Hard water: what is it, what causes it, and how can it be removed?
Hard water is water rich in dissolved calcium (Ca), with some iron (Fe), and magnesium (mg) ions as well. Bare calcium ions cannot exist alone in water. As a result, water molecules or other ions that can donate electrons to it surround it so that the calcium ion can be stable. Calcium ions are generated when carbonic acid (H2CO3) reacts with calcium carbonate(CaCO3). Calcium carbonate is found in nature in the form of chalk, marble, and limestone. And is used to build bones and shells of eggs and many sea creatures. Calcium carbonate dissolves little in water, because of this, when calcium and carbonate ions meet in solution, they easily form a precipitate (solid) that settles to the bottom of their container.
What are types of water hardness?
Water hardness can exist in two forms. These are
- temporary hardness, and
- permanent hardness
What is temporary hardness?
Water is temporary hard when calcium (Ca2+) and bicarbonate (HCO3–) ions are present in it. These ions are produced when calcium bicarbonate (Ca(HCO3)2) dissolves in water. Calcium bicarbonate is formed when carbonic acid (H2CO3) reacts with calcium carbonate. We can write the equation for this reaction as:
- H2CO3 (aq)+ CaCO3s) ↔ Ca(HCO3)2(aq) (1)
The Ca(HCO3)2 dissociates in water as:
Ca(HCO3)2(aq) ↔ Ca2+(aq)+ 2HCO3–(aq)
As you can see from the double arrow, equation one is an equilibrium reaction, and depending on reaction conditions, it can go either left or right. For instance, when carbon dioxide concentration is high, it reacts with water to form carbonic acid. This acid then eats away calcium carbonate to generate calcium bicarbonate. In other words, an increase in carbon dioxide concentration shifts the reaction (1) to the right. The calcium bicarbonate then dissolves in water to form calcium (Ca2+) and bicarbonate(HCO32-) ions: the source of the ions that causes temporary hardness.
And when temperature of water is increased, the carbonic acid breaks down to release carbon dioxide gas and solid deposits of calcium carbonate. In other words, as temperature of water increase, carbon dioxide gas is released, and solid calcium carbonate forms, shifting the reaction to the left.
Where from carbonic acid?
As rain (water) drops from the sky, it reacts with carbon dioxide (CO2) in the air to form carbonic acid. As this water seeps through pores and roots of plants underground, it further reacts with carbon dioxide from respiring microorganisms and decaying organic matter to form more carbonic acid. We can write the equation for this reaction as:
- CO2(g)+ H2O(l) ↔ H2CO3 (aq) (2)
What does this reaction tell us? This reaction tells us that ground water consists of dilute carbonic acid. This acid converts calcium carbonate(CaCO3), which is insoluble in water to calcium bicarbonate (CaHCO3)2, which is soluble in water as shown in equation 1.
How does hard water affect washing?
When we use hard water to wash clothes, soap dissolves in it to form scum, a slimy film or solid. While water deficient in calcium bicarbonate (soft water) dissolves soap to form suds (foam). In other words, soap does not foam with hard water, but it does foam with soft water.
How is temporary hardness removed?
As you may have noticed, calcium carbonate is insoluble in water. As a result, an easy way to remove temporary hardness is to boil hard water. When you boil hard water, the calcium bicarbonate breaks down to form carbon dioxide gas and solid calcium carbonate. The solid calcium carbonate settles to the bottom of the container and can be filtered off, while the water on top is soft. And this soft water will foam easily when soap dissolves in it.
What is permanent hardness?
Water is permanently hard when calcium (Ca2+), magnesium (Mg2+), and sulfate (SO42-) ions are present in it. These ions are generated when calcium sulfate (CaSO4) and magnesium sulfate (MgSO4) dissolve in water.
How is permanent hardness removed?
In temporary hardness, we saw that calcium and carbonate ions easily react to form solid that can be filtered out of water. However, in permanent hardness, the calcium and sulfate ions react to form soluble calcium sulfate. Since the calcium sulfate is soluble in water, it cannot be removed simply by boiling. It can only be removed chemically. To do this, sufficient sodium bicarbonate (NaHCO3) is added to water so that the carbonate will kick out the sulfate and bond with calcium or magnesium. When this happens, the soluble CaSO4 and MgSO4 are replaced with insoluble CaCO3 and MgCO3, which then can easily be filtered out of water.