Authors: Rachel Casiday and Regina Frey room of cg-tower.com, Washington university St. Louis, MO 63130

What is "Hard Water"?

Perhaps you have on occassion i found it mineral store on yourcooking dishes, or rings of insoluble soap scum in her bathtub.These are not indications of bad housekeeping, yet are rather indications ofhard water from the municipal water supply. Difficult wateris water that contains cations through a fee of +2, specifically Ca2+and Mg2+. These ions carry out not pose any health threat,but they have the right to engage in reactions that leave insoluble mineraldeposits. This deposits deserve to make difficult water unsuitable because that manyuses, and also so a selection of means have been developed to"soften" tough water; i.e.,remove the calciumand magnesium ions.

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Problems with hard Water

Mineral store are created by ionic reactions resulting inthe formation of one insoluble precipitate. For example, as soon as hardwater is heated, Ca2+ ions react with bicarbonate (HCO3-)ions to form insoluble calcium lead carbonate (CaCO3), asshown in Equation 1.



This precipitate, known as scale, coats the vesselsin which the water is heated, developing the mineral shop onyour food preparation dishes. In tiny quantities, these deposits space notharmful, but they might be frustrating to try to clean. As thesedeposits develop up, however, they mitigate the efficiency of heattransfer, for this reason food might not chef as same or easily in pans withlarge scale deposits. More serious is the instance in whichindustrial-sized water boilers end up being coated through scale: the costin heat-transfer efficiency can have actually a dramatic effect on yourpower bill! Furthermore, scale can accumulate ~ above the within ofappliances, such together dishwashers, and pipes. As range builds up,water circulation is impeded, and hence appliance parts and also pipes mustbe replaced much more often than if Ca2+ and also Mg2+ions were not current in the water.

Some tactics to "Soften" difficult Water

For large-scale municipal operations, a procedure known as the"lime-soda process" is offered to remove Ca2+and Mg2+ from the water supply. Ion-exchangereactions, similar to those girlfriend performed in this experiment,which an outcome in the formation of an insoluble precipitate, arethe communication of this process. The water is treated through acombination of slaked lime, Ca(OH)2, and also soda ash, Na2CO3.Calcium precipitates as CaCO3, and also magnesiumprecipitates together Mg(OH)2. These solids can becollected, thus removing the scale-forming cations from the watersupply.

To check out this process in an ext detail, permit us consider thereaction for the precipitation of Mg(OH)2.Consultation the the solubility guidelines in the experimentreveals that the Ca(OH)2 of slaked lime is moderatelysoluble in water. Hence, it have the right to dissociate in water to give oneCa2+ ion and two OH- ions for each unit ofCa(OH)2 that dissolves. The OH- ion reactwith Mg2+ ion in the water to kind the insolubleprecipitate. The Ca2+ ions are unaffected by thisreaction, and also so we perform not incorporate them in the network ionic reaction(Equation 2). They are gotten rid of by the different reaction v CO32-ions from the soda ash.



Household water softeners commonly use a various process,known together ion exchange. Ion-exchange devices consist the a bed ofplastic (polymer) beads covalently bound to anion groups, such as-COO-. The an unfavorable charge of this anions is balancedby Na+ cations attached come them. As soon as water containingCa2+ and Mg2+ is passed v the ionexchanger, the Ca2+ and Mg2+ ions space moreattracted to the anion groups than the Na+ ions.Hence, they replace the Na+ ion on the beads, and also sothe Na+ ion (which do not form scale) enter thewater in their place.


Figure 1

When tough tapwater passes with the ion exchanger (left), the calcium ion from the tapwater change the sodium ion in the ion exchanger. The softened water, containing sodium ions in location of calcium ions, can be built up for household use.

Unfortunately, many human being with high blood pressure or otherhealth difficulties must restrict your intake of sodium. Becausewater softened by this type of ion exchange includes many sodiumions, world with restricted sodium intakes have to avoid drinkingwater that has been softened this way. Several new techniques forsoftening water without presenting sodium ion are beginning toappear ~ above the market.

Questions on Water Hardness

1. How can you usage a flame check to distinguish in between hardand soft water? (HINT: for which ions in hard and soft water haveyou performed experimental flame-test monitorings in Experiment2?)

2. After a duration of time, your ion-exchange water softener(the kind defined above) no longer clears hard-water ions. Howcan you "refresh" her water softener, making use of a materialobtained from the grocery store store?

3. The deionized water in the cg-tower.com department is producedby ion exchange that offers H+ ions, fairly than Na+ions, to change the cations uncovered in the water. Briefly, explainwhy the pH of deionized water is frequently found to be approximately 6.

To answer the following three questions, describe thesolubility rule in the lab hand-operated for this experiment.

4. Explain why remedies of Ca2+ and Mg2+in the visibility of carbonate leave deposits (see Equation 1), butNa+ walk not.

5. Compose the network ionic equation for the removal of calciumions through precipitation with carbonate in the lime-soda process.(See the arrival to Experiment 2 for a conversation of netionic equations.)

6. Might sodium ions be removed in the same way as magnesiumions in the lime-soda process (i.e., by enhancement ofhydroxide) for people concerned around their salt intake?Briefly, explain your reasoning.

Additional Link:


Brown, Lemay, and Buster. cg-tower.com: the main Science,7th ed. Upper Saddle River, NJ: Prentice Hall, 1997. P. 681-3.


The authors give thanks to Dewey Holten (Washington University) formany helpful suggestions in the composing of this tutorial.

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The breakthrough of this accuse was supported by a provide fromthe Howard Hughes medical Institute, with the UndergraduateBiological Sciences education program, give HHMI# 71192-502004to Washington University.