Air and Air Pollution

Oxygen in the Atmosphere

Despite occupying just about 21% (one-fifth) by volume of air, oxygen is the most active components of air. It is very important for cellular respiration in all living organisms, and responsible for the rusting of iron and burning (combustion) of a substance in air.

Determination of Proportion of Oxygen in Air

To determine the proportion of oxygen in air, and to show that oxygen supports combustion, the following experiments can be performed.

Burning of Phosphorus in Air

Because phosphorus is a very reactive solid non-metal, it burns readily in oxygen to give an oxide

4P_(s) + 5O_2(g) à 2P₂O_5(s)

Procedure:

i) Fill a graduated glass tube with pure water, and invert into a trough of water. Adjust the water level in the glass tube, so as to be the same as the water level in the trough. Allow the air inside the tube to be saturated with water vapour, then, note the volume of air enclosed above the water level in the tube (x cm³)

ii) Carefully introduce int to the tube, a piece of yellow phosphorus on a wire.

Observation:

As the phosphorus smolders, the water level in the glass tube rises, until oxygen in the enclosed air has been used up, leaving nitrogen, carbon dioxide, and the noble gases.

iii) When the phosphorus ceases to smolder, remove it. Then, carefully adjust the water level in and outside the glass tube until they are the same. Read the volume of mixture of gases left (y cm³)

iv) The volume of oxygen used up is (x – y) cm³

v) the percentage of oxygen in air will be

Within the limit of experimental error, the percentage of oxygen by volume will be about 20.0%.

Using Alkaline Pyrogallol Solution

Alkaline pyrogallol which is a solution of pyrogallol in sodium hydroxide solution, readily absorbs oxygen

Procedure:

a) Put few drops of alkaline pyrogallol solution in a graduated glass tube, and cover it with an air tight glass. Then, shake the set-up for a while.

b) Carefully invert it over the water in a trough; then, remove the glass cover.

Observation:

The water – level rises in the glass tube as the alkaline pyrogallol absorbs the oxygen enclosed.

The volume of water inside the tube corresponds to the volume of oxygen absorbed. It will be about 20% by volume of the air enclosed initially.

Combustion of Substances in air

Most substances burn in air. Burning or combustion is a chemical reaction which is frequently accompanied by the production of light and heat.

To show the oxygen is present in air and supports combustion;

i) Invert a gas jar over a burning candle stick placed in a trough of water. Note the level of water in the jar.

Observation:

The water level falls initially due to the expansion of air in the jar, then, starts to rise. When all the oxygen has been used up, the candle goes out.

ii) the rise in the level of water shows the volume of oxygen in the volume of air enclosed. It will be about 20% by volume of the total air enclosed initially.

The experiment shows that oxygen is the only reactive component of the atmospheric air.

During combustion, flames are produced. A flame can be described as a region where gases combine chemically, with the production of heat and light.

Flames are not homogeneous, but are composed of several defined zones. The type of flame produced depends on the nature of the substance that is burning. A flame may be luminous or non-luminous. The luminosity of a flame is caused by the presence of solid-particles in the flame. An increase in the temperature or pressure of the burning gases also increases the luminosity of a flame.

Examples of Flames

1) Hydrogen flame:

Hydrogen burns with a very faint, non-luminous flame. The structure of the hydrogen flame is simpler, consisting of only two regions, the unburnt gas zone and the zone of complete combustion.

2) Candle Flame:

A candle flame burns with luminous flame. Four zones are identifiable in the flame:

i) the zone of unburnt gas around the wick

ii) the bright yellow luminous zone where there is incomplete burning of the hydrocarbon due to insufficient air supply. The luminosity is caused by the presence of unburnt carbon particles

iii) the barely visible, non-luminous zone on the outside – where complete combustion of carbon particles takes place because of a plentiful supply of air from the atmosphere.

iv) the blue zone at the base of the flame which is also a region of complete combustion

3) Bunsen flame:

A Bunsen flame is usually produced by a Bunsen burner. A Bunsen burner is built with an air inlet at the base of the burner so that a stream of air can be supplied to the flame together with the fuel gas. This supplements the external supply of air, and allows a more complete combustion of the fuel. The air-hole can also be adjusted to give a flame with the required luminosity.

To produce a luminous Bunsen flame, the air-hole at the base of the burner tube should be closed. The flame produced is high and wavy, with a large, bright yellow zone within it. It is not hot, and deposits soot on the surface of any object held in it. Like the candle flame, the luminous Bunsen flame has four zones.

To produce a non-luminous Bunsen flame, the air-hole should be kept open, the flame us non-luminous, much hotter, cleaner and more compact than the luminous Bunsen flame. Only three zones can be seen in the flame:

i) the unburnt gas zone, much reduced in size when compared to that of the luminous Bunsen flame.

ii) the luminous zone, also much smaller

iii) the outermost non-luminous zone which has increased in size because the flame has a sufficient air supply.

Note: if the air-hole of a Bunsen burner is opened too widely or the fuel supply is slowly turned down, the rate of combustion may exceed the rate of supply of the fuel mixture with the result that the flame enters the tube to consume any oncoming fuel. This phenomenon is known as striking-back. It can be corrected by making the air-hole smaller to reduce the air supply.

Corrosion of Metals

The corrosion of metals is as a result of the combined action of atmospheric oxygen and water, which is usually accelerated by the presence of carbon (IV) oxide and gaseous pollutants like sulphur (IV) oxide in the air.

A typical example of metallic corrosion is that of iron. The corrosion of iron is commonly known as rusting. Iron rusts by combining with oxygen in the presence of water to form brown hydrated iron (III) oxide which is commonly known as rust (Fe₂O₃.XH₂O). Rust is soft and easily breaks off. When this happens, the metal below begins to rust too.

Therefore, if iron is exposed to moist air long enough, it will rust completely.

4Fe_(s) + 3O_2(g) + 2XH₂O_(l) à 2Fe₂O₃.XH₂O_(s)

Carbon (IV) Oxide in the Atmosphere

The atmosphere contains about 0.03% by volume of carbon (IV) oxide which is kept fairly constant by a balance in nature i.e., the carbon cycle. The following processes are generally responsible for the balance in the volume of carbon (IV) oxide in the atmosphere:

i) Respiration in Living Organisms:

Respiration which is a catabolic (breaking down) process is the process whereby living things use oxygen from the air to oxidize food substances (mainly glucose) in their body cells to release energy. Carbon (IV) oxide is produced as a waste product and liberated into the surrounding air.

C₆H₁₂O_6(s) +     6O_2(g)       à   6H₂O_(l)   +        6CO_2(g)          +            Energy

             Glucose          Oxygen             Water         Carbon dioxide

ii) Photosynthesis in Plants:

Photosynthesis which is an anabolic (building up) process is a process whereby green plants manufacture carbohydrates (glucose) from atmospheric carbon (IV) oxide and water from the soil, in the presence of sunlight which provides the necessary energy for the reaction. The process occurs in the chlorophyllous cells of the leaves and stems only. Oxygen is produced as a waste product and released in the atmosphere.

Nitrogen in the Atmosphere

Nitrogen occupies about 78% by volume of the atmospheric air, making it the major constituent. Gaseous nitrogen is relatively inactive but it acts as an important diluent of air to slow down combustion and corrosion.

The Noble Gases in the Atmosphere

The noble gases occupy about 1% by volume of air. They include helium, neon, argon, krypton, xenon and radon. Of these gases, argon is most abundant in the atmosphere.

Uses of the Noble Gases

1) A mixture of Krypton and xenon is used in the photographic flash tube for taking high speed pictures

2) Argon is used in gas – filled electric lamps to help prevent the oxidation of the lamp filaments

3) Helium, neon and argon are used in advertisement signs because they usually give out coloured lights when a high – voltage current is passed through them under low pressure. Below is a table with the different colours each will give out:

Noble Gas	Colour Given Out
Neon	Reddish – orange colour
Argon	In the presence of mercury, gives a blue colour
Helium	White light which appears golden through a yellow tube

Note: Neon light are usually used in aerodrome beacons because they have a high power of fog penetration.

4) Radon being radioactive is used in medicine for treating some forms of cancer.

Water Vapour in the Atmosphere

The quantity of water vapour in the atmosphere varies depending on the season and geographical location. The presence of water vapour in the atmosphere is due to the evaporation of water from the water bodies, and can be detected by exposing a substance that absorbs moisture from the atmosphere, e.g., pellets of NaOH (or KOH).

Air in Dissolved Water

All the natural water contains dissolved air because their surfaces are exposed to the atmospheric. Air is sparingly soluble in water, dissolving more readily in cold water than in warm water.

The composition of dissolved air is markedly different from that of atmospheric air. This is due to the different solubility of the constituent gases in water. The percentage composition by volume of dissolved air in water is as follows:

Constituent	Percentage by Volume
Nitrogen	63.9%
Oxygen	33.2%
Carbon (IV) oxide	2.9%

 

Exposure of Certain Substances to Ordinary Air

When certain substances are exposed to the atmosphere, they either gain or lose weight. The three categories of substances are:

1) Deliquescent Substances:

These are substances which usually absorb moisture from the atmosphere and form a saturated solution. Such a substance is said to be deliquescent. Deliquescent substances gain weight, when exposed to the atmosphere, e.g., fused CaCl_2(s), FeCl_3(s); pellets of NaOH(s) or KOH(s), and P₂O_5(s).

2) Hygroscopic Substances:

These are substances that usually absorb moisture from the atmosphere without forming a solution. Such substances are said to be hygroscopic.

Hygroscopic substances gain weight when exposed to the atmosphere, e.g., NaCl, CaO, CuO, fresh iron nails, and CuSO₄ (anhydrous).

3) Efflorescent Substances:

These are crystalline substances which on being exposed to atmospheric air usually lose their water of crystallization to the atmosphere. They are said to be efflorescent.

The commonest hydrated compound, which is efflorescent is Na₂CO₃.10H₂O. On exposure, the crystals lose their water molecules to the atmosphere to form powdered anhydrous compound, Na₂CO₃.H₂O.

Note: Deliquescent and hygroscopic substance are used as drying agents for gases, liquids and solids, since they are capable of absorbing moisture.

Fractional Distillation of Liquid Air

This separation technique is used to obtain nitrogen and oxygen from liquefied air. When atmospheric air is compressed to about 200 atmospheres, heated, and then cooled, and finally allowed to expand, it turns to liquid at – 200^oC. Nitrogen boils first at – 196^oC, being more volatile than oxygen, which boils at – 183^oC.

Air Pollution

Air pollution is a term used to describe a phenomenon in which certain substances that are harmful to living things are present or released in large excess, in the air. Such substances are called pollutants.

Types of Air Pollution

There are two forms of air pollution:

1) Natural Pollution:

Pollution caused by the combustion of fuels (such as coal, petrol, kerosene, and gases), volcanic eruptions, forest fires, and biological activities. The substances involved contain carbon, nitrogen and sulphur.

2) Environmental Pollution:

Pollution due to high human population, agricultural activities, industrialization and chemical warfare.

Air Pollutants

The main air pollutants are

i) Particulate matter (tiny solids)

ii) Oxide of carbon

iii) Oxides of sulphur and nitrogen

iv) gaseous hydrocarbons and chlorofluorocarbons

Particulate Matter (Tiny Solids):

Smoke, soot and dust mainly emitted by industrial processes and the burning of coal and wood fires damage the respiratory system, especially the lungs. Lead and lead (ii) bromide which is emitted from the exhaust of moto-cars using leaded petrol can cause lead poisoning and injury to the nervous system.

Oxides of Carbon:

Carbon (II) oxide which is a colourless, odourless, poisonous gas is produced in large amounts when the carbon in fuel undergoes incomplete combustion.

Oxides of Nitrogen and Sulphur:

The oxides of nitrogen and sulphur dissolves in rain water to produce acids, resulting in acid rain which is harmful to plant and animal life, buildings and metal structures.

Gaseous Hydrocarbons and Chlorofluorocarbons:

Many hydrocarbons are believed to cause cancer. Hydrocarbons reacts with nitrogen oxides in the presence of light, to produce smog or heavily stagnant air.

Chlorofluorocarbons popularly called CFC are a group of man-made chemicals such as Freon which are used as propellants for spray cans, etc. Chlorine atom in CFC is usually broken off by the UV light in the atmosphere, and then react and attacks a molecule of ozone, O₃, repeatedly, thus causing the depletion in the ozone layer which is found in the stratosphere.

Control of Air Pollution

The type of measures to be put in place to control air pollution is determined by the type of air pollution being experienced. Below are some of the measures and regulations that can be enacted to regulate and reduce air pollution.