C21 Water

Question 1

Explain what it means for a molecule to be polar.

Answer 1

Each molecule has a side that has a partial positive charge and a side that has a partial negative charge.

Question 2

Explain why water has unique physical properties.

Answer 2

The partial positive hydrogen atoms and partial negative oxygen atoms of the water molecules are attracted to each other, forming a hydrogen bond. Hydrogen bonds are usually stronger than other intermolecular forces which exist between molecules and this gives water several unique properties.

Question 3

Explain why ice floats in water.

Answer 3

When water is cooled down to 4 degrees Celcius it contracts and becomes denser. However, if it is cooled below 4 degrees Celcius it starts to expand and continues to expand until it freezes at 0 degrees Celcius. The volume of ice is greater than the volume of water from which it formed and it floats on the liquid as it forms.

Question 4

Aquatic organisms can live on the bottom of a lake in winter even though the surface is frozen because:

A. water has a relatively low melting point
B. the maximum density of water occurs at 4 degrees Celcius
C. water is not very volatile
D. water has a high specific heat capacity

Answer 4

D. water has a high specific heat capacity

Question 5

How is water’s high heat of vaporisation useful to living organisms?

A. it makes things dry faster
B. it makes sweating an effective method of cooling organisms.
C. it holds heat near the surface of skin

Answer 5

B. it makes sweating an effective method of cooling organisms

Question 6

Water has its maximum density at what temperature?

Answer 6

Four degrees Celcius

Question 7

List the advantages of water having good solvent properties.

Answer 7

Water dissolves chemicals in cells so that chemical reactions can take place.
Water dissolves many useful substances so that they can be absorbed and transported around the bodies of organisms.
Water dissolves waste products so that they can be excreted.

Question 8

Which of these are disadvantages of water having good solvent propertues?

I. Water dissolves chemicals.
II. Water can become polluted.
III. Mineral salts can be leached out of the soil.

A. I only
B. III only
C. I, II, and III
D. II and III only

Answer 8

D. II and III only

Question 9

What is hard water and how is it formed?

Answer 9

Hard water is water that does not lather easily with soap, whereas soft water lathers easily when soap is added. It is caused by dissolved calcium and magnesium salts.

Question 10

True or False: Scum is formed when soap reacts with permanently hard water.

Answer 10

True.

Question 11

What is temporary hardness?

Answer 11

Temporary hardness of water is hardness that can be removed by boiling.

Question 12

What is the chemical equation that summarises how temporary hard water is formed?

Answer 12

CaCO3 (s) + H2O (l) —-> Ca(HCO3)2 (aq)

Question 13

What is permanent hardness?

Answer 13

Permanent hardness of water is hardness that cannot be removed by boiling.

Question 14

What are the main pollutants of water?

Answer 14

Heavy metal ions like mercury and lead, mainly from industry; sulfur dioxide and oxides of nitrogen which are produced during combustion dissolve in water to form acid rain; nitrate and phosphate ions present in fertilisers and detergents; pesticides used in agriculture; organic waste from untreated sewage and farmyards.

Question 15

What is leaching?

Answer 15

Leaching is the loss of water soluble substances from the soil as water passes through it.

Question 16

Which of the following lists the stages involved in the large-scale treatment of water in the correct order?

A. flocculation, sedimentation, filtration, chlorination
B. sedimentation, flocculation, filtration, chlorination
C. chlorination, flocculation, sedimentation, filtration
D. flocculation, chlorination, sedimentation, filtration

Answer 16

A. flocculation, sedimentation, filtration, chlorination

Question 17

Describe ways in which water can be treated at home.

Answer 17

Filtration: fibre filters can be used to remove suspended sediment from water. Carbon filters can remove dissolved organic compounds, odours and unpleasant tastes.
Chlorination: Adding sodium chlorate (I) solution or calcium chlorate (I) tablets to the water. Ten drops of chlorine bleach to 5 dm3 of water, stir and leave for 30 minutes.
Boiling: Boiling water for 15 minutes is sufficient to kill any microorganisms in the water. After boiling, the water should be cooled before drinking.

Question 18

Suggest one reason why it is useful to remove hardness from water.

Answer 18

When soap is added to hard water, it makes scum. Soft water is a more efficient cleaning agent, because instead of creating scum, it rinses the surface properly.

Question 19

Write an equation to show the removal of temporary hardness by boiling.

Answer 19

Ca(HCO3)2 (aq) –heat–> CaCO3 (s) + H20 (l) + CO2 (g)

*calcium carbonate is also known as limescale.

Question 20

Write an equation to show the removal of permanent hardness using sodium carbonate.

Answer 20

CaSO4 (aq) + Na2CO3 (aq) —-> CaCO3 (s) + Na2SO4 (aq)

Question 21

What is the compound mainly responsible for temporary hardness?

Answer 21

calcium hydrogencarbonate Ca(HCO3)2

Question 22

What compounds are mainly responsible for permanent hardness?

Answer 22

calcium sulfate CaSO4 and magnesium sulfate MgSO4

Question 23

Permanent hardness can be removed by:
I adding sodium carbonate to the water or distillation
II passing the water though an ion exchange column
III boiling the water

A. II only
B. I and II only
C. I and III only
D. I, II and III

Answer 23

B. I and II only

Question 24

Briefly describe how reverse osmosis works to soften hard water.

Answer 24

Water is forced through a semi-permeable membrane under pressure. Only water molecules are forced through the membrane and all dissolved substances remain behind.