Unit2Topic2 - Water

Question 1

What % of the earth is covered in water?

Answer 1

70%

Question 2

Tests for presence of water

Answer 2

Anhydrous copper sulfate (white->blue)

Cobalt chloride paper (blue->pink)

Question 3

Test for pure water

Answer 3

Melts at 0°C and boils at 100°C

Question 4

What is hard water?

Answer 4

Hard water is water that doesn’t lather easily with soap

Question 5

Which ions cause water to be hard?

Answer 5

Calcium and magnesium cations (Ca2+ and Mg2+)

Question 6

What happens with ions in hard water areas?

Answer 6

The calcium ions combine with the stearate ions in soap to form insoluble calcium stearate (scum).

Question 7

Formation of scum word equation

Answer 7

soap(sodium stearate) + hard water -> scum(calcium stearate) + sodium ions

Question 8

Formation of scum symbol equation

Answer 8

2C17H35COONa (aq) + Ca2+ (aq) -> (C17H35COO)2Ca (s) + Na+ (aq)

Question 9

Test to identify hard/soft water / comparison

Answer 9

Add 10cm3 of each water sample in separate conical flasks.

Add 1cm3 of soap solution from burette, put a bung in the conical flask and shake for 30 seconds.

Continue adding 1cm3 of soap solution and shake until a permanent lather lasting more than 30 seconds is formed.

Record volume of soap solution added.

The more soap required, the harder the water.

Question 10

What is temporary hardness caused by?

Answer 10

These salts dissolved in water…

• calcium hydrogencarbonate
• magnesium hydrogencarbonate

Question 11

How is temporary hardness formed?

Answer 11

Rainwater is naturally acidic (weak carbonic acid). The acid reacts with the limestone to form these soluble salts.

Question 12

Temporary hard water word equation

Answer 12

Carbonic acid + calcium carbonate -> calcium hydrogencarbonate

Question 13

Temporary hard water symbol equation

Answer 13

H2CO3 (aq) + CaCO3 (s) -> Ca(HCO3)2 (aq)

Question 14

How is permanent hardness formed?

Answer 14

Permanent hardness is formed by rain water passing over gypsum rocks containing calcium compounds (eg calcium sulfate) which can dissolve into the water.

Question 15

Features of a hard water area

Answer 15

Pot-holes
Caverns
Stalactites
Stalagmites

Question 16

What happens when temporary hard water is boiled?

What is the symbol equation?

Answer 16

The calcium hydrogencarbonate decomposes and insoluble calcium carbonate, water and carbon dioxide are formed.

Ca(HCO3)2 (aq) -> CO2 (g) + H2O (l) + CaCO3 (s) (limescale)
same can be written with Mg2+ ions

Boiling the water causes the precipitation of solid calcium carbonate.
this removes the calcium/magnesium ions from the water, removing the hardness.

Question 17

What is permanent hard water?

Answer 17

Hard water that cannot be softened by boiling

Question 18

How is permanent hard water caused?

Answer 18

Caused by other types of calcium or magnesium salts in water (such as calcium chloride CaCl2 or magnesium sulfate MgSO4). These cannot be broken down by boiling.

Question 19

Methods used to soften water

Answer 19

Washing soda

Ion exchange column

Question 20

Washing soda

Answer 20

Hydrated sodium carbonate Na2CO3.xH2O

A precipitation reaction involves the carbonate ions from the washing soda reacting with the calcium ions from the hard water to make an insoluble substance (calcium carbonate)
*Ca2+ (aq) + ^CO32- (aq) -> CaCO3 (s)
*from calcium chloride
^from sodium carbonate

Question 21

What is a precipitation reaction?

Answer 21

A precipitation reaction is when aqueous ions from 2 solutions form an insoluble solid.

Question 22

Ion exchange column

Answer 22

Hard water contains calcium/magnesium ions

An ion exchange column is a glass tube filled with a resin with sodium ions attached

Calcium/magnesium ions become replaced by sodium ions as water flows through the column

The Ca/Mg ions have been removed from the water so it’s no longer hard

The sodium ions do not cause hardness in water

Question 23

How is an ion exchange column regenerated?

Answer 23

Pouring sodium chloride solution (salt water) through it to replace Ca ions with Na ions

Question 24

Advantages of hard water

Answer 24

• tastes better
• provide Ca ions for growth of bones and teeth
• better for tanning leather
• better for brewing beer

Question 25

Disadvantages of hard water

Answer 25

• wastes soap (produces scum); increased costs as more soap is required for washing
  (Also has no effect on detergents eg cillit bang)
• expensive; dishwasher salt must be used to remove the lime scale
• fur and scale in kettles and pipes (kettles less efficient over time, blocks pipes). The scale is insoluble calcium carbonate formed during heating water with temporary hardness.

Question 26

Drying agents with moist air

Answer 26

Silica gel

Calcium chloride

Question 27

What is a desiccant?

Answer 27

A substance which absorbs moisture

Question 28

What is a deliquescent?

Answer 28

A substance that absorbs so much moisture from the air that it dissolves in it forming a solution

Question 29

What is silica gel used for?

Answer 29

Used for packaging of equipment where moisture would be a problem

Question 30

Objections to fluoridation of drinking water

Answer 30

• many disagree with mass medication
• not effective enough to justify cost
• dental fluorosis (excessive consumption)

Question 31

Medical use of barium sulfate

Answer 31

Barium compounds are very toxic.
Barium sulfate doesn’t allow x-rays to pass through it.
Medical experts use barium sulfate on patients with stomach and intestinal problems to x-ray the stomach or bowel.
Barium sulfate is chosen as a safe barium compound because it has a low solubility in water so cannot act as a poison.