Experiment Based on Hard and Soft Water: As we know, a major part of living organisms is made up of water. With water being very crucial for the survival of life on Earth, it is extremely important to understand the essential components present in it. While water in its purest form contains only two hydrogen and one oxygen atom, sometimes there are other dissolved solids, gases and elements present in it due to various reasons. Rainwater, for example, is considered one of the purest forms of water and even though it might contain some dissolved gases from the atmosphere. However, when the rainwater flows along the surface of the Earth, it starts collecting salts of several kinds. Water, being the universal solvent, dissolves all these salts and takes them along.

When chloride, sulphate and hydrogen carbonate salts of magnesium and calcium are dissolved in water, the water becomes hard. Water-from these dissolved salts is called soft water. There are certain ways in which the hard and soft water can be differentiated, and the hard water can be softened. This article will look into the difference between hard water and soft water and different experiments by which one can soften the hard water.

Hard and Soft Water

Natural water, such as rainwater, contains dissolved salts. Water, therefore, can be classified as hard water and soft water, depending upon its behaviour towards soap solution, and is considered one of the most efficient ways of gauging whether water is soft or hard. 

Water can be classified as soft water and hard water, depending upon the way it forms lather with the salt. 

Soft Water:  Water that readily produces lather with soap is called soft water. Some of the best examples of soft water are rainwater, demineralised water and distilled water.
Hard Water: Water that does not produce lather readily with the soap is called hard water. Seawater, spring water, water from the rivers, lake water, etc., can be termed as hard water depending upon the salts dissolved in it. 

The hardness of water is due to the dissolved bicarbonate, sulphate and chloride salts of calcium and magnesium. The presence of these salts prevents the water from forming lather. 

Experiment to Check the Hardness in Water

Soap is the potassium or sodium salts of higher fatty acids such as palmitic acid, stearic acid or oleic acid or other such acids. When hard water is mixed with soap solution, the calcium and magnesium ions present in the water react with the anions of the fatty acid and form scum or a curdy-white precipitate. This scum prevents the lather from forming. That is why hard water cannot produce lather with soap.

The reaction is given by:

\(\mathop {2{{\rm{C}}_{17}}{{\rm{H}}_{35}}{\rm{CO}}{{\rm{O}}^ – }{\rm{N}}{{\rm{a}}^ + } + {\rm{CaC}}{{\rm{l}}_2}}\limits_{{\rm{Sodium Stearate (Soap)}}} \to \mathop {{{\left( {{{\rm{C}}_{17}}{{\rm{H}}_{35}}{\rm{COO}}} \right)}_2}{\rm{Ca}} \downarrow + 2{\rm{NaCl}}}\limits_{{\rm{Calcium Stearate (Curdy white ppt)}}} \)

\(2{{\rm{C}}_{17}}{{\rm{H}}_{35}}{\rm{CO}}{{\rm{O}}^ – }{\rm{N}}{{\rm{a}}^ + } + {\rm{MgS}}{{\rm{O}}_4} \to {\left( {{{\rm{C}}_{17}}{{\rm{H}}_{35}}{\rm{COO}}} \right)_2}{\rm{Mg}} \downarrow + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\)

When all the \({\rm{C}}{{\rm{a}}^{2 + }}\) and \({\rm{M}}{{\rm{g}}^{2 + }}\) ions precipitate out as stearate salts, the resulting water becomes soft and can form lather with soap. However, hard water is not suitable for washing since it will take up a lot of soap before the lather is formed.

Types of Hardness in Water

There are two types of hardness of water:

1. Temporary Hardness

Temporary hardness in water is a result of the presence of bicarbonate salts of calcium and magnesium in water. The presence of \({\rm{Ca}}{\left( {{\rm{HC}}{{\rm{O}}_3}} \right)_2}\) and \({\rm{Mg}}{\left( {{\rm{HC}}{{\rm{O}}_3}} \right)_2}\) results in temporary hardness, which can be easily removed by boiling and filtering the water. Due to the presence of bicarbonates. The residue formed is the insoluble carbonates of calcium and magnesium. The filtrate formed is the soft water.

\({\rm{Ca}}{\left( {{\rm{HC}}{{\rm{O}}_3}} \right)_2}\left( {{\rm{aq}}} \right){\rm{Boil}} \to {\rm{CaC}}{{\rm{O}}_3}\left( {\rm{s}} \right) + {\rm{C}}{{\rm{O}}_2}\left( {\rm{g}} \right) + {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right)\)

\({\rm{Mg}}{\left( {{\rm{HC}}{{\rm{O}}_3}} \right)_2}\left( {{\rm{aq}}} \right){\rm{Boil}} \to {\rm{MgC}}{{\rm{O}}_3}\left( {\rm{s}} \right) + {\rm{C}}{{\rm{O}}_2}\left( {\rm{g}} \right) + {{\rm{H}}_2}{\rm{O}}\left( {\rm{l}} \right)\)

2. Permanent Hardness

The dissolved chlorides and sulphates of calcium and magnesium create permanent hardness in water. These salts are soluble, are hard to remove by just boiling. The permanent hardness is caused by salts such as \({\rm{CaC}}{{\rm{l}}_2},{\rm{CaS}}{{\rm{O}}_4},{\rm{MgC}}{{\rm{l}}_2}\) and \({\rm{MgS}}{{\rm{O}}_4}.\) Since they are caused by chlorides and sulphates of calcium and magnesium, this type of hardness is called as non-carbonate hardness.

Removal of non-carbonate hardness is done by the following method:

Boiling with Washing soda: When the hard water containing salts of chloride and sulphates of magnesium and calcium is treated with a predetermined and calculated amount of washing soda \(\left( {{\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3}} \right),\) the calcium and magnesium sulphates and chlorides precipitate out as insoluble carbonates of magnesium and calcium. They are then filtered off to get soft water.

\({\rm{CaC}}{{\rm{l}}_2} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{CaC}}{{\rm{O}}_3} \downarrow + 2{\rm{NaCl}}\)

\({\rm{MgS}}{{\rm{O}}_4} + {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{MgC}}{{\rm{O}}_3} \downarrow + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\)

There are other processes such as the lime-soda process, the Calgon process and ion-exchange methods which are used to remove both permanent and temporary hardness in water.

Water, once softened by the above processes, can be used for household and laundry purposes. The permutit process, which employs inorganic resins, is an efficient and cheap process and can remove both permanent and temporary hardness in water.

Test for Degree of Hardness

The degree of hardness in water can be defined as:

“The number of parts of calcium carbonate or equivalent to the calcium and magnesium salts present in parts per million of water by mass”.

The degree of hardness is expressed as parts per million or ppm.

As an example, to calculate the total hardness in terms of parts of \({\rm{CaC}}{{\rm{O}}_3}\) per \(106\) parts of water by mass in one kg of a sample of hard water containing \(1\,{\rm{mg}}\) of \({\rm{CaC}}{{\rm{l}}_2}\) and \(1\,{\rm{mg}}\) of \({\rm{MgC}}{{\rm{l}}_2}:\)

a. Molecular mass of \({\rm{CaC}}{{\rm{l}}_2} = 111\,{\rm{grams}}/{\rm{mol}}\)
111 grams of \({\rm{CaC}}{{\rm{l}}_2}\) is approximately equal to \(100\,{\rm{grams}}\) of \({\rm{CaC}}{{\rm{O}}_3}.\)
Therefore, \(1\,{\rm{mg}}\) of \({\rm{CaC}}{{\rm{l}}_2} = 100/111 \times 1{\rm{mg}}\) of \({\rm{CaC}}{{\rm{O}}_3} = 0.9\,{\rm{mg}}\) of \({\rm{CaC}}{{\rm{O}}_3}.\)
b. Similarly: Molecular mass of \({\rm{MgC}}{{\rm{l}}_2} = 95\;{\rm{g}}/{\rm{mol}}.\)
\(95\) grams of \({\rm{MgC}}{{\rm{l}}_2}\) is approximately equal to \(100\) grams of \({\rm{CaC}}{{\rm{O}}_3}.\)
Therefore, \(1\;{\rm{mg}}\) of \({\rm{MgC}}{{\rm{l}}_2} = 100/95 \times 1{\rm{mg}}\) of \({\rm{CaC}}{{\rm{O}}_3} = 1.05\,{\rm{mg}}\) of \({\rm{CaC}}{{\rm{O}}_3}\)
Hence, \(1\,{\rm{kg}}\) of hardwater contains: \(0.90 + 1.05 = 1.95\,{\rm{mg}}\) of \({\rm{CaC}}{{\rm{O}}_3}\)
\(1\,{\rm{kg}}\) of water \( = {10^6}\,\,{\rm{mg}}\) of water.
\({10^6}\,\,{\rm{mg}}\) of hard water contain: \(1.95\,\,{\rm{mg}}\) or \(1.95\,\,{\rm{ppm}}\) of \({\rm{CaC}}{{\rm{O}}_3}\)
Therefore, Degree of Hardness \( = 1.95\,\,{\rm{ppm}}\)

To estimate hardness due to \({\rm{C}}{{\rm{a}}^{2 + }}\) and \({\rm{M}}{{\rm{g}}^{2 + }},\) volumetric analysis is usually used. A known buffer solution of \({\rm{pH}}\,{\rm{10}}\) is added to the hard water and is titrated against a standard EDTA (Ethylenediamine tetra acetic acid disodium salt) solution. Eriochrome black T is used as an indicator. Both the magnesium and calcium ions form a complex with EDTA and the colour changes from wine red to blue, when all the ions are consumed by the EDTA solution.

It is very important to determine the hardness of water content before using it for drinking, washing or for other usages, as hardness in water can result in:

• Clogged water pipes and sediments on vessels and other materials
• Increase in consumption of soap and improper washing
• Dry and itchy scalp and skin in some instances.

Summary

Water in its purified form as rain collects salts as dissolved salts in due course. These salts, in turn, convert soft rainwater into hard water.  Hard water does not form lather with soap, while soft water can easily lather up when treated with soap. There are two types of hardness, temporary hardness resulting from bicarbonate salts of calcium and magnesium and permanent hardness resulting from chloride and sulphate salts of Magnesium and calcium. While temporary hardness can be removed by boiling the water and filtering, permanent hardness requires different experimental processes to remove them. The amount of hardness in water is measured by an entity called the degree of hardness.

FAQs

Q1. What is the relationship between hard and soft water?
Ans: Hard water contains dissolved bicarbonate, chloride and sulphate salts of magnesium and calcium and does not lather up when treated with soap. Soft water, on the other hand, is the pure form of water without any dissolved salts and, therefore, can form lather easily with soap.

Q2. How do you classify soft and hard water-based on the soap test?
Ans: When treated with soap, hard water does not form lather. It forms a white precipitate called scum which is a stearate salt of calcium and magnesium. Soft water can react easily with soap to form lather.

Q3. What is the significance of the hardness of water experiment?
Ans: The hardness of water calculation can help in determining the dissolved salts present in water. It can help in treating the water before it is used for household or drinking purposes.

Q4. What is the formula of hard water and soft water?
Ans: The formula of water is \({{\rm{H}}_2}{\rm{O,}}\) and it is the same for both hard and soft water. The dissolved salts make soft or pure water hard. However, it does not change the formula or molecular composition of water.

Q5. How is hardness in water determined?
Ans: Hardness of water or the ‘degree of hardness’ in water can be determined by titrating a solution of hard water and a \({\rm{pH}}\,{\rm{10}}\) buffer with EDTA solution and using Eriochrome Black T as an indicator.

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