Download Organization of unit 2

Unit 2 The atmosphere
Teaching Notes
2.2
Classification of matter: pure substance and mixture
Homogeneous and heterogeneous mixtures
Mixtures can be homogeneous or heterogeneous. Homogeneous mixtures have uniform composition
and properties throughout. For example, when sugar dissolves in water, it forms a solution in which the
sugar and water are uniformly mixed. The mixture is therefore homogeneous. A container of air is
another homogeneous mixture.
Heterogeneous mixtures are those which are not uniform in composition and properties throughout.
For example, granite is a heterogeneous mixture consisting of the minerals quartz, feldspar and mica. It
is usually possible to identify these components by the eyes. Concrete is a heterogeneous mixture
consisting of cement, sand and rock.
2.3
Elements and compounds
Main differences between a mixture and a compound
Summary of difference between a mixture and a compound
Mixture
Compound
1 Composition by mass
The amounts of the different
substances in the mixture can vary
The amounts of the elements in
the compound are in a fixed ratio
2 Separating the
constituents
Constituents can be separated by
simple physical methods, such as
evaporation or distillation
Can only be separated into its
constituents by chemical methods
3 General properties
Properties are similar to the
substances making up the mixture
Properties are very different from
the elements in it
4 Melting point and
boiling point
Does not have a sharp melting point
or boiling point
With sharp melting point and
boiling point
5 Energy change during
formation
No or little energy change when a
mixture is made
Energy is usually given out or
absorbed when a compound is
formed
2.4
The atmosphere
The four layers of the Earth’s atmosphere
The four layers of the Earth’s atmosphere are the troposphere, the stratosphere, the mesosphere and
the ionosphere.
We live in the troposphere with trips into the stratosphere. Only astronauts pass out of the
atmosphere.
The atmosphere in balance
All living things depend on oxygen, nitrogen, carbon dioxide and water vapour. These substances are
recycled.

The process of photosynthesis and respiration recycle oxygen.
oxygen in air
oxygen taken in
during respiration
oxygen given off
during photosynthesis
plants build up
starches from glucose

animals break down
plant starches to glucose
during respiration
Nitrogen circulates between the air, the soil and living things in the nitrogen cycle.
nitrogen
in air
proteins in
living things
bacteria,
lightning,
fertilizers
decay
nitrates in soil
denitrifying
bacteria

Carbon dioxide circulates between the air, the soil and living things ?and in and out of the ocean ?in
the carbon cycle.
carbon dioxide in air
bruning
fossil fuels
respiration
Photosynthesis
deca
y
carbon compounds
in plants
carbon compounds
in animals
carbon compounds in fossil fuels

Water circulates between the air, the ocean and living things in the water cycle.
water vapour
in air
condensation
evaporation
rain
ocean
In the last 200 years (ever since the Industrial Revolution) the balance of carbon dioxide has been
upset by the burning of too much fossil fuel. Most scientists agree that this is the cause of global
warming.
2.6
Separating oxygen and nitrogen from the air
Uses of oxygen

Manufacturing steel - Oxygen is blown through the molten iron to remove carbon and sulphur
impurities. The carbon and sulphur are converted to carbon dioxide and sulphur dioxide and escape
as gases.

Welding and cutting - Oxygen is used in oxy-acetylene welding and cutting. When acetylene burns
in oxygen, the temperature reaches 3 200 ℃. This is hot enough to melt most metals which can
then be cut or welded together.

Breathing apparatus - Oxygen is used in life-support machines in hospitals. It is also mixed with
anaesthetizing gases during surgical operations. Mountaineers and deep sea divers also use
oxygen.

Space rockets and shuttles - Oxygen is carried in space rockets so that the hydrogen and kerosene
fuels can burn. Space shuttles use oxygen gas in fuel cells which convert chemical energy into
electrical energy.
Uses of nitrogen

Manufacturing ammonia and nitric acid - Nitrogen is used in large quantities in the production of
ammonia gas, which is used to produce nitric acid. Nitric acid is used in the manufacture of dyes,
explosives and fertilizers.

As a refrigerant - Liquid nitrogen is used as a refrigerant. Its low temperature makes it useful for
freezing food quickly.

Providing an inert atmosphere - Nitrogen is used as an inert atmosphere for some processes
because it is unreactive. For example, empty oil tankers are filled with nitrogen to prevent fires.
2.9
Air quality in Hong Kong
Chemistry and Society
The disappearing ozone layer
Why is the ozone layer so important?
Ozone is a gas found in the Earth’s atmosphere some 10 to 30 km above the Earth. This layer of ozone
gas absorbs some of the sun’s radiation that is harmful to living things. Too much of the harmful radiation
can cause skin cancer, as well as other health problems.
The ozone layer is breaking down
In 1986, scientists discovered two holes in the ozone layer - a small hole over the North Pole, and a
much larger hole over Antarctica. Scientists reported that the holes have grown bigger and the ozone
layer has become thinner all around the Earth.
October 1979
October 1990
October 1996
October 1982
October 1991
October 1997
October 1984
October 1992
October 1998
October 1986
October 1993
October 1999
October 1988
October 1994
October 2000
Dobson units
100
500
Chlorofluorocarbons
A possible cause of the holes is a group of chemicals called chlorofluorocarbons (CFCs). CFCs are used
in automobile air conditioners, in some aerosol sprays, in refrigerators and foam packaging for fast-food
containers. CFCs enter the atmosphere when these products are made and used. In the upper
atmosphere, CFCs are broken down by ultraviolet radiation, releasing chlorine atoms. These chlorine
atoms destroy ozone.
Government action and future trends
International concerns regarding ozone depletion has led governments around the world to phase out
the use of CFCs. Recently, CFCs have been replaced as aerosol propellants by hydrocarbons and the
use of manual pressure packs. Alternative chemicals have been developed for use in refrigeration and
air-conditioning systems. However, some scientists say that even if we stop using CFCs today, the ozone
layer would continue to thin for the next 10 years.
How can you help?
You may feel that you cannot change laws or influence the policies of international companies. However,
politicians and business people can be affected by the way you act. If people stopped buying products
containing CFCs, companies that make them would need to find an alternative ?and that process is
already starting. Imagine you have to buy a box of eggs. If you buy eggs packed in a recycled paper
container, instead of in a foam container that contains CFCs, you have made a decision that will help the
environment.
Questions
1
Suggest how you can help to protect the ozone layer.
2
Visit the website of the Environmental Protection Department of Hong Kong Government
http://www.epd.gov.hk/epd/
Find out the status of phasing out of ozone depleting substances in Hong Kong.
Notes for questions
1
Students may suggest the following points:

Buy air-conditioning equipment that does not use CFC as refrigerant.

Conduct regular inspection and maintenance of air-conditioning and refrigeration appliances to
prevent and minimize refrigerant leakage.

Replace air-conditioning and refrigeration appliances operating on CFC refrigerant with those
that use non-CFC refrigerant.
2
Phase out schedule of ozone depleting substances in Hong Kong
Ozone depleting substance
Schedule
Halons
Import for local consumption banned by 1.1.1994
CFCs
Carbon tetrachloride
1,1,1-trichloroethane
Import for local consumption banned by 1.1.1996
HCFCs
Freeze consumption at base level starting 1.1.1996
Import for local consumption banned by 1.1.2030