Metal ion solutions, image 1 of 2. From left to right are solutions of calcium (Ca), magnesium (Mg), copper (Cu), iron (II) (Fe2+), iron (III) (Fe3+), cobalt (Co), nickel (Ni), manganese (Mn), chromium (Cr), silver (Ag), Zinc (Zn), lead (Pb) and aluminium (Al) ions. To discover the identity of metal ions in solution, sodium hydroxide is added. The resulting metal hydroxides have unique characteristics, for example colour and degree of solubility. See A500/419 for completed reactions.