O Level Notes : Chemistry -  Environmental Chemistry

Air and Oxygen: 

We all know that oxygen is important for human and animal life. It is even important for plant life. The process by which living organisms produce energy from their food is called respiration, and oxygen is essential for this process.

Oxygen is one of the main constituent of air. 20% of air is oxygen. The nitrogen in air is almost inert at room temperature, and after that the highest composition is of oxygen. So when any reaction says ‘react with air’, it actually means react with O₂. However, we have to study other things about oxygen and the air today.

Combustion:

This is the process of burning. When anything burns, it requires fuel, heat and oxygen. Combustion actually means burning in the presence of oxygen.

Oxides:

When oxygen reacts with elements (metals or non-metals), the oxide of that element is formed. Oxygen is very reactive, and reacts with most metals.

For example, when oxygen O₂ reacts with sodium Na, it forms sodium oxide Na₂O.

When oxygen reacts with carbon C, it forms carbon’s oxide which is known as carbon dioxide, CO₂.

Air Pollution:

The addition of poisonous gases in the environment is called air pollution. These gases include carbon monoxide CO, sulphur dioxide SO2, etc, and are called pollutant gases. Some pollutants like the oxides of sulphur and nitrogen are also acidic oxides and water soluble, so they cause acid rain and thus cause plants to die and buildings to be eaten away.

The main source of these pollutant gases entering the air is from the burning of fuels. All oils and fuels contain sulphur, and when they are burnt the sulphur reacts with the oxygen and forms sulphur dioxide gas.

When fuels like petrol and diesel are burnt in an internal combustion engine, the amount of oxygen present is limited. The carbon in these fuels reacts with the limited oxygen to form carbon monoxide, which is poisonously fatal. Had there been an excess of the oxygen, carbon dioxide would have been formed, which is a lot harmless compared to carbon monoxide. Also, the oxides of nitrogen that are released from car exhausts cause major pollution problems.

Don’t worry though; you won’t have to stop using your cars in order to save the environment! Engineers have already thought of this problem and figured out a way to reduce the pollution that our cars produce. They have made catalytic converters that are fit to car exhausts. Inside the converter is a special metal, mostly platinum, which acts as a catalyst. The function of this converter is to turn the poisonous exhaust gases we discussed earlier into harmless gases. Carbon monoxide is converted into carbon dioxide, while all oxides of nitrogen are converted into nitrogen gas. It does this by transferring oxygen atoms from the oxides of nitrogen to the carbon dioxide.

Ozone:

Ozone gas is actually three atoms of oxygen combined in form of a molecule. Even though it is useful in the atmosphere high up, it is a pollutant and harmful at ground level. It is formed when an electric spark passes through oxygen. Levels of ozone are high near railway tracks and electrified wires. It is also produced in car engines.

Ozone is harmful to the plants, and an irritant for nose and throat for humans. It reacts with ultraviolet radiation in sunlight to produce a photochemical smog, which is not fit for humans. However, up in the atmosphere it is beneficial as it helps keep out ultraviolet radiation from the sun.

Lead Compounds:

Lead compounds are usually added to petrol to make it heavier and thus more efficient. When this petrol burns, the lead particles stay as they are. These particles are then emitted out of the vehicle, and into the air. Such particles, when breathed in, can build up inside the body, and are toxic and poisonous. So these lead particles are also pollutants.

Hydrogen:

We all know by now that hydrogen is the first element of the periodic table. It is the simplest of all atoms. It just has one proton. It is the only atom that has no neutron.

The hydrogen atom has only one electron. It combines with another hydrogen atom to form a complete duplet, and thus makes a molecule of hydrogen H₂. Hydrogen in its original state exists always as a diatomic molecule, i.e. it is always in the form of H₂.

If we need to test for hydrogen, we take a lighted splint near the mouth of the suspected hydrogen. If there is a loud ‘pop’ sound, it means that the gas is hydrogen.

Water:

Water is the most common of all liquids. Life is not possible without water. It is essential for human and animal life, and also for plant growth.

Water is an extraordinary solvent, and dissolves many things. This has its advantages as well as disadvantages. Beneficial dissolved substances in water are mineral salts and oxygen. However, here are also some water pollutants that may get dissolved in this water. These include metal compounds, sewage, nitrates from fertilizers, phosphates from detergents, and crude oil from spillage. These water pollutants must be removed before consumption.

Due to these and other water pollutants, there is a need for water purification, as we cannot consume dirty water. The place where dirty contaminated water is treated and purified is called waterworks.

There are few main stages in the purification of water. Let us go through these.

In the first stage, the easily removable impurities are removed. Large impurities like leaves, sticks, etc are removed through screening, while a settling agent like aluminium sulphate is used for smaller particle impurities, so that they form small clumps and settle at the bottom, so that screening them out is easier. The water containing these clumps is left for sometime in sedimentation tanks so that the large solid particles may settle down. Next, the water is filtered through layers of sand and gravel to remove any fine solid particles. Sometimes the water is passed over beds of activated charcoal to remove tastes and odors.

After this, the water is chlorinated. The purpose of chlorine is to kill any harmful bacteria that might be present. However, it must be added carefully and in a controlled quantity so that it cannot be detected and is thus harmless if the water is consumed. Sometimes even fluoride salts are added to the water to prevent tooth decay.

Finally, the purified water is stored in places from where it can be easily pumped or gravity-fed to the taps of the consumer.

But this method is not practiced everywhere. So the water from seas and oceans is also used in many countries. But as you know, this water is salty and cannot be used in that state; therefore it must be desalinated first. To desalinate it, fractional distillation is used. We have already learnt how that is carried out.

Uses for Water:

I’ll list them up for you.

• Essential for life
• Cooling in power stations and producing steam for generators.
• Cooking and cleaning in the home
• Manufacture of food and drinks
• Building and construction industry

That is pretty much all for this chapter. I thank you students for your concentration. Good day!