Limewater is the common name for saturated calcium hydroxide solution. It is sparsely soluble. Its chemical formula is Ca(OH)2. Since calcium hydroxide is only sparsely soluble, i.e. ca. 1.5 g per liter at 25 °C, there is no visible distinction to clear water. Attentive observers will notice a slightly earthy smell. It is clearly distinguishable by the alkaline taste of the calcium hydroxide. The term lime refers to the mineral, rather than the fruit. When exposed with carbon dioxide, lime water turns into a milky solution.
While limewater is a clear solution, milk of lime on the other hand is a suspension of calcium hydroxide particles in water. These particles give it the milky aspect. It is commonly produced by reacting quicklime (calcium oxide) with an excess of water - usually 4 to 8 times the amount of water to the amount of quicklime. Reacting water with quicklime is sometimes referred to as "slaking" the lime. The calcium oxide will convert to the hydroxide according to the following reaction scheme:
- CaO + H2O → Ca(OH)2
This reaction is strongly exothermic and will generate enough heat to bring the suspension to a scalding temperature. At a ratio of 2 parts water to 1 part lime (by weight), the generated heat is sufficient to bring the suspension, i.e. the water in it, to boil.
Milk of lime is an alkaline with a pH of 12.3. It is commonly used in the chemical industry and as a neutralizing agent in municipal waste water treatment. While it has a multitude of other uses, it is best known in its (historical) use as a paint: lime wash or whitewash.
Limewater can be made by mixing excess calcium hydroxide with distilled water, or deionized water. The mixture needs to be shaken to ensure the solution is saturated with calcium hydroxide. It is then left to settle and the clear "saturated" solution is siphoned off the sediment.
Uses of limewater
- [[Ca(OH)2 (aq)]] + [[CO2 (g)]] → [[CaCO3]] (s) + [[H2O (l)]]
If excess CO2 is added, the following reaction takes place: CaCO3 + H2O + CO2 --> Ca(HCO3)2 (colourless)
Limewater is also used in experiments involving aerobic or anaerobic respiration, to determine whether carbon dioxide was produced, by first boiling the lime water and then pouring the solute into the boiling lime water. When limewater reacts with CO2 it becomes milky, because of the calcium carbonate, or chalk, produced.
When the old vaudeville-style medicine shows promoting various patent medicines were in full swing, lime water was often used as a part of the act. The salesperson would have an audience member blow through a straw into a glass of lime water. Since the exhaled gas is carbon dioxide, the water would turn cloudy; the huckster then announced that this reaction proved that the audience member suffered from some ailment. If too much carbon dioxide comes into contact with the cloudy lime water, it will cause the calcium carbonate precipitate to redissolve to form soluble calcium bicarbonate.
CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2(aq)
The huckster had a patent medicine bottle filled with vinegar or some similar acid. He then would pour some of the acid into the glass of cloudy lime water. The acid reacted with the calcium carbonate, and the water would instantly clear. This demonstrated the potent effect of the nostrum he was selling to eliminate the "disease" demonstrated by the audience member.
Limewater is used to make sugar from sugar beets.
In buon fresco painting, limewater is used as the colour solvent to apply on fresh plaster.
Limewater is widely used by marine aquarists and is a primary supplement of calcium and alkalinity for reef aquariums. Corals of order Scleractinia build their endoskeletons from aragonite (a polymorph of calcium carbonate) and by doing so, usually form a reef. Calcium concentration in natural seawater is about 420 ppm. When used for this purpose, lime water is also referred to as Kalkwasser. Lime water, or some variation of it, is also used in tanning and making parchment. The lime is used as a dehairing agent based on its alkaline solution.
- ^ ´Solubility of Inorganic and Metalorganic Compounds - A Compilation of Solubility Data from the Periodical Literature´, A. Seidell, W. F. Linke, Van Nostrand (Publisher), 1953
- ^ Hambidge KM; Krebs NF, Westcott JL, Sian L, Miller LV, Peterson KL, Raboy V (1 July 2005). "Absorption of calcium from tortilla meals prepared from low-phytate maize.". The American Journal of Clinical Nutrition 82 (1): 84–7. PMC 1592687. PMID 16002804. http://www.ajcn.org/cgi/content/full/82/1/84. Retrieved 2006-10-02.
- ^ "How to make konjac foods (shirataki noodles or konnyaku) myself". http://www.konjacfoods.com/konjac.htm.
- ^ "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH". http://reefkeeping.com/issues/2006-06/rhf/index.php.
- ^ "The Nature and Making of Parchment" by Ronald Reed pub. 1975
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