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.
Nickel compounds
Nickel hydroxides are used in nickel–cadmium and Nickel–metal hydride batteries.
Nickel(II) hydroxide Ni(OH), the main hydroxide of nickel is coloured apple green. It is known as the mineral theophrastite.
β-NiO(OH) is a black powder with nickel in the +3 oxidation state. It can be made by oxidising nickel nitrate in a cold alkaline solution with bromine. A mixed oxidation state hydroxide NiO(OH) is made if oxidation happens in a hot alkaline solution. A Ni hydroxide: nickel peroxide hydrate NiO, can be made by oxidising with alkaline peroxide. It is black, and unstable and oxidises water.