HARDNESS OF WATER

The Concept of Hardness of Water

As water flows over the land, it dissolves many mineral substances. The dissolved minerals are deposited together with water in rivers, lakes and oceans, Water is said to be hard if it contains some specific type of dissolved minerals, It is important to note that not all dissolved salts make water hard As you learned early, water is treated in water purification plants before being piped to your home. The treatment removes only the insoluble particles and kills bacteria. So the water still is

not pure. It contains natural compounds dissolved from rocks and soil. It may also contain traces of chemicals dumped from homes, farms and factories Water obiained from an area where the rocks contains chalk, limestone, dolomite or gypsum, contains dissolved calcium and magnesium sulphates and hydrogencarbonates, These salts make the water hard (One can distinguish between hard and soft water when washing with soap. Hard water does not

form lather easily. Instead, it forms a precipitate or scum It requires mach soap to react with all the dissolved minerals before enough lather is formed. Therefore, hard water wastes soap during washing When soap is used with hard water a “scum” forms on the surface. This is a result of a precipitation reaction between calcium and/or magnesium ions and soap. Soaps are the sodium or potassium salts of long-chain organic acids. Soaps are made from animal fats by treatment with

alkali (NaOH or KOH) Ordinary washing soap is a compound of stearic acid, Ci7H;sCOOH TThe nature of such soaps isthe salt, sodium stearate, Cj HssCOONa’. Sodium stearate ts soluble in water but calcium stearate is not When soap is mixed with hard water, the calcium or magnesium salts inthe hard water react with soap and precipitates as scum, The nature of scum is either caleium stearate or magnesium stearate Ca?jaq) + 2C17H3sCOONaiag) > Ca(C17H3sCOO)y@) + 2Na“faqy

scum Mg? aq) + 2C17HssSCOONAgaq) > Mg(C17HssCOO):@) + 2NA™ eq) seum Soap will not form any lather with water until all the caleium and magnesium ions have been precipitated. Hard water, therefore, wastes soap. This means that more soap may be used for an efficient washing. The amount of soap needed to just produce froth can be used to estimate the hardness of water.

The problem of scum formation only occurs with soaps. Soapless detergents donot produce scum. The trade names for some soapy detergents sold in Tanzania include Komoa, Kuku, Tatfa Mbuni, Mshindi, Changu, Jamaa and several other bar soaps. The trade names for some soapless detergents include Omo, Foma, Tesa, Toss, Dynamo, Swan, ete.

Causes of Permanent and Temporary Hardness in Water

Water is generally said to be hard if it contains soluble salts of calcium and magnesium. The salts are calcium and magnesium sulphates and hydrogencarbonates. Hardness of water is caused by higher than usual levels of caleium (Ca”) and magnesium (Mg”) ions in water.

AAs rain water passes through the atmosphere, it dissolves carbondioxide to form a weak earbonic acid. FO + COxg — H2CO3aq) carbonic acid As this solution passes over and through rocks containing chalk, limestone or dolomite, the rainwater very slowly dissolves them: HCO xu) CaCO.) > Cal HCO) 9) The calcium hydrogenearbonate formed is soluble in water and is responsible for the presence of calcium (Ca) ions in water Some of the rocks may contain gypsum (CaS0s2H20), anhytite (CaS0y), Kieserite

(MgS0,.H:0) or dolomite (CaCOs. MgCOs) which ean dissolve to a limited extent in water. The presence of these dissolved substances also causes the water to be hard. These substances dissoive spatingly in water to form Ca?* and Mg?” dons wilich are respondibWe or water hardness as stated early

Activity 1

Investigation of the causes of mater hardness Materials:

• Test tube rack-
• Five clean test hes
• Measuring eylinder (100em')
• Calcium sulphate solution (1-mol dm)
• Soap solution
• Magnesium sulphate solution (I mot dm?)
• Sodium sulphate solution (1 mol den”)
• Potassium sulphate solution (1 mol dim”)
• Distilled water

Procedure

• Label five clean and dry test tubes as A,B, C, D and E.2
• Add 10 em’ of 1.0M calcium, magnesium, sodium and potassium sulphate solutions and distilled

water in each of the test tubes respectively

• Add Sm ofsoap in each test tube
• Shake the test tubes well and place them in 2 test tube rack
• Observe the amount of father formed in each test tube, and if there is any precipitate (scum)

formed. Results: Results of experiment showing minerals which cause water hardness Ions

Lather or Water

Test present in Salt present scum hand or tube solution of formed? soft? salt caleium qe Sum is

A C?*, SOF Hard

sulphate formed magnesium = Mg" scum is, B ¥ a hard sulphate So¥ formed sodium lather is c Na’, SO.” soft sulphate formed otassium > lather is 5 © Kk’ so2 soft sulphate formed distilled lather is E no ions soft water formed Interpretation of the results From the result of experiment, we ean conclude that scum is produced when either calcium or magnesium salt is present in water. So, high levels of calcium or magnesium ions in water are

responsible for water hardness. When the concentration of either of these minerals is over 150 milligrams per cubic decimeter (150 mgidm’), water is considered to be hard. The upper limit allowed is 300 mg/dm*

‘TYPES OF HARDNESS OF WATER,

There are two types of hardness of water, namely Temporary and Permanent hardness of water Hardness caused by caleium hydrogen carbonate is called temporary hardness because it can be removed by boiling the water Ca (ACOs) jy + H20 «+ COs ) + CACO 1 Hardness caused by other calcium and magnesium compounds is called Permanent Hardness.

This is because boiling does not affect it.

‘TREATMENT AND PURIFICATION OF HARD WATER

Because of the problem it causes, hard water is often softened for use in fetoris, industries and homes. That means removing the dissolved caleium and magnesium ions. Described below are the methods of treating and purifying hard water Boiling Boiling removes temporary hardness in water, as you saw carly. Boiling causes calcium carbonate to precipitate, The hydrogencarbonate in water are decomposed to carbonates, which

are insoluble in water sJoreq) Heat Ca(HCO3)raq) —M > CaCOs) +H20q + COrg), In this way, the calcium is removed, since the calcium carbonates being insoluble, takes no further part in the reaction, An insoluble calcium salt cannot cause hardness. However, this method uses a lot of fuel, which makes it expensive to do om a large Seale Distillation Distillation removes all impurities from water. This gets rid of both temporary and permanent

hardness. In distillation, the water is boiled and the steam collected, cooled and condensed. Distilled water is pure and softest water. All the dissolved substances have been removed, Like boiling, itis an expensive option in terms of fuel used. But itis essential for some purposes, for

example for laboratory experiments and for making drags,

Addition of caleium hydroxide Addition of calculated amounts of calcium hydroxide can remove temporary hardness, The quantity to be added should be properly calculated because excess would cause hardness on its ‘own account. The amount of calcium hydroxide to be added is calculated based on knowledge of the hardiness of water and the capacity of the reservoir (Clark's method). The caleium hydroxide reacts with the hydrogencarbonates dissolved in water and precipitates as the insoluble calcium

carbonates, Ca{OH)ayy slightly soluble + Ca(HCOs)si4) “7 2CaCOs@ + 2H:O insoluble Addition of sodium earbonate (washing soda) Washing soda removes both temporary and permanent hardness by precipitating calcium carbonate. It reacts with calcium hydrogencarbonate (which eauses temporary hardness) to form sodium hydrogencarbonate like this: NaxCOsjgj# Ca(HCOs)a.97> 2NAHCO sayy CACO) {also reacts with calcium sulphate (which causes permanent hardness) to form sodium sulphate

thus: NayCOsjaq7+ CASO.jug7—* CACO 3+ NaS These sodium salts are soluble, but do not cause the water to be hard The calcium and magnesium ions are precipitated as insoluble calcium and magnesium carbonates. lonically, the situation is like this: Ca agi COS y+ CCOs.

Mg agi CO%" ug MECO%)

Ton Exchange

Another method that removes both temporary and permanent hardness in water is the use of fon exchange resin. A typical ion exchanger is @ container full of small beads. These beads are made ‘of special plastic called ion exchange resin The resin beads are porous znd contain sodium ions When hard water flows through the resin, the calcium and magnesium ions in the water are exchanged for the sodium ions and attach themselves to the resin. This process, therefore

removes calcium and magnesium ions from the water. They are replaced by sodium ions, which {do not make the water hard iy our hard water with [Gon exchange a mis don exchange " sof water with ca2*+andmg?*ions : resin ee Nations An ion exchange column removes ca and mg’ ions from the water and replaces them with Nat’ ions When all sodium ions have been removed from the resin, it is regenerated by pouring concentrated solution of sodium chloride through it.The sodium ions remove the calcium and/or

magnesium ions off the resin and the ion exchanger is ready for the use again, Other ions could also be used instead of sodium for the resin. But sodium chloride is normally used to supply the sodium ions because salt is cheap Use of softeners Many modern washing powders now have softeners added to them. The softeners are often phosphates. The phosphates ions react with calcium ions to form calcium phosphate and remove

the hardness 3Ca™ (y+ 2POs*agy-* Cax(POs)) The Importance of Hard Water Treatment and Purification Describe the importance of hard water treatment and purification The significance of water in daily life is well known to everyone. The water we obtain from natural sources is never pure. It contains dissolved minerals which render the water unfit for direct uses, The water from some sources contains calcium and magnesium compounds

dissolved in it These compounds are responsible for water hardness. To make the water fit for various uses, it is imperative to remove the hardness, The following points state why it is important to treat and purify hard water Hard water wastes soap. To get enough lather with hand water, it requires more soap than it does with soft water. So itis important to soften the water in order to save the soap and hence reduce

the cast of washing. Laundry uses less soap and can be done at lower temperatures, Treating and purifying hard water climinates the possibility of forming limescale deposits in water boilers, kettles, washing machines, water heaters, shower heads and dish washers. The scale formed around the heating elements can cause the element to overheat and fail Treated and purified water leaves no scum on clothes during washing. Scum spoils the finishing

of some fabrics. It forms nasty deposits (marks) on clothing that has been washed Softened water has the advantage of not blocking the water pipes. In industry, deposits of scales ccan block the pipes in boilers. This is a safety hazard as it could cause pressure to build up until there is an explosion. A similar coating can occur in hot water pipes at home and in central heating systems

THE IMPORTANCE OF HARD WATER IN DAILY LIFE

Hard water is not always disadvantageous. The following points explain the importance of hard water:

• The dissolved calcium and magnesium saits improve the taste of water. Distilled water is

tasteless and quite unpleasant to drink This is why water-processing plants add some salts in the distilled water to make it tasteful

• Caleium dissolved in hard water is an essential mineral for growth of bones and teeth. It makes

‘our teeth and bones hard, strong and resistant to shear and pressure

• In-some places, old lead pipes are used for water supply. Lead is very poisonous, and a little of it

can dissolve in soft water. But the earbonate (CO;*} or sulphate (SO,*) ions present in hard water reacts with lead fo form a coating of lead carbonate or lead sulphate that prevents lead from dissolving, This prevents lead poisoning

• A coating of calcium carbonate inside pipes, boilers and radiators helps to prevent corrosion.
• In recent years, it has been suggested that drinking hard water helps to prevent heart discases. 6

[thas also been found that hatd water is good for brewing beer.