Flashcards in Water Deck (37):
What is hard water?
It's water that will not easily form a lather with soap
What is the hardness caused by?
Calcium and magnesium ions
What are soap molecules?
Sodium or potassium salts of long chain carboxylic acids such as stearic acid eg, sodium stearate
What occurs when soap is dissolved in hard water
Magnesium and calcium ions will immediately precipitate with the dissolved stearate ions forming insoluble calcium stearate or magnesium stearate which appear as scum
What occurs when soap is continuously added to the water?
All the calcium and magnesium ions will eventually be precipitated with stearate ions and then soap molecule will at last be available to from a lather
What are the disadvantages of hard water?
Difficult to from a lather with soap
Scum may be formed in the reaction with soap, wasting the soap
Lime scale forms inside kettles, which wastes the energy whenever you need to boil the kettle
How can we avoid these problems?
Methods of removing hardness, softening the water may be used such as ion exchange and distillation
What do the methods of removing hardness involve? [calcium and magnesium sulfates and chlorines]
Remove calcium and magnesium ions to prevent them from precipitation with the soap anions
How do you remove hardness caused by dissolved calcium hydrogen carbonate and magnesium hydrogen carbonate?
Removed by boiling such hardness is temporary
What is temporary hardness?
Temporary hardness is hardness in water that can be removed by boiling
Where is temporary hard water found?
Parts of the country where there is limestone ie, calcium carbonate is found in the earth
What are the advantages of hard water?
Some people prefer the taste
Calcium ions in the water are good for bones and teeth
What causes temporary hardness?
Calcium ions are not readily available to cause hardness as calcium carbonate is insoluble in water. However as rain fails, the water comes in contact with the co2 in the atmosphere. Co2 is fairly soluble in water. Some of this solution then reacts with the water forming the weak acid, carbonic acid [h2co3] which results in a dilute solution of carbonic acid falls on the ground and seeps in the earth. Calcium carbonate reacts with the carbonic acid forming soluble calcium hydrogencarbonate
Calcium hydrogencarbonate is soluble in water and so the presence of dissolved calcium ions now means the water is hard
How do you remove temporary hardness?
Ion exchange and by distillation and also boiling
How is temporary hardness removed by boiling?
If a hydrogencarbonate is heated, it decomposes and becomes a carbonate, co2 and water eliminated
What does temporary hardness mean? [ions]
Containing calcium hydrogen carbonate and magnesium hydrogen carbonate
Disadvantages of limescale
Wasting heat, or even explosions, lead to pipes being completely clogged
How do you remove permanent hardness?
Ion exchange and by distillation but cannot be removed by boiling
What is ion exchange?
It involves the water being passed through a cation-exchange resin. The resins used are are usually complex sodium compounds [RNa] the calcium and magnesium ions in the hard water swap places with the sodium ions in the resin [do not cause hardness]
Anions are unaffected by this process
How does deionisation work? [distilling]
Removes all ions from water which has a cation and anion exchanger. Cation exchanger : replaces metal ions in water by hydrogen ions [h+] and an anion exchanger which replaces anions in water by hydroxide ions
Why is deionised water not quite as pure as distilled?
The ion-exchange region merely removes dissolved ions from water. Distilling the water removes all dissolved and suspended solids and dissolved gases from the water
What is flocculation?
It's the coming together of small-suspended solids in water
Name a flocculating agent
Flocculating reagent is added to the weather which causes small suspended particles to stick together to from larger heavier particles known as flocs. It is mixed thoroughly with water and is then pumped slowly into sedimation tanks
What is sedimentation?
The suspended particles settle to the bottom and clear water at the top flows over the edges of the tank to be collected
What is filtration?
Remaining suspended particles are removed. The water is allowed fall through beds of graded sand and gravel and the sand in the filter bed acts as a filter and removes tiny particles from water
What occurs in filtration after a long period of time
The filter bed gets clogged up from the matte and is renewed by forcing an upward flow of clean water which flushes particles that had been collected
What is chlorination?
Water is likely to contain disease causing bacteria and chlorination kills micro-organisms by oxidation
Why isn't it advised to use chlorine directly?
It's unpleasant and dangerous process
What is used instead of chlorine?
Both chlorine and sodium hypochlorite react with water to form chloric acid - active disinfecting agent
Why must care be taken in chlorination
Add a suitable quantity of chlorine to give continued protection until use, but not so much as to subject the consumer to too large a dose
What is fluoridation?
The fluoride ion F- is added, usually using sodium fluorosilicate
To prevent tooth decay
Necessary to adjust ph of water before it leaves treat plant, tap water should have a PH in the range 6-8. It's PH may be outside this range after most stages of the treatment process have finished
Why would the ph of the water be acidic
Original source [from a Peary acidic region] or because of chemicals added from the treatment
What occurs if tap water is too acidic?
It can cause corrosion of pipes
What is added in water treatment if the PH is too acidic?