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.
Hydroxide
The mineral malachite is a typical example of a basic carbonate. The formula, CuCO(OH) shows that it is halfway between copper carbonate and copper hydroxide. Indeed, in the past the formula was written as CuCO·Cu(OH). The crystal structure is made up of copper, carbonate and hydroxide ions. The mineral atacamite is an example of a basic chloride. It has the formula, CuCl(OH). In this case the composition is nearer to that of the hydroxide than that of the chloride CuCl·3Cu(OH). Copper forms hydroxyphosphate (libethenite), arsenate (olivenite), sulfate (brochantite), and nitrate compounds. White lead is a basic lead carbonate, (PbCO)·Pb(OH), which has been used as a white pigment because of its opaque quality, though its use is now restricted because it can be a source for lead poisoning.