Hard water
LI – to know the causes, effects and cures for hard water Hard water is hard to lather and forms a scum with soap It is caused by the presence of dissolved calcium or magnesium compounds Detergents are not affected by hard water There are 2 types of hardness; temporary and permanent Hardness is tested for using soap solution The greater volume of soap needed the harder the water
Temporary hardness Temporary hard water is formed by rain water reacting with the limestone. Rain water is slightly acidic due to the presence of dissolved carbon dioxide Rain water is a weak solution of Carbonic acid, H2CO3 Calcium + Water + Carbon = Calcium carbonate dioxide hydrogen carbonate CaCO3(s) + H2O (l) + CO2(g) = Ca(HCO3)2(aq)
Features of hard water areas Stalactites and stalagmites are found in limestone regions. These are formed by the decomposition of the calcium hydrogen carbonate inside the limestone caves
Temporary hardness Temporary hardness can be removed by boiling Calcium hydrogen carbonate decomposes on heating Ca(HCO3)2 = CaCO3 + H2O + CO2 Limescale is formed when temporary hard water decomposes
Permanent Hardness Permanent hard water is caused by Calcium or magnesium sulphate Calcium sulphate and magnesium sulphate is slightly soluble and dissolve when rain water runs over the rocks like granite Permanent hard water is not softened by boiling
Q: Explain why water in limestone regions is hard when limestone is insoluble. Rain water contains dissolved carbon dioxide which makes it acidic (carbonic acid). The acidic rain water reacts with the calcium carbonate (limestone) and forms soluble calcium hydrogen carbonate. CaCO3(s) + H2O(l) + CO2(g) = Ca(HCO3)2(aq)
Hard water can be good and can be a nuisance Good points Tastes better Contains calcium which is good for teeth & bones Good for tanning leather Bad points Wastes soap Leaves a fur inside kettles so harder to boil Leaves limescale inside pipes
Curing Hardness Boiling – removes temporary hardness only Adding washing soda (sodium carbonate) –removes both types of hardness Ion exchange –changes the calcium ions for sodium ions
Equations for curing hardness - Higher Boiling: Ca(HCO3)2 = CaCO3 + H2O + CO2 Washing soda (sodium carbonate) Na2CO3 + CaSO4 = Na2SO4 + CaCO3 Ion Exchange Ca 2+ swapped for 2Na+
Exam questions - Higher Q: Explain in terms of ions how washing soda softens water A: The calcium ions from the hard water reacts with the carbonate ions from the washing soda to form insoluble calcium carbonate
Exam questions - Higher Q: Explain in terms of ions how an ion exchanger softens water A: The calcium ions from the hard water are swapped for sodium ions from the exchanger The water no longer contains calcium ions and therefore is no longer hard