the presence of metal ions in water. - divalent metal cations such as magnesium, iron and calcium Hard water leaves [[limescale]] residue ## Water hardness scale ![[hard-water-scale-chart.jpg]] ## Temporary hardness insoluble **carbonates** ![[Carbonate.png]] Carbonates can be removed by boiling, Ca2+ and Mg2+ ions are removed by filtration ## Permanent hardness insoluble **chlorides and sulfates** Cannot be removed by boiling - requires [[chemical methods of removing water hardness|chemical methods]]