Question

select all that apply
which substances exist in high concentrations in hard water?
magnesium
calcium
sulfur
iron
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Explanation:

Hard water is water that has high mineral content. Specifically, it contains high concentrations of divalent cations like calcium ($\text{Ca}^{2 + }$) and magnesium ($\text{Mg}^{2 + }$). Sulfur and iron are not the main substances that define hard - water in terms of its hardness - related mineral content.

Answer:

magnesium, calcium