How can calcium hydroxide be used to test for carbon dioxide?

Answer 1

Calcium hydroxide can be dissolved to create a saturated aqueous solution, which is sometimes referred to as limewater. When carbon dioxide is bubbled through the limewater, the carbon dioxide reacts with water to form carbonic acid. The carbonic acid is first deprotonated to form a bicarbonate anion, which is then deprotonated once more to form the carbonate anion. The carbonate anion then reacts with a calcium cation to form calcium carbonate (the compound in limestone). Calcium carbonate is insoluble in water, and thus precipitates out. The formerly clear limewater solution has thus become cloudy, giving an indication that carbon dioxide is likely present. Reaction scheme is given below:

CO2(g) + H2O --> H2CO3

H2CO3 + OH- --> HCO3-+ H2O

HCO3- + OH- --> CO32- H2O

CO32- + Ca2+ --> CaCO3(S)

Note that if too much carbon dioxide is added or too little calcium hydroxide is used, then calcium bicarbonate will form instead of calcium carbonate. Calcium bicarbonate is water soluble, and thus will appear to produce a negative test.