ZOL2601 EXAM PREP
UNIT 1.1 How water and air differ as
mediums
Are all greenhouse gases equally effective at trapping heat?
No. Greenhouse gases differ in their ability to trap heat.
How long do greenhouse gases last in the air?
Once it's added to the atmosphere, it hangs around, for a long time: between 300 to 1,000 years.
Is climate change the same things as global warming?
Global warming refers only to the Earth's rising surface temperature, while climate change includes
warming and the “side effects” of warming—like melting glaciers, heavier rainstorms, or more
frequent drought.
1.1.1 Assessment
1. What is the composition, given in percentages, of dry atmospheric air?
Oxygen 20.95 %
Carbon dioxide 0.03 %
Nitrogen 78.09 %
Argon 0.93 %
2. Describe and explain how the composition of air (percentage-wise) is influenced by
a) The combustion of fossil fuels:
It is estimated that a doubling of the atmospheric carbon dioxide content would increase
world temperature by 1.3 °C if atmospheric water remained constant
b) microhabitats such as burrows:
oxygen as low as 15% or even less. The carbon dioxide content is increased, but not
necessarily to the same extent. However, carbon dioxide may rise to above 5%,
c) soil composition
Soil may contain oxidizable material that can severely deplete the oxygen such as
organic matter and iron sulphide. These oxidation processes depend on temperature,
humidity, and other factors, as well as on the amount of exchange with the atmosphere
d) relative humidity
When air is saturated with water vapor, we say that the relative humidity (r.h.) is 100%.
If the air contains less water vapor, the humidity can be expressed as a percent of the
amount required for saturation at that temperature;
e) altitude
, At sea level, where atmospheric pressure is 760 mm Hg and 20.95% of this is oxygen, the
partial pressure of oxygen in dry air is 159 mm Hg (21.2 kPa). At 6000 m the atmospheric
pressure is half that at sea level, or about 380 mm Hg (50.7 kPa). The partial pressure of
oxygen is also half that at sea level, or about 80 mm Hg or 10.6 kPa (20.95% of 380 mm
Hg or 50.7 kPa)
3. The lower the temperature, the less water vapour air contains The coldest valleys on the
Antarctic continent are the driest places on Earth – even drier than warm deserts like the
Namib and the Sahara! Note in table 1.1.1 the partial pressure of water vapour (PH2O) in
millimetres mercury (mm Hg) at 0°C, 20°C, 37°C and 100°C. What are some of the
implications you can think of for humans in cold environments?
the absolute amount of water in cold air is small. Therefore, if saturated outside air in winter
enters our houses and is heated, the indoor relative humidity will be extremely low
(although the absolute humidity of the air is unchanged), and we say that “the air is very
dry”. This dryness causes moist surfaces and mucous membranes to dry out, often to great
discomfort of sensitive persons.
4. How would a 60% increase in the relative humidity (RH) of the atmosphere affect
atmospheric composition?
5. Explain why humans cannot use even pure oxygen to breathe at altitudes higher than 19 000
m above sea level.
1. Give the solubilities of O2, N and CO2
Oxygen 34.1 ml O2 per litre
Nitrogen 16.9ml CO2 per litre
Carbon Dioxide 1019 ml CO2 per litre
2. Name and discuss the four factors that determine how much gas can dissolve in water
Solubility. Solubility of gases in water are important because it determines the difficulty or
ease with which organisms are able to extract oxygen from the medium.
Type of gas
The effect of pressure Henrys law
Temperature. The colder the temperature, the more gas is dissolved
3. Calculate the quantity of oxygen in a bucket of water (sea level, 10.7 l, 15°C). How long will it
take for a striped bass (Morone saxatilis) of 1.5 kg to consume all the oxygen in the bucket if
it consumes oxygen at a rate of 100 ml hr-1 kg-1?
, 4. Refer to figure 1.1.3 and answer the following questions:
a. To what can the difference in solubility be ascribed?
b. Why is there a greater density of animals and plankton in the colder oceans (such as along
the West Coast of Southern Africa) compared to the warmer oceans?
This is why phytoplankton prefer higher density water (cold and salty water): high density
water helps them float or, failing that, it pushes them upwards harder, so they sink very
slowly.
5. Discuss the quantity of CO2 in sea water
In sea water, which is slightly alkaline (pH = ca 8.2), the total amount of carbon dioxide may
range between 34 and 56 ml CO2 per liter sea water, yet the amount of carbon dioxide
present as dissolved gas is still that which is in equilibrium with the atmosphere, about 0.3
ml CO2 per liter;
6. Graphically represent and explain the changes in pH, PO2 and PCO2 in a tidal pool during a
full day-night cycle
When the pool was isolated from the sea during the day, the PO2 increased from the normal
level of sea water to as much as 555 mm Hg by mid-afternoon. The carbon dioxide used for
photosynthesis was taken from the sea water, but as the amount dissolved is small, most of
