Unit 8: Hard and Soft Water

Hard water is water that has a high concentration of dissolved mineral ions. Specifically, it is defined by the presence of dissolved calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$) ions. Iron ions ($Fe^{2+}$) can also contribute to hardness. Soft water, by contrast, has a very low concentration of these ions.

Water becomes hard as it flows over or through rocks and soil containing calcium and magnesium compounds, such as limestone (calcium carbonate, $CaCO_3$) and chalk. These minerals dissolve into the water, increasing its ion concentration.

Hard water causes two main problems in domestic and industrial settings:

1. Formation of Limescale

When hard water is heated, dissolved calcium hydrogencarbonate decomposes to form calcium carbonate ($CaCO_3$), which is insoluble. This solid deposit is known as limescale.
$Ca(HCO_3)_2(aq) \xrightarrow{\Delta} CaCO_3(s) + H_2O(l) + CO_2(g)$

Limescale builds up inside pipes, kettles, boilers, and washing machines. It is a poor conductor of heat, reducing the efficiency of heating elements and potentially causing them to overheat and break. It can also block pipes.

2. Formation of Soap Scum

Soap works because its molecules have a soluble "head" and an insoluble "tail". In hard water, the dissolved $Ca^{2+}$ and $Mg^{2+}$ ions react with the soap molecules to form an insoluble grey precipitate called scum.
$Ca^{2+}(aq) + 2C_{17}H_{35}COO^-(aq) \rightarrow Ca(C_{17}H_{35}COO)_2(s)$ (Scum)

This reaction removes the soap from the water, preventing it from forming a lather and cleaning effectively. It wastes soap and leaves a residue on clothes and skin. Note that modern detergents are formulated to work effectively in hard water without forming scum.

Despite the disadvantages, hard water also has some benefits:

• Source of Essential Minerals: Calcium is vital for the development of strong bones and teeth. Drinking hard water can be a useful dietary source of calcium and magnesium.
• Taste: Many people prefer the taste of hard water over soft water, which can sometimes taste "flat".
• Reduced Risk of Heart Disease: Some studies have suggested a correlation between drinking hard water and a lower risk of heart disease.

Water hardness can be classified into two types:

• Temporary Hardness: This is caused by the presence of dissolved calcium hydrogencarbonate ($Ca(HCO_3)_2$) and magnesium hydrogencarbonate. It is called "temporary" because it can be removed simply by boiling the water. Boiling decomposes the hydrogencarbonate to form insoluble carbonate precipitates (limescale), which can be removed.
• Permanent Hardness: This is caused by dissolved calcium sulfate ($CaSO_4$) and magnesium sulfate ($MgSO_4$). These salts are not decomposed by boiling, so this type of hardness cannot be removed by this method. It requires other methods of water softening.

Softening water involves removing the dissolved $Ca^{2+}$ and $Mg^{2+}$ ions. Several methods can be used:

1. Boiling: Only removes temporary hardness by precipitating calcium carbonate.
2. Precipitation (Adding Washing Soda): Adding sodium carbonate ($Na_2CO_3$), known as washing soda, removes both temporary and permanent hardness. The carbonate ions react with the calcium and magnesium ions to form insoluble precipitates.
   $Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)$
3. Distillation: This is the most effective method for producing pure water. Water is boiled to produce steam, leaving all dissolved ions behind. The steam is then condensed back into pure, soft liquid water. It is very effective but uses a lot of energy.
4. Ion Exchange: This is a common method used in domestic water softeners. Hard water is passed through a column containing a resin. The resin is packed with sodium ions ($Na^+$). As the water flows through, the $Ca^{2+}$ and $Mg^{2+}$ ions, which are more strongly attracted to the resin, displace the $Na^+$ ions. The hard water ions are trapped on the resin, and sodium ions are released into the water, making it soft.

A simple experiment can be done to compare the hardness of different water samples. This involves adding soap solution drop by drop to a known volume of each water sample and shaking after each addition.

The harder the water, the more soap solution will be required to produce a permanent lather (one that lasts for at least 30 seconds). Soft water will produce a lather with just a few drops of soap solution.