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