Water

Question 1

What is hard water?

Answer 1

It’s water that will not easily form a lather with soap

Question 2

What is the hardness caused by?

Answer 2

Calcium and magnesium ions

Question 3

What are soap molecules?

Answer 3

Sodium or potassium salts of long chain carboxylic acids such as stearic acid eg, sodium stearate

Question 4

What occurs when soap is dissolved in hard water

Answer 4

Magnesium and calcium ions will immediately precipitate with the dissolved stearate ions forming insoluble calcium stearate or magnesium stearate which appear as scum

Question 5

What occurs when soap is continuously added to the water?

Answer 5

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

Question 6

What are the disadvantages of hard water?

Answer 6

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

Question 7

How can we avoid these problems?

Answer 7

Methods of removing hardness, softening the water may be used such as ion exchange and distillation

Question 8

What do the methods of removing hardness involve? [calcium and magnesium sulfates and chlorines]

Answer 8

Remove calcium and magnesium ions to prevent them from precipitation with the soap anions

Question 9

How do you remove hardness caused by dissolved calcium hydrogen carbonate and magnesium hydrogen carbonate?

Answer 9

Removed by boiling such hardness is temporary

Question 10

What is temporary hardness?

Answer 10

Temporary hardness is hardness in water that can be removed by boiling

Question 11

Where is temporary hard water found?

Answer 11

Parts of the country where there is limestone ie, calcium carbonate is found in the earth

Question 12

What are the advantages of hard water?

Answer 12

Some people prefer the taste

Calcium ions in the water are good for bones and teeth

Question 13

What causes temporary hardness?

Answer 13

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

Question 14

How do you remove temporary hardness?

Answer 14

Ion exchange and by distillation and also boiling

Question 15

How is temporary hardness removed by boiling?

Answer 15

If a hydrogencarbonate is heated, it decomposes and becomes a carbonate, co2 and water eliminated

Question 16

What does temporary hardness mean? [ions]

Answer 16

Containing calcium hydrogen carbonate and magnesium hydrogen carbonate

Question 17

Disadvantages of limescale

Answer 17

Wasting heat, or even explosions, lead to pipes being completely clogged

Question 18

How do you remove permanent hardness?

Answer 18

Ion exchange and by distillation but cannot be removed by boiling

Question 19

What is ion exchange?

Answer 19

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

Question 20

How does deionisation work? [distilling]

Answer 20

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

Question 21

Why is deionised water not quite as pure as distilled?

Answer 21

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

Question 22

What is flocculation?

Answer 22

It’s the coming together of small-suspended solids in water

Question 23

Name a flocculating agent

Answer 23

Aluminium sulphate

Question 24

Describe flocculation

Answer 24

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

Question 25

What is sedimentation?

Answer 25

The suspended particles settle to the bottom and clear water at the top flows over the edges of the tank to be collected

Question 26

What is filtration?

Answer 26

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

Question 27

What occurs in filtration after a long period of time

Answer 27

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

Question 28

What is chlorination?

Answer 28

Water is likely to contain disease causing bacteria and chlorination kills micro-organisms by oxidation

Question 29

Why isn't it advised to use chlorine directly?

Answer 29

It's unpleasant and dangerous process

Question 30

What is used instead of chlorine?

Answer 30

Sodium hypochlorite | Both chlorine and sodium hypochlorite react with water to form chloric acid - active disinfecting agent

Question 31

Why must care be taken in chlorination

Answer 31

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

Question 32

What is fluoridation?

Answer 32

The fluoride ion F- is added, usually using sodium fluorosilicate Na2SiF6 To prevent tooth decay

Question 33

Ph adjustment

Answer 33

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

Question 34

Why would the ph of the water be acidic

Answer 34

Original source [from a Peary acidic region] or because of chemicals added from the treatment

Question 35

What occurs if tap water is too acidic?

Answer 35

It can cause corrosion of pipes

Question 36

What is added in water treatment if the PH is too acidic?

Answer 36

Calcium hydroxide

Question 37

What is added if the ph is too basic?

Answer 37

Sulfuric acid is added