Hydroxide
The solubility in water of the other hydroxides in this group increases with increasing atomic number. Magnesium hydroxide Mg(OH) is a strong base (up to the limit of its solubility, which is very low in pure water), as are the hydroxides of the heavier alkaline earths: calcium hydroxide, strontium hydroxide, and barium hydroxide. A solution or suspension of calcium hydroxide is known as limewater and can be used to test for the weak acid carbon dioxide. The reaction Ca(OH) + CO Ca + + OH illustrates the basicity of calcium hydroxide. Soda lime, which is a mixture of the strong bases NaOH and KOH with Ca(OH), is used as a CO absorbent.
Aluminium hydroxide
Gibbsite is also known as hydrargillite, named after the Greek words for water (') and clay ('). The first compound named "hydrargillite" was thought to be aluminium hydroxide, but was later found to be aluminium phosphate; despite this, both "gibbsite" and "hydrargillite" are used to refer to the same polymorphism of aluminium hydroxide, with "gibbsite" used most commonly in the United States and "hydrargillite" used more often in Europe. In 1930, it was referred to as "α-alumina trihydrate" to contrast it with bayerite, which was called "β-alumina trihydrate" (the alpha and beta designations were used to differentiate the more- and less-common forms respectively). In 1957, a symposium on alumina nomenclature attempted to develop a universal standard, resulting in gibbsite being designated γ-Al(OH), bayerite becoming α-Al(OH), and nordstrandite being designated Al(OH). Based on their crystallographic properties, a suggested nomenclature and designation is for gibbsite to be α-Al(OH), bayerite to be designated β-Al(OH), and both nordstrandite and doyleite are designated Al(OH). Under this designation, the α and β prefixes refer to hexagonal, close-packed structures and altered or dehydrated polymorphisms respectively, with no differentiation between nordstrandite and doyleite.