Since they are the two most widespread and troublesome ions in hard water, it is often said that hardness is
caused by calcium (Ca2+) and magnesium (Mg2+) ions dissolved in water. However, hardness can be caused
by several other dissolved metals as well, including strontium (Sr2+), iron (Fe2+), and manganese (Mn2+).
You will notice that all of the hardness-causing ions are divalent cations, meaning that they have a charge of
positive two. Metals such as sodium (Na+) and potassium (K+) with a charge of positive one do not cause
hardness