Hard Water and Soft Water

Hard water is defined as water having a high concentration of dissolved minerals—mostly calcium and magnesium. Such minerals are picked up as water percolates through deposits of limestone, chalk, or gypsum. Contrarily, soft water is water having low concentrations of these minerals. These minerals can, therefore, trigger the formation of scale and soap scum in hard water, which reduces the efficiency of cleaning agents and lowers the life expectancy level for plumbing systems and appliances.

This Story also Contains

1. Type of Hardness and Their Removal
2. Temporary hardnessTemporary hardness is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed by:
3. Permanent Hardness Removal
4. Importance and Applications
5. Some Solved Examples
6. Summary

Water is said to be soft if it produces sufficient lather with soap and water is described as being hard if it forms an insoluble scum before it forms lather with soap. The hardness of natural water is generally caused by the presence of the bicarbonates and sulfates of calcium and magnesium but all soluble salts that form a scum with soap cause hardness.

$\begin{array}{r}{\mathrm{Ca}}^{2+}(\mathrm{aq})+2{\mathrm{C}}_{17}{\mathrm{H}}_{35}{\mathrm{COO}}^{-}(\mathrm{aq})\to {\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}\right)}_2\mathrm{Ca}\\ {\mathrm{Mg}}^{2+}(\mathrm{aq})+2{\mathrm{C}}_{17}{\mathrm{H}}_{35}{\mathrm{COO}}^{-}(\mathrm{aq})\to {\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}\right)}_2\mathrm{Mg}\end{array}$

Soap will not produce lather with water until all the calcium and magnesium ions have been precipitated. Hard water thus wastes soap. The hardness of water is of two types:

• Temporary hardness

Temporary water hardness is due to dissolved bicarbonate minerals that get precipitated out by boiling.

• Permanent hardness

Permanent water- hardening, on the other hand, is caused by sulfate or chloride compounds of calcium and magnesium, that cannot be removed through boiling.

Type of Hardness and Their Removal

Softening of Water - Removing Temporary Hardness

There is removable temporary hardness, which is removed by boiling the water. Upon boiling, the bicarbonates break down into carbonate, water, and carbon dioxide. The carbonates drop out of the solution as scale. One other method used to remove temporary hardness is to add lime — calcium hydroxide — that reacts with the bicarbonate to form insoluble carbonates, which may be filtered out. These methods are effective and relatively simple; thus they are suitable for household purposes.

Temporary hardness
Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed by:

• Boiling: During boiling, the soluble Mg(HCO₃)₂ is converted into insoluble Mg(OH)_2, and Ca(HCO₃)₂ is changed to insoluble CaCO₃.It is because of the high solubility product of Mg(OH)₂ as compared to that of MgCO₃, that Mg(OH)₂ is precipitated. These precipitates can be removed by filtration. The filtrate thus obtained will be soft water.$\mathrm{Mg}{\left({\mathrm{HCO}}_3\right)}_2\stackrel{\text{Heating}}{\to }\mathrm{Mg}(\mathrm{OH}{)}_2\downarrow +2{\mathrm{CO}}_2\uparrow$
• Clark’s method: In this method calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.$\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2+\mathrm{Ca}(\mathrm{OH}{)}_2\to 2{\mathrm{CaCO}}_3\downarrow +2{\mathrm{H}}_2\mathrm{O}$

Permanent Hardness Removal

Permanent hardness calls for a stronger means of removing the dissolved calcium and magnesium compounds. Such tested areas include one based on ion exchange, whereby the water passes through a resin, which ultimately substitutes calcium and magnesium ions with sodium or potassium ions. Another way could be that of chemical precipitation, whereby certain chemicals such as washing soda react to form insoluble precipitates with calcium and magnesium ions. These techniques thus become immensely important in the industrial and municipal water treatment processes.

Permanent hardness
It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness is not removed by boiling. It can be removed by the following methods:

• Washing soda: It removes both the temporary and permanent hardness by converting soluble calcium and magnesium compounds into insoluble carbonates.${\mathrm{CaCl}}_2+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{CaCO}}_3+2\mathrm{NaCl}$
• Permutit: Permutit is the technical name given to certain hydrated silicates of aluminum and sodium. It is obtained by fusing sodium carbonate, china clay, silica, or quartz. The crystalline mass obtained is sodium alumino-silicate or sodium zeolite (Na₂Al₂Si₂O₈.xH₂O) and is technically called as permutit. The sodium ions of permuit are exchanged with calcium and magnesium ions when hard water is passed through it thus removing the hardness of the water
  ${\mathrm{Na}}_2{\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\cdot {\mathrm{xH}}_2\mathrm{O}+{\mathrm{Ca}}^{2+}\to {\mathrm{CaAl}}_2{\mathrm{Si}}_2{\mathrm{O}}_8\cdot {\mathrm{xH}}_2\mathrm{O}+2{\mathrm{Na}}^{+}$
  These ions can be re-exchanged by treating them with brine(NaCl) solution.
  This method is useful for the removal of both temporary and permanent hardness of water.
• Calgon: The complex salt of metaphosphoric acid, sodium hexametaphosphate(NaPO₃)₆, is known as Calgon. It is represented as Na₂[Na₄(PO₃)₆]. Calcium and magnesium salts present in hard water react with Calgon to give complex salts.$2{\mathrm{CaSO}}_4+{\mathrm{Na}}_2\left[{\mathrm{Na}}_4{\left({\mathrm{PO}}_3\right)}_6\right]\to {\mathrm{Na}}_2\left[{\mathrm{Ca}}_2{\left({\mathrm{PO}}_3\right)}_6\right]+2{\mathrm{Na}}_2{\mathrm{SO}}_4$
• By ion exchange resins: Ion exchange resins are the most popular water softeners these days. These resins are synthetic substances. The cation exchanger consists of granular insoluble organic acid resin having giant molecules with -SO₃H or -COOH groups while the anion exchanger contains giant organic molecules with basic groups derived from amines.

Importance and Applications

Knowing how to distinguish between hard and soft water has several important practical applications. In houses, a water softener can browse off-scale buildup in pipes and appliances, extending their lifetime by operating more efficiently. Industry must similarly treat water to remove its hardness to avoid the scale that forms in boilers and cooling towers associated with energy inefficiency and equipment damage.

The academic study of hard and soft water forms the basis for both chemistry and environmental studies. This would give learners an understanding of hard and soft water, thus enabling them to understand the solubility of minerals and the chemical reactions of species in aqueous solutions. It also explains how the hardness of water affects the environment—for instance, areas, where hard water occurs naturally, need to deal with scale in plumbing and heating system equipment, while regions with soft water may add mineral content to the water before distribution to prevent the corrosion of pipes.

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Some Solved Examples

Example 1

Question: The concentration of fluoride, lead, nitrate, and iron in a water sample from an underground lake was found to be 1 ppm, 40 ppm, 100 ppm, and 0.2 ppm, respectively. This water is unsuitable for drinking due to a high concentration of:

1) Fluoride

2) Lead

3) Nitrate

4) Iron

Solution:

To determine which constituent makes the water unsuitable for drinking, we compare the given concentrations with the maximum permissible concentrations:

- Fluoride: 1 ppm (permissible: 1 ppm)

- Lead: 40 ppm (permissible: 50 ppm)

- Nitrate: 100 ppm (permissible: 50 ppm)

- Iron: 0.2 ppm (permissible: 0.2 ppm)

The nitrate concentration exceeds the permissible limit of 50 ppm. Therefore, the water is unsuitable for drinking due to a high concentration of nitrate. Hence, the answer is option (3).

Example 2

Question: Calgon is used for water treatment. Which of the following statements is NOT true about Calgon?

1) It does not remove \( Ca^{2+} \) ion by precipitation.

2) Calgon contains the 2nd most abundant element by weight in the earth's crust.

3) It is also known as Graham's salt.

4) It is a polymeric compound and is water soluble.

Solution:

First, we identify the correct statements about Calgon:

1. Calgon removes \( Ca^{2+} \) ions by complexation, not precipitation. (True)

2. The second most abundant element in the earth's crust is silicon (Si), which is not present in Calgon. (False)

3. Calgon is also known as Graham's salt. (True)

4. Calgon (sodium hexametaphosphate) is a polymeric and water-soluble compound. (True)

Hence, the incorrect statement is option (2).

Example 3

Question: Which of the following methods is NOT suitable for the removal of the permanent hardness of water?

1) Clark’s method

2) Ion-exchange method

3) Calgon's method

4) Treatment with sodium carbonate

Solution:

The permanent hardness of water is caused by dissolved calcium and magnesium salts (sulfates and chlorides) and cannot be removed by simple boiling. The methods suitable for removing permanent hardness include:

- Ion-exchange method

- Calgon's method

- Treatment with sodium carbonate

Clark's method is used to remove temporary hardness, which is caused by bicarbonates of calcium and magnesium. Therefore, it is not suitable for the removal of permanent hardness.

Hence, the answer is option (1).

Summary

Hard water is, therefore, defined as water having a high content of calcium and magnesium ions that form scale and soap scum. Otherwise, soft water does not contain such substances. Temporary hardness can be removed either by boiling or by the addition of slaked lime, and permanent hardness requires methods involving ion exchange or chemical precipitation. In this respect, the creation of awareness on the understanding and management of such parameters in domestic, industrial, and academic applications is very paramount for efficiency, cost-effectiveness, and sustainability.