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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

Aluminium sulphate


Describe flocculation

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?

Sodium hypochlorite
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


Ph adjustment

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?

Calcium hydroxide


What is added if the ph is too basic?

Sulfuric acid is added