The atmosphere

The atmosphere is a layer of air surrounding the earth. Air is a mixture because; it has variable composition; its constituents or components can be separated by physical means and its formation is not accompanied by heat or light (energy changes).
The composition of the atmospheric air
The atmospheric air consists of

  1. A mixture of gases mainly oxygen, nitrogen, carbon dioxide and noble gases or inert of rare gases.( the noble gases include; helium, neon, argon, krypton and xenon). It also consists of minute quantities of other gases such as sulphurdioxide, hydrogen dioxide, carbon monoxide, traces of methane and ozone.
  2. Water vapor
  3. Solid particles such as pollen grains, soot, smoke, dust particles etc.
  4. Micro organisms such as bacteria.
    The percentage composition of air by volume

How to detect the presence of atmospheric gases and water vapor

  1. Carbondioxide .When carbondioxide is passed through lime water, the lime water turns milky.
  2. Water vapor . The presence of water vapor in the atmosphere can be detected using anhydrous copper (II) sulphate or cobalt chloride paper. When water vapor gets in to contact with anhydrous copper(II) sulhate, it turns from white to blue or when water gets into contact with blue cobalt chloride paper, it turn to pink.
  3. Oxygen . Since oxygen supports burning, its presence in the atmosphere can be detected using a glowing splint. When a glowing splint is in contact with oxygen, it relights of rekindles. As well, burning e.g. of a candle takes place in the open due to the presence of oxygen in the atmosphere.
  4. Nitrogen. Nitrogen is inert and very difficult to detect. However, when oxygen and carbondioxide have been removed from the atmosphere, the remaining gases constitute mainly nitrogen.

Uses of the atmosphere
The atmosphere supports life in many ways:

The atmosphere filters off the harmful ultra violet radiations from the sun, thus protecting life.

It provides oxygen needed for respiration and combustion (burning).

Nitrogen is inert and therefore slows down various chemical reactions such as burning and rusting. Nitrogen is also needed by plants.

Carbondioxide is needed for photosynthesis by green plants.

Water vapor is necessary for rain formation. It also provides moisture to sooth our bodies and make plants cool.

Noble gases provide inert conditions for various chemical reacrtions. Other uses of noble gases include:
i) Helium is used to fill balloons because its light.
ii) Neon is used in electric sign boards for advertisement.
iii) Argon is used in electric bulbs.
Air pollution
Air pollution is the introduction of substances in the atmosphere that directly or indirectly endanger the lives of living organisms. Air pollutants (substances introduced in the atmosphere that endager life of living organisms) include:
i) Carbondioxide which is responsible for green house effect.
ii) Carbonmonoxide which is very poisonous reacting with blood and forming a stable compound (carboxyheamoglobin) and stopping it from carrying oxygen.
iii) Oxides of sulphur e.g. Suphur dioxide and sulphur trioxide which are toxic.
iv) Oxides of nitrogen e.g. nitrogen monoxide, nitrogen dioxide and dinitrogen oxide.
v) Compounds of heavy metals like leadf, mercury and cadmium are toxic and poisonous.
vi) Radioactive particles which cause abnormalities to plant and animals as a result of mutation.
Sources of air pollutants
i) Burning of fossils (coal, coke, oil fuel) produce oxides of carbon and sulphur, hydrogen sulphide and unburnt oil vapor.
ii) Industrial and domestic depositions. These produce oxides of carbon, sulphur. They may produce methane and oher heavy metals like mercury.
iii) The exhaust fumes of locomotives e.g cars contain carbon monoxide, carbondioxide, nitron dioxide, unburnt fuel and some heavy metals like lead.
iv) Nuclear explosions produce radioactive particles.
v) Tobacco smoking
vi) Natural processed like volcanic eruption.

vii) Gases released from manure in farm land.

Effects of atmospheric pollution

  1. Causes respiratory diseases e.g. cough, lung cancer e.t.c.
  2. Formation of fog which distracts vision and may also be poisonous.
  3. Formation of acid rain due to the presence of sulphur dioxide, sulphur trioxide, carbon dioxide e.t.c.
  4. Darkens buildings especially those painted using lead paints.
  5. Causes global warming, particularly carbondioxide and other green house gases like methane.

COMBUSTION
This is a chemical reaction in which a substance combines with air to form energy. Combustion can be slow (with no flame) or rapid (rapid combustion produces flame). Examples of rapid combustion are: burning bush and burning charcoal while examples of slow combustion include: digestion of food, rusting of iron, and fermentation/decay.
Experiments on combustioon
Experiment to determine the percentage of air used in combustion
i) Using phosphorus
Drawing of setup

Procedure
Place a small piece of phosphorus in a crucible, float the crucible in a water trough and place a bell jar over it.
Measure and record the height of air (X cm) in the bell jar.
Ignite the phosphorus

Observation
The phosphorus burns with a bright yellow flame producing dense white fumes. The water level drops for a while due to the increased pressure as a result of expansion of air, then rises as the white fumes dissolve in the water.
When phosphorus stops burning and the water level comes to rest, measure the height of air left in the bell jar (Ycm).

image 7


Conclusion
The percentage volume of air used in the burning of phosphorus was 20.8% indicating that the gas used was oxygen. The remaining gas in the gas jar was mainly nitrogen.
(Use this data and calculate the percentage of air used up in the combustion of phosphorus: Y= 55.8cm, X= 70.5cm)

image 4

Procedure
Place some copper turnings (small pieces of copper) in a combustion tube and connect the combustion tube to two syringes (each 100cm3 in volume) as shown above. One of the syringes must be full of air and the other empty.
Record the volume of air, Y cm3 in the syringe.
Heat the copper turnings strongly while passing air over it from one syringe to another until there is no further change in the volume o air in the syringes.
N.B. The glass wool allows air to easily be blown from one syringe to another without the copper turnings entering the syringes.
Allow the apparatus to cool and record the remaining volume of air in the syringe, Y cm3.

Observation
The copper turnings turn from brown to black as the copper reacts with ooxygen to form copper(II) oxide which is black. The volume of air in the combustion tube and the syringes reduced as part of the air is used up during the process of combustion.
Results

image 6


Conclusion
The percentage volume of air used up during the combustion was 20.8%, indicating that the gas was oxygen since it occupies approximately 21% of the air. Oxygen is there fore the active component of air used during combustion.
(Use this data and calculate the percentage of air used up in the combustion of copper: Y= 100cm3, X= 80cm3)
Experiment to determine products formed when hydrocarbons burn in air

Experiment to determine products formed when hydrocarbons burn in air
A hydrocarbon is a compound containing hydrogen and carbon only. Examples of hydrocarbons include: candle, natural gas like methane (CH4),petrol and paraffin.
Products of combustion of a candle
Set up of apparatus

Procedure
The arrangement of the apparatus is made as above and the candle lit.
As the candle burns, the pump is switched on to suck the gaseous products of the burning candle through the glasas funnel into the setup.
Observation
A colorless liquid is collected in the U tube that turns white anhydrous copper (II) sulphate to blue, indicating that it is water. Lime water in the test tube turned milky indicating the presence of carbondioxide.

Conclusion
When a candle burns water and carbondioxide are produced as the only products showing that the gas used up in the combustion is oxygen.