Hardness of Water

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Hard Water

Soft water forma a lather easily when mixed with soap, but hard water does not

  • Hardness in water is caused by dissolved magnesium ions and calcium ions
  • These can get into the water when it comes into contact with limestone and other rocks that contain calcium compounds
  • One way of this happening is when rainwater flows over these rocks on its way to a reservoir

Permanent hardness is caused by dissolved calcium sulfate. Unlike temporary hardness, it is not removed by boiling the water

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Temporary Hardness

Temporary hardness is caused by dissolved calcium hydrogencarbonate

  • Rainwater is naturally acidic because it contains dissolved carbon dioxide from the air
  • It reacts with calcium carbonate in rocks to form calcium hydrogencarbonate, which is soluble
  • calcium carbonate + water + carbon dioxide --> calcium hydrogencarbonate
  • Temporary hardness is removed by boiling the water
  • When this happens, the soluble calcium hydrogencarbonate decomposes to form calcium carbonate, water and carbon dioxide
  • calcium hydrogencarbonate --> calcium carbonate + water + carbon dioxide
  • The insoluble calcium carbonate forms a layer of limescale
  • This may coat the heating element in kettles and irons, making them less efficient
  • Limescale is unslightly and it clogs up hot water pipes and boilers
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Softening Water

Washing soda

  • Sodium carbonate, also known as washing soda, can soften both temporary and permanent hard water
  • It is soluble in water and adds a large amount of carbonate ions to the water
  • These react with dissolved calcium ions forming a precipitate of calcium carbonate

Calcium ions come from the water and carbonate ions come from the washing soda

Ion exchange

  • Ion exchange resins can soften both permanent and temporary hard water
  • As hard water passes through the column, sodium ions come off the resin and go into the water, while calcium ions come out of the water and stick to the resin
  • Dishwashing machines ccontain ion exchange resin to soften the water used to was the dishes. The resin needs recharging with dishwasher salt once it becomes full of calcium ions
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