In the previous section, we completed a discussion on hydrocarbons. In this chapter, we will see environmental chemistry.
• We know that, by the word environment, we mean our surroundings.
• The surroundings include:
♦ living things (plant and animals)
♦ non living things (air, water, soil, sunlight, buildings, factories etc., )
• Many chemical species are present in the environment. They may be:
♦ Naturally occurring (oxygen, nitrogen, nitric oxide are examples)
♦ Artificial (cement, sulphuric acid, petrol, diesel are examples)
◼ Environmental chemistry deals with five questions:
(i) How the chemical species originate in the environment ?
(ii) How the chemical species are transported from one place to another ?
(iii) How the chemical species react with each other ?
(iv) What are the effects of these chemical species on the environment ?
(v) Where do these chemical species end up, after completing their role in the environment ?
Environmental pollution
• Sometimes undesirable effects occur in our surroundings. Such effects may be harmful for plants and animals.
• For example, toxic wastes released from a factory may reach a nearby river. The aquatic life in that river will be certainly affected. It may indirectly affect humans also. In such situations, we say that environmental pollution has occurred.
◼ Let us see what a pollutant is and it’s main features. It can be written 4 in steps:
1. The substance causing the pollution is known as pollutant.
2. Pollutants can be solid, liquid or gaseous substances.
3. Scientists have specified certain upper limits.
♦ The quantities of various substances should not exceed those limits.
♦ If the quantity of a substance exceed the limit, it becomes a harmful substance and is classified as a pollutant.
4. Pollutants may occur in an environment due to natural causes or human activities.
The disastrous effect of pollution can be demonstrated by a simple example. It can be written in 3 steps:
1. Imagine that, 2 milligrams of pollutants is present in each kilogram of food that we consume.
2. Also imagine that, 2 milligrams of pollutant is present in each kilogram of air that we breathe.
3. An average human being requires 12-15 times more air than food.
• So if a person consumes 1.5 kg food in a day, he will be breathing about 18 kilograms of air.
• Based on the pollution level written in (2), he will be breathing 36 milligrams of pollutants in a day. This much pollutants per day can make him seriously ill.
◼ Let us see the two types of pollutants. It can be written in 2 steps:
1. Degradable pollutants quickly break down into simpler substances by the action of microorganisms, fungi etc., Kitchen wastes like fruit and vegetable peels is an example of degradable pollutants.
2. Non degradable pollutants do not break down into simpler substances. They remain as such for decades. Toxic substances leach out from them causing harmful effects on soil and water. Plastic materials, heavy metal, nuclear wastes etc., are examples of non degradable pollutants.
Atmospheric pollution
This can be written in 5 steps:
1. The air in our atmosphere do not have the same density every where. As the height from the sea level increases, the density of air decreses.
2. There are several layers in our atmosphere. Some images can be seen here.
3. The bottom most layer is called troposphere.
• The upper boundary of this layer is at a distance of about 12 km from the sea level.
♦ In other words, the thickness of this layer is about 12 km.
4. The layer just above troposphere is called stratosphere.
• The upper boundary of this layer is at a distance of about 50 km from the sea level.
♦ In other words, the thickness of this layer is about (50 - 12) = 38 km
5. When we study atmospheric pollution, we mainly study the pollution in the troposphere and stratosphere.
Tropospheric pollution
Troposphere is the lowest layer in the atmosphere. It’s thickness is low when compared to other layers. But about 75% of the total mass of the atmosphere is concentrated on this layer. So density of this layer is high. The weather experienced by humans occur in this layer. This layer contains most of the water vapour in the atmosphere. It also contains large quantities of dust particles. The flow of air in this layer is very turbulent.
• Pollution of the troposphere is caused by two types of pollutants. They are: gaseous pollutants and particulate pollutants.
1. Gaseous pollutants:
Oxides of sulphur, oxides of nitrogen, oxides of carbon, hydrogen sulphide, hydrocarbons etc., are the main gaseous pollutants.
2. Particulate pollutants:
• These are solid or liquid particles.
• There size is less than 10 micro meters. (diameter of a human hair is 50-70 micro meters).
• Such small particles can easily reach our lungs and some times into the blood stream.
• Dust, mist, fumes, smoke, smog etc., are examples of particulate pollutants.
cont..
In the next section, we will see more details about tropospheric pollution.
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