2.6  Air

What is air? Air is the general term that we use to refer to the mixture of gases that is our atmosphere. Now you may ask what is atmosphere?

Atmosphere is a layer of different types of gases that sort of covers our planet like a blanket. The atmosphere of earth is held in place by gravity. It is thanks to our atmosphere (and other conditions) that life on earth has flourished.

Composition of Atmosphere

Air consists of several different types of gases. The most abundant gas in air is nitrogen. Approximately 78% of unpolluted dry air is nitrogen. 21% is oxygen, 0.9% argon and 0.01% carbon dioxide. It also contains very small amounts of neon, helium, xenon, hydrogen and krypton. If the air is polluted (air in busy cities and factories)  then it will contain other types of gases like carbon monoxide, sulfur dioxide and water vapor. Water vapor in the air accumulates to form clouds in the atmosphere.

The availability of oxygen in our atmosphere is one of the several components that has helped life to survive and evolve on this planet. The energy production system (respiration) and other major reactions in our body are based on this gas and without it we will die in a matter of minutes. The amount of oxygen in the atmosphere is just right for ecosystems to thrive.

Carbon dioxide is another important gas that we depend on for our survival. It is a green house gas; it absorbs infrared radiation that is reflected from the earth's surface. By absorbing this radiation it heats up and transmits this heat in all directions into our atmosphere and warms it. There is just enough CO2 in our atmosphere to warm up the earth for life to survive. However, due to human-caused pollution, CO2 has risen over the years. This causes more heat to be absorbed, which results in the earth heating up further (global warming). This is why it is important to be aware of how our actions can negatively impact the conditions set on earth for life to thrive.

It is important to note that the above numbers are only approximate because the composition of our atmosphere changes depending on location and climate. Some areas are below sea level while others are above sea level. Some areas are cold while others are hot. Therefore, the composition is not standard for everywhere on earth.

We discussed earlier that gravity holds our atmosphere is place. But gravity's strength decreases the further we move away from the earth's core. Therefore as you travel higher and higher, gravity's hold on the different gases in our atmosphere weakens. This is why our atmosphere thins out, air compositions change and temperature decreases as you climb higher and higher.

The atmosphere can be divided into several layers:

Troposphere

The troposphere is the lowest part of our atmosphere. It is where us and all living things live. The clouds, seasons, temperature, amount of oxygen are all just right for us to survive. This layer is approximately 10 to 18 km (depending on your location) thick measured from the earth's surface.

Stratosphere

The stratosphere layer has an outer boundary that is about 50 km away from the earth's surface. This layer contains the ozone (O3) layer, which protects us and other living things from harmful radiation emitted from the sun. The ozone absorbs these high frequency rays and prevents them from passing through. Since it absorbs all this radiation, the temperature in this region increases with height (i.e the closer it gets to the sun, the higher the sun's intensity) to a maximum of about -2.5 C . As a result, the ozone also prevents the atmosphere below it from heating up. This is why it is important that we do our best to preserve this natural protection by reducing pollution.

Mesosphere

This layer's outer boundary is about 80 to 90 km from the earth's surface. There is no ozone in this layer to absorb radiation; therefore, the temperature decreases to a minimum of about -86.5 C.

Thermosphere

The thermosphere's outer boundary extends to about 600 km above the earth's surface. This layer, similar to the troposphere, heats up with height. It has a high temperature of about 1200 C which much greater than that of the troposphere. This layer actually absorbs most of the harmful radiation emitted by the sun. It makes the ozone's job a lot easier. 

You may wonder however that if temperature rises up to 1200 C, how can rockets and satellites actually pass through this layer safely without combusting. They pass through safely because of the weak gravity at such heights. We discussed earlier that as gravity gets weaker with height the amount of air or atmosphere it can keep wrapped around earth decreases. This means that the density of the atmosphere in the higher layers of atmosphere is very small compared to the lower layers of the atmosphere. Since there is very few particles up there, heat is not transferred very efficiently. This means that you would have very small areas that are extremely hot but the rest of the larger areas are cold since energy is not transferred from the hot points.  So if you would go up there without any protective clothing you will not burn to death but probably freeze to death.

Exosphere

This layers starts from about 600 km above the earth's surface and extends to probably a maximum of 190,000 km in some areas. This layer has a very small amount of particles since gravity is weakest in this layer. The particles in this layer either break free and float away or is attracted by the layers below the exosphere since they have stronger gravity.

% of Oxygen

We talked earlier about how the % of oxygen in our atmosphere is 21. But can we show or prove this fact? We can prove this by doing some simple experiments in the lab. 

We know that a lot of elements and compounds react and combust with oxygen to form oxides and other compounds. So in theory, if we can place such a reactant into a container with a fixed amount of air and heat it, it should react and use up all the oxygen. When all the oxygen is used up the reaction would stop. We can then measure the amount of air left and calculate how much oxygen was present in air.

One such experiment involves using copper. Take a small tube and place some pieces of copper wire in it. It is very important to pack the tube with copper so that the air in the tube is displaced. Then take two syringes. Pull the plunger in one syringe to fill it with 100 cm^3 of air.

 

Leave the plunger pushed in for the other syringe so that it doesn't contain any amount of air. Then connect the two syringes to either end of the tube. Now heat the copper in the tube by using a Bunsen flame or other heat source.

 

While it is heating start pushing the plunger in the filled syringe. As you push, air will flow through the tube into the empty syringe. Now push the plunger of the syringe that was initially empty. Do this back and forth  for about 3 minutes or until you see that all the copper is starting to turn black.

 

After that duration push all the air left into one syringe and you will notice that there is a drop in the volume. Repeat this process until there is no change in the volume of air. If you observe the final volume, you would notice that there is about 79 cm^3 of air left. The rest of the 21 cm^3 was the oxygen that was used by copper to burn and turn into copper (II) oxide.

A similar experiment to this is done by using damp iron fillings. The damp iron fillings are allowed to rust by reacting with the oxygen present in the fixed amount of air in the system. Rusting is a a slow reaction; therefore, we will have to leave it for about a week before we can measure the amount of air left.

Extraction and Uses

The different components in air are very useful. Therefore extracting them from air is very important for the manufacturing and chemical industries. Doing this in the most cost effective way is essential if they are to make profits. The best way to this currently is by using fractional distillation. We discussed this separation method previously in the 'Compounds and Mixtures' section under 'Principles of Chemistry'. If you need to revise separations you can read that chapter.  

Lets looks at at some of the gases we find in our atmosphere:

Nitrogen

Nitrogen is used to make fertilizer and ammonia. Ammonia is very important compound used in the production of pesticides, dyes, refrigerant gases, plastics and even explosives.

Since it is a noble gas it is used as a protective layer to preserve substances that could react with air. For example, it is widely used in food packaging to prevent biscuits and crisps from going stale. Since it also has a very low boiling point it can be used to quickly freeze foods. 

It is also used to freeze water pipes before making repairs on them.

Oxygen

Very important part of rocket fuel.

Stored in pressurized tanks for use in hospitals, life support systems and use in areas with a very low or zero supply of oxygen.

It is also used when converting soft iron to steel.

Pollution

This is one of the major problems that earth has had to face due to human activity. Pollution caused by plastics and waste has polluted fresh water rivers and destroyed countless ecosystems. Air pollution caused by burning fossil fuels has degraded the quality of air we breathe and also negatively affected the earth's climate.

 

Even though developed countries are capable of handling these climate changes (hurricanes, torrential rains and flooding) to a certain extent, developing countries struggle to deal with these disasters. Countless families become homeless, crops are destroyed and expensive damages to the country's infrastructure are incurred. Therefore, it is important that we become more conscious about how our actions could impact the world. We should try to make an effort to reduce the amount of pollution that we cause individually. We could take shorter showers, use water sparingly, use the air conditioner only when necessary, turn off electricity when not in use, try to walk or bike when travelling short distances, recycle more, always throw away garbage into the designated bins etc. 

Now let us look at some the pollutants released into our atmosphere:

Greenhouse Gases

Greenhouse gases such as methane and CFCs (chlorofluorocarbons) are very bad for the environment. They prevent heat from escaping the troposphere, causing temperatures to rise (global warming).

 

Methane is actually the cleanest fossil fuel but when released into the atmosphere without burning, its chemical properties enable it to absorb a lot of the sun's radiation that is reflected from the earth's surface. It keeps all that heat trapped within our atmosphere. Large landfill sites and animal farms release a lot of methane.

 

CFCs are the gases that are used in refrigerators, air conditioners and aerosol cans. These gases are actually nonreactive and safe for humans but when it leaks into the atmosphere and it cause the same problem as methane.

Sulfur Dioxide

SO2 is an acidic and colorless gas. It is formed as a byproduct in power plants and volcanoes when sulfur is burned. This gas reacts with water vapor (clouds) and air to form sulfuric acid. This leads to acidic rain which corrodes buildings and also reduces pH in rivers harming aquatic life. It is also absorbed by plants which could damage and kill them.

Volcanoes were the major source of SO2 pollution before the industrial revolution.

Carbon Monoxide

CO is a colorless and odorless gas that is primarily formed when there is not enough oxygen during combustion. When a combustion reactions such as burning of fossil fuels is allowed to happen in an area with low amounts of oxygen ( car engines, indoor furnaces, etc.), CO cannot be oxidized into CO2. Therefore, CO concentrations increase in such cases.

CO is extremely toxic to animals in that it starves the respiratory system of oxygen. CO binds with hemoglobin and prevents it from carrying oxygen. Therefore, even low concentrations of this gas in the presence of enough oxygen could suffocate you to death.

Nitrogen Oxides

These gases are acidic. They are mainly produced in electric power plants, diesel engines and other industrial activities. The two main gases from this group that is very harmful to the earth are nitric oxide and nitrogen dioxide. 

They are also greenhouse gases that trap heat in our atmosphere. Similar to sulfur dioxide they can dissolve in water vapor to give acidic rain. They can damage our respiratory system by causing infections and irritations.

End

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