Water (chpt.19)

Question 1

Hard water

Answer 1

Water that will not easily form a lather with soap

Question 2

What causes hardness in water

Answer 2

Ca 2+ and Mg 2+ ions dissolved in water

Question 3

Temporary hardness

Answer 3

Hardness that can be removed by boiling the water

Question 4

Permanent hardness

Answer 4

Hardness that cannot be removed by boiling the water

Question 5

Methods of removing hardness from water

Answer 5

1. Distillation → boiling water, then condensing vapour
   ↳ expensive due to cost of boiling
2. Using washing soda → exchanges Na 2+ ions for Ca 2+ ions
3. Ion-exchange resin → swaps positive ions for H+ ions and swaps negative ions for OH-

Question 6

Advantages of hard water

Answer 6

• Provides calcium for teeth and bones
• Nicer taste
• good for brewing & tanning leather

Question 7

Disadvantages of hard water

Answer 7

• blocks pipes, leaves scale on kettles and boilers
• wastes soap
• produces scum

Question 8

Distilled water

Answer 8

Purest form of water
All dissolved and suspended solids as well as dissolved gases have been removed from the water

Question 9

Why is deionised water not as pure as distilled water?

Answer 9

Deionised water contains dissolved gases

Question 10

Is deionised or distilled water purer?
Explain

Answer 10

Distilled more pure

Deionised water contains dissolved gases

Question 11

Flocculation

Answer 11

The coming together of small suspended particles in water

Question 12

Flocculating agent

Answer 12

A chemical added to water to coagulate suspended particles and this help sedimentation take place

Question 13

Steps in water treatment

Answer 13

1. Screening → water passed through wire mesh to remove debris
2. Flocculation → small suspended particles brought together by flocculating agents
3. Sedimentation → water passed into settlement tanks, over 90% of suspended particles removed
4. Filtration → water passed through large beds of sand, removing any remaining suspended solids
5. Chlorination → chlorine added to water to sterilise
6. Fluoridation → small quantities of fluorine compounds added to water, helps reduce tooth decay
7. pH adjustment → optimum pH is 7.2

Question 14

How is the pH of water raised?

Answer 14

Add calcium hydroxide

Question 15

How is the pH of water lowered?

Answer 15

Add sulfuric acid

Question 16

Example of flocculating agent

Answer 16

Aluminium sulfate

Question 17

Example of fluorine compound added to water during fluoridation

Answer 17

Sodium fluoride NaF

Hexafluorosilic acid H2SiF6

Question 18

Buffer solution

Answer 18

A solution that resists changes in pH in order to keep pH constant

Question 19

Complexometric titration

Answer 19

A titration involving the formation of a complex between metal ions and a reagent such as edta

In this type of titration the end point is marked by a sharp decrease in the concentration of free metal ions

Question 20

Biochemical oxygen demand (B.O.D)

Answer 20

The amount of dissolved oxygen consumed by biological action when a sample of water is kept at 20°c in the dark for five days