Calcium hydroxide

• 1. Introduction

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  Calcium hydroxide is made at a scale of roughly 125 million tons a year, yet few people could name it.

  The recipe behind it is one of the oldest chemical reactions known. Evidence of prehistoric production reaches back to at least 7000 BCE. All it takes is quicklime, which is calcium oxide, and water.

  The white powder that results goes by many names: slaked lime, hydrated lime, caustic lime, builders' lime, pickling lime, and simply cal. Food labels know it as E526.

  Why does a clear solution of it turn milky when carbon dioxide passes through? How does one compound end up in tortillas, century eggs, paper mills, and reef aquariums? And what did the writer Diodorus Siculus notice about the hair of the Celts?

  19822

• 2. Limewater and Its Moods

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  A litre of water at 25 °C can dissolve only about 1.5 grams of calcium hydroxide, and that thin solution is called limewater. Pure limewater is clear and colorless. It has a slight earthy smell and a bitter, astringent taste. Its pH is 12.4, which makes it firmly basic. Despite the name, it is named after limestone, not the lime fruit.

  Push past saturation by adding more calcium hydroxide than the water can hold, and the liquid clouds over. Raising the temperature enough does the same. Tradition calls this white suspension milk of lime.

  Temperature behaves backward here. At 0 °C the solubility is 1.89 g/L, and at 100 °C it drops to 0.66 g/L. Adding extra hydroxide or a source of calcium cuts solubility sharply, a pattern known as the common-ion effect.

  Carbon dioxide gives the classic demonstration. Passed through limewater, it makes the solution turn milky as insoluble calcium carbonate forms. Keep supplying the gas and the cloudiness clears again, because calcium bicarbonate dissolves in water. Strong heat, meanwhile, can run the slaking reaction in reverse.

  211422

• 3. Layers and Reactions

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  At 512 °C, the pressure of water vapor in equilibrium with calcium hydroxide reaches 101 kPa, which is normal atmospheric pressure. At that point the compound breaks down into calcium oxide and water.

  Inside the solid, calcium hydroxide forms a layered polymeric structure, as all metal hydroxides do. It matches the brucite structure of Mg(OH)2, which follows the cadmium iodide motif. Strong hydrogen bonds link the layers.

  Aluminium reacts with calcium hydroxide, and that reaction is the basis of aerated concrete. Iron and steel are a different case. They are not corroded, because their surfaces passivate.

  Hydrochloric acid turns the compound first into calcium hydroxychloride and then into calcium chloride. Sulphur dioxide reacts with limewater in a process called sulfation.

  Like any strong base, calcium hydroxide can burn skin, but it is not acutely toxic. That mild hazard profile helps explain where it turns up next.

  5973

• 4. Workhorse of Industry

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  Lime mortar is one of the most familiar jobs for calcium hydroxide. A larger job is in water and sewage treatment, where it acts as a flocculant. It forms a fluffy, charged solid that gathers fine particles and leaves clearer water. Low cost and low toxicity make this practical.

  In fresh-water treatment, the compound raises the pH of acidic water so that pipes will not corrode. It is self-regulating and does not push the pH too high. Limewater also drives lime softening, which reduces water hardness, and it neutralizes municipal waste water.

  Paper mills use it too. In the causticizing step of the Kraft process for making pulp, burned lime is added to green liquor. Green liquor is a solution mainly of sodium carbonate and sodium sulfate, made by dissolving smelt, the molten form of those chemicals from the recovery furnace. Calcium hydroxide is an intermediate in making sodium hydroxide there.

  Orchards use it against disease. Sprayed as lime water, it prevents cankers caused by the fungal pathogen Neonectria galligena. Growers spray in winter while the trees are dormant, to avoid toxic burns from the highly reactive compound. The European Union and the United Kingdom authorise this under Basic Substance regulations.

  Endodontics, a dental specialty, relies on calcium hydroxide for its antibacterial properties and because it induces hard-tissue deposition. Its mildness lets it into the kitchen as well.

  221210119

• 5. Kitchen Chemistry

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  Sugar refiners clarify raw juice from sugarcane and sugar beets with slaked lime, in a step called carbonatation. Salt makers use it to clear a brine of calcium and magnesium carbonates, for salt destined for food and pharmaceutical uses.

  Drink makers treat the process water for alcoholic beverages and soft drinks with it. It is also a calcium supplement in fruit drinks such as orange juice, and in infant formula.

  Bakers add it to speed up Maillard reactions, the browning chemistry behind pretzels. Cooks in South Asia use it in place of baking soda when making papadam. Pickle makers use it on cucumbers and other foods, and it is part of how Chinese century eggs are made.

  It appears in USDA-certified food production for plants and livestock. It removes carbon dioxide from controlled-atmosphere produce storage rooms. It even goes into the preparation of mushroom-growing substrates.

  One food use stands apart from this list, because it shaped a whole cuisine.

  2213121410

• 6. Nextli and the Maize

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  In Nahuatl, the language of the Aztecs, calcium hydroxide is called nextli. Maize cooked with nextli becomes nixtamal, also known as hominy. The process is called nixtamalization.

  The chemistry changes what the body can take from the grain. Nixtamalization significantly increases the bioavailability of niacin, or vitamin B3, which prevents pellagra. The treatment also removes the cellulose hulls from the kernels. Nixtamal is considered tastier and easier to digest.

  Ground into flour, nixtamal becomes masa. Masa is the dough for tortillas and tamales, which cannot be made from maize that has not been nixtamalized.

  The same compound plays a quieter role beside coca leaves.

  151322

• 7. Companion to Stimulants

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  Chewers of coca leaves usually take calcium hydroxide alongside. It keeps the alkaloid stimulants chemically available for absorption by the body.

  Native Americans traditionally chewed tobacco leaves with calcium hydroxide derived from burnt mollusc shells, to enhance the effects. Some indigenous South American tribes use it in yopo, a psychedelic snuff prepared from the beans of some Anadenanthera species.

  Across South and Southeast Asia, the compound goes by names including chuna, choona, apog, and soonis. It is added to paan, also called buyo, a bundle of areca nut and betel leaf. There it keeps the alkaloid stimulants available to enter the bloodstream through sublingual absorption.

  Naswar, also called nass or niswar, is a dipping tobacco made from fresh tobacco leaves, choona, and wood ash. It is consumed most among the Pathan diaspora in Afghanistan, Pakistan, India, and Bangladesh.

  Choona also gives petha its characteristic crunchy, firm texture. Villagers in Afghanistan, Pakistan, and India use it to paint their mud houses. Traditionally in Taiwan and China, lime water preserved persimmons and removed their astringency.

  161713

• 8. Hair, Hides, and Stars

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  Diodorus Siculus wrote that the Gauls were "always washing their hair in lime-water." He described them as tall, white of skin, and naturally blond, and said the practice deepened that color. They pulled the hair back from the forehead to the nape of the neck. He said the result was so heavy and coarse that it "differs in no respect from the mane of horses." Limewater does stiffen and bleach hair, and it kills lice or mites living there.

  Leather workers use lime for its alkalinity as a dehairing agent. The leather process called liming depends on it. The same property serves in tanning and in making parchment.

  In buon fresco painting, limewater is the colour solvent applied on fresh plaster. Historically the same material is known as the paint whitewash.

  Marine aquarists favor limewater as a primary supplement of calcium and alkalinity for reef aquariums. Corals of the order Scleractinia build their endoskeletons from aragonite, a polymorph of calcium carbonate. In this hobby, limewater is usually called Kalkwasser.

  Far from any kitchen or tank, the ion CaOH+ has been detected in the atmosphere of S-type stars.

  19182022