Natural-occurring Gypsum or Calcium Sulfate
Some reactions involve two starting chemicals which swap parts (called ions) to produce two different products. It is not rare that one of those products is insoluble, driving the potentially reversible equation forward, eliminating its reversibility. The reaction between solutions of aluminum sulfate and calcium hydroxide, however, produces---not one but two---insoluble products, aluminum hydroxide and calcium sulfate.
The Reaction
The chemical reaction between these two solutions may be written
Al2(SO4)3 + 3 Ca(OH)2 ---> 2 Al(OH)3 + 3 Ca(SO4)
This is because aluminum has a valence of plus three, but calcium has a valence of plus two. For every two aluminum ions there must be three calcium ions to balance out the reaction.
Product One: Calcium Sulfate
Calcium sulfate is sometimes called plaster or plaster of Paris and sometimes it is called gypsum. It is used in the manufacture of wallboard and other building materials. Under ordinary conditions, calcium sulfate precipitates from water as finely-divided particulate. One interesting use for calcium sulfate is in bone grafts and for bone graft substitutes. It is also useful in soil amendment.
Product Two: Aluminum Hydroxide
Aluminum hydroxide that results from the reaction between aluminum sulfate and calcium hydroxide is a gelatinous mess that takes a long time to separate. It is hard to filter out and will prevent the calcium sulfate from settling to the bottom. Filtration efforts will merely clog the filter. Aluminum hydroxide is sometimes referred to as hydrated alumina. Alumina, prepared according to specifications, is a useful tool in separation, in ceramics, and in furnace applications.
Water Purification
Both aluminum sulfate and calcium hydroxide are used in the purification of water at treatment plants. The interaction caused by simultaneous use of these two chemicals, however, can cause havoc, since the reaction would produce a lot of unnecessary precipitate, which could clog filtration devices and cost a lot of money in both time and materials.
Advisories
Although both products from this reaction are relatively harmless, the same cannot be said for the starting materials. Be sure to read the material safety data sheets for both aluminum sulfate and calcium hydroxide before attempting the reaction described in this article.
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