C3 - Water

Question 1

What is hard water?

Answer 1

Water that contains dissolved compounds such as calcium and magnesium salts.

Question 2

What does hard water cause?

Answer 2

Scum

Question 3

How does scum form?

Answer 3

Soap reacts with dissolved salts.

Question 4

Why isn’t scum a problem when washing clothes?

Answer 4

Modern detergents are soapless.

Question 5

How is hard water formed?

Answer 5

When streams or rivers flow over rocks containing calcium or magnesium compounds, the compounds dissolve in the water.

Question 6

How does limestone produce hard water?

Answer 6

• As rain falls, carbon dioxide dissolves in the water making the water in rivers slightly acidic.
• CaCO3 reacts with the acidic solution forming calcium hydrogencarbonate.
• Ca2+ ions get into the water making it hard.

Question 7

What is the equation for forming hard water with limestone?

Answer 7

CaCO3 + H2O + CO2(aq) —-> Ca2+ + 2HCO3-

Question 8

What is the equation for calcium reacting with soap?

Answer 8

sodium stearate + Ca2+ —-> calcium stearate precipitate + Na+ ions

Question 9

Why is using hard water expensive?

Answer 9

More soap is needed since some is wasted reacting with the ions in the water.

Question 10

How is limescale formed?

Answer 10

When water containing calcium ions is heated.

Question 11

Give two advantages of hard water.

Answer 11

• Calcium ions help in the development of strong bones.

- Hard water could help reduce heart disease.

Question 12

What is soft water?

Answer 12

Water that does not contain ions that produce scum or scale.

Question 13

What is temporary hard water?

Answer 13

Water that can have its hardness removed by heating.

Question 14

What is permanent hard water?

Answer 14

Water that remains hard after boiling.

Question 15

What types salts produce permanent hard water?

Answer 15

Sulfates

Question 16

What is the equation for heating temporary hard water?

Answer 16

Ca2+ 2HCO- —-> CaCO3 + H2O + CO2

Question 17

What are the two methods for softening water?

Answer 17

• Washing Soda

- Ion Exchange System

Question 18

What does washing soda contain?

Answer 18

Sodium Carbonate

Question 19

How does washing soda soften water?

Answer 19

The soluble carbonate ions precipitate out calcium and magnesium ions.

Question 20

How do ion-exchange columns work?

Answer 20

The sodium ions replace the magnesium or calcium ions when hard water passes through a resin.

Question 21

How can ion-exchange systems be recharged?

Answer 21

Washing with sodium chloride solution.

Question 22

What does chlorine do in water treatment?

Answer 22

Kill microbes

Question 23

What does a water filter usually contain?

Answer 23

• Carbon
• Ion exchange resin
• Silver

Question 24

What does the carbon in a water filter do?

Answer 24

• Reduces the levels of chlorine

- Reduces amounts of pesticides and other impurities.

Question 25

What does the ion-exchange resin in a water filter do?

Answer 25

Removes calcium, magnesium, lead, copper and aluminium ions.

Question 26

What does the silver in a water filter do?

Answer 26

Discourages growth of bacteria.

Question 27

What is the process for supplying water once collected in a reservoir?

Answer 27

- The water passes through a fine sand filter to remove grit and mud. - Large objects are trapped by metal screens. - A settlement tank allows sand and soil to settle out. - Aluminium sulfate and lime are added to make small particles of dirt clump together making them fall to the bottom. - Chlorine is added to kill bacteria. - The pH is checked to be neutral.

Question 28

Give an advantage of softening water.

Answer 28

- Scale isn't produced. | - Less soap needs to be used.

Question 29

Give three advantages of hard water.

Answer 29

- Hard water is good for bones and teeth. - Hard water can prevent high blood pressure. - Softening can cost money.

Question 30

Give an alternative to chlorine for killing microbes in water.

Answer 30

Ozone

Question 31

Give five advantages of fluoridation.

Answer 31

- There are no obvious health risks. - It can reduce tooth cavities. - It can protect people who don't visit the dentist. - It can kill bacteria that can cause heart disease. - Fluoride is added in very small quantities.

Question 32

Give five disadvantages of fluoridation.

Answer 32

- Fluorosis can cause bones to weaken. - The benefit of fluoridation isn't considered to be significant. - People should have a right to choose treatments. - Excess fluoride can affect the brain. - Safe limits of fluoride can't be set without controlling intake.