Hard waterHard water

LI – to know the causes, LI – to know the causes, effects and cures for effects and cures for hard waterhard water Hard water is Hard water is

hard to lather hard to lather and forms a and forms a scum with soapscum with soap

It is caused by It is caused by the presence of the presence of dissolved calcium dissolved calcium or magnesium or magnesium compoundscompounds

Detergents are Detergents are not affected by not affected by hard waterhard water

There are 2 types There are 2 types of hardness; of hardness; temporary and temporary and permanentpermanent

Hardness is tested Hardness is tested for using soap for using soap solutionsolution

The greater The greater volume of soap volume of soap needed the harder needed the harder the waterthe water

Temporary hardnessTemporary hardness

Temporary hard water is formed by rain Temporary hard water is formed by rain water water reactingreacting with the limestone. with the limestone.

Rain water is slightly Rain water is slightly acidicacidic due to the due to the presence of dissolved carbon dioxidepresence of dissolved carbon dioxide

Rain water is a weak solution of Rain water is a weak solution of Carbonic Carbonic acidacid, H, H22COCO33

Calcium + Water + Carbon = CalciumCalcium + Water + Carbon = Calcium

carbonatecarbonate dioxide hydrogen carbonate dioxide hydrogen carbonate

CaCOCaCO3(s)3(s) + H + H22O O (l)(l) + CO + CO2(g)2(g) = Ca(HCO = Ca(HCO33))2(aq)2(aq)

Features of hard water Features of hard water areasareas

Stalactites and stalagmitesStalactites and stalagmites are are found in limestone regions. These found in limestone regions. These are formed by the decomposition are formed by the decomposition of the calcium hydrogen of the calcium hydrogen carbonate inside the limestone carbonate inside the limestone cavescaves

Temporary hardnessTemporary hardness

Temporary hardness can be Temporary hardness can be removed by boilingremoved by boiling

Calcium hydrogen carbonate Calcium hydrogen carbonate decomposes on heatingdecomposes on heating

Ca(HCOCa(HCO33))22 = CaCO = CaCO3 3 + H+ H22O + COO + CO22

Limescale is formed when Limescale is formed when temporary hard water decomposestemporary hard water decomposes

Permanent HardnessPermanent Hardness

Permanent hard water is caused Permanent hard water is caused by Calcium or magnesium by Calcium or magnesium sulphatesulphate

Calcium sulphate and magnesium Calcium sulphate and magnesium sulphate is slightly soluble and sulphate is slightly soluble and dissolve when rain water runs dissolve when rain water runs over the rocks like graniteover the rocks like granite

Permanent hard water is not Permanent hard water is not softened by boilingsoftened by boiling

Q: Q: Explain why water in Explain why water in limestone regions is hard when limestone regions is hard when limestone is insoluble.limestone is insoluble.

Rain water contains dissolved carbon Rain water contains dissolved carbon dioxide which makes it acidic (carbonic dioxide which makes it acidic (carbonic acid). The acidic rain water reacts with acid). The acidic rain water reacts with the calcium carbonate (limestone) and the calcium carbonate (limestone) and forms soluble calcium hydrogen forms soluble calcium hydrogen carbonate.carbonate.

CaCOCaCO3(s)3(s) + H + H22OO(l)(l) + CO + CO2(g)2(g) = Ca(HCO = Ca(HCO33))2(aq)2(aq)

Hard water can be Hard water can be good and can be a good and can be a nuisancenuisance

Good pointsGood points

Tastes betterTastes better Contains calcium Contains calcium

which is good for which is good for teeth & bonesteeth & bones

Good for tanning Good for tanning leatherleather

Bad pointsBad points

Wastes soapWastes soap Leaves a fur Leaves a fur

inside kettles so inside kettles so harder to boilharder to boil

Leaves limescale Leaves limescale inside pipesinside pipes

Curing HardnessCuring Hardness

Boiling – removes temporary Boiling – removes temporary hardness onlyhardness only

Adding washing soda (sodium Adding washing soda (sodium carbonate) –removes both types carbonate) –removes both types of hardnessof hardness

Ion exchange –changes the Ion exchange –changes the calcium ions for sodium ionscalcium ions for sodium ions

Equations for curing Equations for curing hardness - Higherhardness - Higher Boiling: Boiling:

Ca(HCOCa(HCO33))22 = CaCO = CaCO3 3 + H+ H22O + COO + CO22

Washing soda (sodium carbonate)Washing soda (sodium carbonate)

NaNa22COCO33 + CaSO + CaSO44 = Na = Na22SOSO44 + + CaCOCaCO33

Ion ExchangeIon Exchange

Ca Ca 2+2+ swapped for 2Na swapped for 2Na++

Exam questions - Exam questions - HigherHigher

Q:Q: Explain in terms of ions how Explain in terms of ions how washing soda softens waterwashing soda softens water

A:A: The The calcium ionscalcium ions from the from the hard water reacts with the hard water reacts with the carbonate ionscarbonate ions from the washing from the washing soda to form soda to form insoluble calcium insoluble calcium carbonatecarbonate

Exam questions - Exam questions - HigherHigherQ:Q: Explain in terms of ions how Explain in terms of ions how

an ion exchanger softens wateran ion exchanger softens water

A:A: The The calcium ionscalcium ions from the from the hard water are hard water are swappedswapped for for sodium ionssodium ions from the exchanger from the exchangerThe water no longer contains The water no longer contains calcium ions and therefore is no calcium ions and therefore is no longer hardlonger hard