volume 1, p. 329.

NOTES

Directions for this demonstration normally call for undiluted lime water.
However, the use of this solution causes the re-dissolution of the calcium
carbonate to be inconveniently delayed.