Permanent Hardness
What is permanent hardness defined as?
Hardness that cannot be removed by boiling
What is temporary hardness defined as?
Hardness that can be removed by boiling
What is the purpose of adding chlorine for water treatment?
Kill microorganisms (by oxidation)
How many stages typically of sewage treatment are there?
3
What is water hardness defined as?
Water that cannot easily form a lather with soap
How can permanent hardness be removed
distillation and ion exchange
How can temporary hardness be removed?
Boiling, ion exchange and distillation
What are the six stages of water treatment? (in order)
Flocculation, sedimentation, filtration, chlorination, fluoridation, adjustment of pH
During the primary stage of sewage treatment how is the sewage filtered and broken up?
Metal screens
Which two ions cause water hardness?
Ca+ and Mg+
How does ion exchange work?
Calcium and magnesium ions in the water are exchanged for sodium ions which do not cause water hardness
How does temporary water hardness cause limescale in kettles?
During boiling, the calcium hydrogencarbonate forms insoluble calcium carbonate, carbon dioxide and water. Calcium carbonate build up is called limescale
How is water fed into the sedimentation tanks and why is this done?
From the bottom of the tank to allow the clean water at the top of the tank to not get mixed with the dirty water
Why is it important if possible to carry out the tertiary stage of sewage treatment?
How can limescale be tested?
Reacted with an acid and monitor the production of carbon dioxide using lime water
Why does distillation create purer water than ion exchange?
What two molecules (chemical name or chemical formula) cause temporary hardness?
Calcium hydrogencarbonate and magnesium hydrogencarbonate
Ca(HCO3)2 and Mg(HCO3)2
How are filter beds prevented from becoming blocked?
By pushing cold water up through the beds
What happens during the secondary stage of sewage treatment?
Microorganisms oxidise the nutrients present in the effluent
How does water hardness prevent lather forming with soap?
Calcium or magnesium ions bind to the active part of soap preventing it from lathering and forming an insoluble scum instead
What four molecules (chemical name or chemical formula) cause permanent hardness?
magnesium chloride, calcium chloride, calcium sulphate, magnesium sulphate
MgCl2, CaCl2, MgSO4, CaSO4
How does temporary water hardness occur? (need two sentences describing)
carbon dioxide dissolved in rainwater forms carbonic acid. Falls into soil and reacts with limestone rock in soil to form soluble calcium hydrogencarbonate
What can be used as a flocculating agent?
Synthetic polyelectrolytes
Aluminium sulfate
How are phosphates removed from the water during the tertiary stage?
Precipitation- they react with a compound like aluminium sulfate which forms an insoluble salt that can then be removed by filtration
What is the active part of soap?
The stearate ion