The calcium hydroxide is an inorganic compound whose chemical formula is Ca (OH)two. It is a white powder that has been in use for thousands of years, during which time it has earned several traditional names or nicknames; among them, we can mention slaked, dead, chemical, hydrated or fine lime.
In nature it is available in a rare mineral called portlandite, of the same color. Because of this Ca (OH)two It is not obtained directly from this mineral, but from a heat treatment, followed by hydration, of the limestone. From this, lime, CaO, is obtained, which is later quenched or hydrated to produce Ca (OH)two.
Ca (OH)two it is a relatively weak base in water, as it can hardly dissolve in hot water; but its solubility increases in cold water, because its hydration is exothermic. However, its basicity continues to be a reason to be careful with it during handling, as it can cause burns to any part of the body..
It has been used as a pH regulator for different materials or foods, as well as being a good source of calcium in regards to its mass. It has applications in the paper industry, in the disinfection of sewage, in depilatory products, in foodstuffs made from corn flour..
However, its most important use has been as a construction material, since lime hydrates when mixed with the other ingredients in plaster or mortar. In these hardened mixtures, Ca (OH)two absorbs carbon dioxide from the air to consolidate sand crystals together with those formed from calcium carbonate.
Currently, research is still being carried out with the aim of developing better construction materials that have Ca (OH)two directly in its composition as nanoparticles.
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In the upper image we have the ions that make up calcium hydroxide. Its same formula Ca (OH)two points out that for each cation Catwo+ there are two OH anions- that interact with it through electrostatic attraction. The result is that both ions end up establishing a crystal with a hexagonal structure.
In such hexagonal crystals of Ca (OH)two the ions are very close to each other, which gives the appearance of being a polymeric structure; Although there is no formal Ca-O covalent bond, still given the notable difference in electronegativity between the two elements.
The structure generates octahedra CaO6, that is, the Catwo+ interacts with six OH- (ACtwo+-Oh-).
A series of these octahedra make up a layer of the crystal, which can interact with another by means of hydrogen bonds that keep them intermolecularly cohesive; however, this interaction vanishes at a temperature of 580 ºC, when Ca (OH) is dehydratedtwo to CaO.
On the high pressure side, there is not much information in this regard, although studies have shown that at a pressure of 6 GPa the hexagonal crystal undergoes a transition from the hexagonal to the monoclinic phase; and with it, the deformation of the octahedra CaO6 and its layers.
The crystals of Ca (OH)two They are hexagonal, but that is not an impediment for them to adopt any morphology. Some of these structures (such as strands, flakes or rocks) are more porous than others, robust or flat, which directly influences their final applications..
Thus, it is not the same to use crystals from the mineral portlandite, than to synthesize them so that they consist of nanoparticles where a few rigorous parameters are followed; such as the degree of hydration, the concentration of CaO used, and the time the crystal is allowed to grow.
White, odorless, powdery solid with a bitter taste.
74.093 g / mol
580 ° C. At this temperature, it decomposes, releasing water, so it never vaporizes:
Ca (OH)two => CaO + HtwoOR
2,211 g / cm3
A saturated aqueous solution of the same has a pH of 12.4 at 25 ºC.
The solubility of Ca (OH)two in water it decreases with an increase in temperature. For example, at 0 ºC its solubility is 1.89 g / L; while at 20 ºC and 100 ºC, these are 1.73 g / L and 0.66 g / L, respectively.
This indicates a thermodynamic fact: the hydration of Ca (OH)two is exothermic, so obeying Le Chatelier's principle the equation would be:
Ca (OH)two <=> ACtwo+ + 2OH- + Q
Where Q is the heat released. The hotter the water, the more equilibrium will tend to the left; that is, less the Ca (OH) will dissolvetwo. It is for this reason that in cold water it dissolves much more than in boiling water..
On the other hand, said solubility increases if the pH becomes acidic, due to the neutralization of the OH ions.- and the displacement of the anterior equilibrium to the right. Even more heat is released during this process than in neutral water. In addition to acidic aqueous solutions, Ca (OH)two it is also soluble in glycerol.
5.5 10-6. This value is considered small and is consistent with the low solubility of Ca (OH)two in the water (same balance as above).
1,574
Ca (OH)two remains stable as long as it is not exposed to COtwo from the air, as it absorbs it and forms calcium carbonate, CaCO3. Therefore, it begins to become contaminated into a solid mixture of Ca (OH) crystals.two-Thief3, where there are CO anions3two- competing with the OH- to interact with Catwo+:
Ca (OH)two + COtwo => CaCO3 + HtwoOR
In fact, this is the reason why concentrated solutions of Ca (OH)two they become milky, as a suspension of CaCO particles appears3.
Ca (OH)two It is obtained commercially by reacting lime, CaO, with a two- to three-fold excess of water:
CaO + HtwoO => Ca (OH)two
However, in the process carbonization of Ca (OH) can occurtwo, just as explained above.
Other methods to obtain it is to use soluble calcium salts, such as CaCltwo or Ca (NO3)two, and basify them with NaOH, so that Ca (OH) precipitatestwo. By controlling parameters such as water volumes, temperature, pH, solvent, degree of carbonization, maturation time, etc., nanoparticles with different morphologies can be synthesized.
It can also be prepared by selecting natural and renewable raw materials, or waste from an industry, that are rich in calcium, which when heated and processed, their ashes will consist of lime; and from here, again, the Ca (OH) can be preparedtwo by hydrating these ashes without the need to waste limestone, CaCO3.
For example, agave bagasse has been used for this purpose, assigning added value to waste from tequila industries..
Calcium hydroxide is present in many foods in some of its preparation stages. For example, pickles, such as gherkins, are dipped in an aqueous solution of it to make them more crisp when packed in vinegar. This is because the proteins on its surface absorb calcium from the medium..
The same occurs with corn grains before transforming them into flour, as it helps to release vitamin B3 (niacin) and makes it easy to grind. The calcium it provides is also used to add nutritional value to certain juices.
Ca (OH)two It can also substitute for baking powder in some bread recipes, and clarify the sugary solutions obtained from sugar cane and beets..
The clarifying action of Ca (OH)two It is because it acts as a flocculating agent; that is, it increases the size of the suspended particles until they form flocs, which later settle or can be filtered.
This property has been used to disinfect sewage, destabilizing its colloids that are unpleasant to the sight (and smell) of spectators..
Ca (OH)two it is used in the Kraft process to regenerate the NaOH used to treat wood.
Ca (OH)two used to remove COtwo of closed spaces or in environments where their presence is counterproductive.
In formulations for depilatory creams Ca (OH)two It is tacitly found, since its basicity helps in the weakening of the keratin of the hairs, and thus, it is easier to remove them.
Ca (OH)two It has been present since time immemorial, integrating the masses of plaster and mortar used in the construction of Egyptian architectural works such as the pyramids; also buildings, mausoleums, walls, stairs, floors, supports, and even to rebuild dental cement.
Its fortifying action is due to the fact that when "breathing" the COtwo, the resulting crystals of CaCO3 finish integrating the sands and the other components of such mixtures to a better degree.
Ca (OH)two It is not a strongly basic solid compared to other hydroxides, although it is more so than Mg (OH)two. Even so, despite not being reactive or flammable, its basicity is still aggressive enough to cause minor burns..
Therefore, it must be handled with respect, as it is capable of irritating the eyes, tongue and lungs, in addition to triggering other illnesses such as: loss of vision, severe alkalization of the blood, skin rashes, vomiting and sore throat.
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