A saturated solution of calcium hydroxide
(slaked lime, Ca(OH)2). It is used to detect carbon dioxide (CO2),
which forms a milky white precipitate of calcium carbonate (CaCO3)
when bubbled through lime water. Excess carbon dioxide changes the calcium
carbonate into calcium hydrogencarbonate (calcium carbonate, Ca(HCO3)2),
which is soluble and so the white precipitate disappears.