Question

What causes the temporary and permanent hardness of water?

Short answer

Temporary hardness is caused by bicarbonates of calcium and magnesium, removable by boiling. Permanent hardness is due to sulfate and chloride salts, requiring chemical treatment.

Step-by-step solution

Understanding Hardness of Water

Water hardness is determined by the presence of dissolved minerals, specifically calcium and magnesium ions. Hard water is characterized by these ions, which affect its behavior with soap and its ability to form scale.

Define Temporary Hardness

Temporary hardness in water is primarily due to the presence of dissolved bicarbonate minerals, such as calcium bicarbonate \( \text{Ca(HCO}_3)_2 \) and magnesium bicarbonate \( \text{Mg(HCO}_3)_2 \). It is called "temporary" because it can be removed by boiling the water, which decomposes the bicarbonates into carbonates, leading to the precipitation of calcium carbonate \( \text{CaCO}_3 \) and magnesium carbonate \( \text{MgCO}_3 \).

Define Permanent Hardness

Permanent hardness is caused by the presence of non-bicarbonate calcium and magnesium salts, such as sulfates \( \text{CaSO}_4, \text{MgSO}_4 \) and chlorides \( \text{CaCl}_2, \text{MgCl}_2 \). Unlike temporary hardness, permanent hardness cannot be removed by boiling, and requires chemical treatment such as ion exchange to remove these ions from water.

Key concepts

Temporary Hardness

Temporary hardness in water occurs when water contains dissolved bicarbonate minerals. These are primarily calcium bicarbonate \( \text{Ca(HCO}_3)_2 \) and magnesium bicarbonate \( \text{Mg(HCO}_3)_2 \). This type of hardness is referred to as "temporary" because it can be easily removed through boiling.

When water is boiled, the heat causes these bicarbonates to decompose into carbonates. As a result, compounds such as calcium carbonate \( \text{CaCO}_3 \) and magnesium carbonate \( \text{MgCO}_3 \) are formed. These carbonates are less soluble, meaning they will precipitate or fall out of solution.

This precipitation helps soften the water, as the problematic ions are removed. Temporary hardness is seen as less of a problem because of how easily it can be addressed by just heating the water. It is, however, a common cause of scale formation in kettles and boilers as the precipitates form solid deposits.

Permanent Hardness

Permanent hardness is different from temporary hardness in that it is not removed by boiling. This hardness is due to the presence of non-bicarbonate minerals, such as calcium sulfate \( \text{CaSO}_4 \), magnesium sulfate \( \text{MgSO}_4 \), calcium chloride \( \text{CaCl}_2 \), and magnesium chloride \( \text{MgCl}_2 \).

These compounds do not decompose into a less soluble form when heated; therefore, the ions remain in the water even after boiling.

Addressing permanent hardness requires different methods compared to temporary hardness. One common technique is using an ion exchange process, where calcium and magnesium ions are replaced with sodium or potassium ions, which do not contribute to hardness. Other methods might include using chemical softeners that alter the composition of the water.

Permanent hardness can be a more significant problem in domestic and industrial settings as it leads to longer-term scale buildup and can affect the efficiency of boilers, pipes, and even household appliances. Unlike temporary hardness, which you can treat with simple heat, permanent hardness needs more technical solutions.

Calcium and Magnesium Ions

Calcium and magnesium ions are the primary contributors to water hardness. Both of these ions are found naturally in the earth's crust and often enter water sources through the dissolution of minerals.

• Calcium ions ([Ca]^{2+}) typically come from limestone, marble, and other calcium-rich minerals.
• Magnesium ions ([Mg]^{2+}) often originate from minerals like dolomite and magnesite.

These ions don't generally pose health risks at typical concentrations but can affect water's interaction with soap and detergents.

That's because they form insoluble salts with fatty acids in soap, reducing cleaning effectiveness. Furthermore, they cause scale buildup, which can harm appliances by leading to inefficient operation and even damages.

To manage the impact of these ions, water treatment processes such as lime softening or reverse osmosis can be used to reduce their concentration. Whether addressing temporary or permanent hardness, understanding the roles of calcium and magnesium is crucial to managing and treating hard water effectively.