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Air and Water

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Air and Water
« on: September 08, 2014, 10:34:07 AM »
Air and Water Water: Water is perhaps the most known substance.  This is perhaps because of its abundance and numerous uses. Water is, H2O, is  the most popular solvent for chemical reaction.
 Tests for Water:  There are several tests for water, the  easiest one which you can perform at home is physical and doesn’t involve any  reaction. It is testing its boiling point. Pure water boils at 100°C sharply,  and freezes at 0°C sharply.
 
There are chemical reactions which could be applied to test for water. For example if you add water to anhydrous copper  sulphate powder which is white in colour, it forms a blue solution and may
give heat out. If you crystallise the solution you will obtain blue crystals of  hydrated copper sulphate (see the picture on the left).
Another test for water is adding it to  anhydrous cobalt chloride which is blue in colour, if water is added to it the  anhydrous salt forms a pink solution. If you crystallise the solution of cobalt
chloride you will obtain pink hydrated cobalt chloride crystals (see the picture on the right).
 
 Uses of Water: The uses of water are many, from drinking and cleaning to irrigating crops and landscapes. Water is used for cooling, for recreation, and dust control. Water is needed for restaurants, most industrial processes, and even some religious ceremonies. On another level, the splash and flow of water in streams and fountains soothes and inspires.
In one way or another, water is a part of almost everything humans make and do. Washing a load of laundry uses 40 gallons, filling a backyard pool takes about 25,000 gallons, growing a pound of cotton consumes 1,000 gallons, while producing a pound of copper uses 20 gallons.
Uses where water is consumed, usually through evaporation or plant growth, are consumptive uses. Examples include water used for irrigation or in evaporative coolers. Non-consumptive uses, such as bathing, hydropower generation and recreation, do no t use up water. Used non-consumptively, the same water can be used again and again, although some uses lower the quality of the water. Once used, wastewater can be treated and used again as reclaimed water or effluent.
The main categories of water use are agricultural, municipal and industrial. Municipal and industrial uses currently are much less, but are growing rapidly. Mining activities and cooling towers used for power generation account for most of the remaining water use.
 Water Purification:  Water that exists naturally in earth is  never pure. There are always impurities in it, sometimes in large amounts. In  fact water could very well be contaminated with diseases and bacteria. This is  why water has to be purified before it is put to use. Water purification involves two processes (Filtration & Chlorination) done in several steps:
 
  • Water is taken from reservoirs or any other  source to the water treatment plant
  • Water is passed through filters to remove  large, floating objects such as pieces of rocks or mud
  • Smaller particles are removed by adding  aluminum sulfate which makes them stick together in large pieces and settle  down
  • Water is passed through sand and gravel  filters which filter off small particles and may kill some bacteria (filtration  is done)
  • Chlorine gas is bubbled through the water  to kill all bacteria living in the water making the water sterile
  • The water may end to be slightly acidic,  small amounts of sodium hydroxide are added to treat this. Fluoride might be added  to because it helps in preventing tooth decay
  • Water is then delivered to homes
 
 Air:  Air is a mixture of gases that makes up the  atmosphere of earth.
 Composition of Clean Air:  Clean air is made up of nitrogen, oxygen  and traces of other gases including carbon dioxide and noble gases. There are  also traces of water vapor in air.  Noble  gases present in air are mostly argon and some helium, neon, krypton and xenon.
Fractional Distillation of Liquid Air:  Fractional distillation of liquid air is  used to separate gases of air, specially nitrogen and oxygen. Like fractional  distillation of petroleum, it is based on the boiling points of the components  of air.
 
GASBoiling Point:Percentage Of Air
Carbon Dioxide-32 °C<1%
Xenon-108 °C<1%
Krypton-153 °C<1%
Oxygen-183 °C20%
Argon-186 °C<1%
Nitrogen-196 °C79%
Neon-246 °C<1%
Helium-249 °C<1%
 
 
  •   Clean air is cooled till -80°C, carbon  dioxide sublimes into solid and is collected, water vapour condenses than freezes into ice and is collected too
  •   The cold air is now put into a compressor  which increases its pressure to 100 atm.   This causes the air to warm up so it has to be cooled down again
  •   The re-cooled, compressed air is then  allowed to expand and lose its pressure, this causes it cool further
  •   The air is now recompressed then expanded  again to keep cooling it. This stage is repeated until all gases liquefy, this is at a temperature below -200°C
  •   Then the cold liquid air brought in a  fractionating column and left to warm slowly
  • Gases separate one after another according  to their boiling points. The gas with the lowest boiling point evaporates  first, followed by the gas of the second lowest boiling point and so on
  •   The three main gases of air (nitrogen,  oxygen and argon) evaporate in the following order:
     1. Nitrogen (-196 °C)
     2. Argon (-186 °C)
     3. Oxygen (-183 °C)
  • Gases are collected and stored separately.
 
 Air Pollution:  Pollution is the presence of harmful  substances. Air pollution is the presence of pollutant gases in the air. A  pollutant is a substance that causes pollution. These are:
 
  • Carbon monoxide
  • Oxides of nitrogen
  • Sulphur dioxide
  • Lead compounds
Carbon Monoxide: Carbon monoxide (CO) is  one of the poisonous pollutants of air. It is considered a pollutant because it  can kill living organisms. The main source of carbon monoxide is factories  which burn carbon-containing fossil fuels since CO is one of the products of  the incomplete combustion of fossil fuels. Carbon monoxide could be treated by  installing catalytic converters in chimneys of the factories.
Sulphur Dioxide: Sulphur dioxide (SO2) is  considered a pollutant since it contributes to acidic rain. Sulphur dioxide is  a product of two process, these are combustion of sulphur –containing fossil  fuels and extraction of metals from their sulphide ores (such as zinc  sulphide). The problem associated with sulphur dioxide is that when it rises in  the air from chimneys of factories, it mixes with water vapour of clouds and  air. This results in the formation of sulphuric acid (H2SO4). When it rains,  rain water which falls becomes acidic. Acid rain causes death to water  creatures since it makes water acidic, acidifies soil causing death to plants  and deforestation, reacting with limestone from buildings and sculptures corroding it, and may also cause lung cancer. Sulphur dioxide could be treated  before it leaves chimneys of factories by reacting it with limestone which is a  neutralisation reaction. This process is called desulphurisation.
SO2 + CaCO3 → CaSO3 + CO2
Oxides of Nitrogen (NO & NO2): Nitrogen  oxides are formed at high temperatures as a result of nitrogen and oxygen  reacting. In cars, engines have a very high temperature; this creates a chance  for nitrogen and oxygen present in air in the engine to react forming nitrogen  monoxide.
N2 + O2 → 2NO
The produced carbon monoxide is released  through the exhaust with other waste fumes. Nitrogen monoxide reacts with more  oxygen from air producing nitrogen dioxide.
2NO + O2 → 2NO2
The problem associated with nitrogen  dioxide is similar to that of sulphur dioxide. It rises up in the air and mixes  with rain water forming nitric acid. This causes acid rain. Nitrogen oxides can  also cause health respiratory problems to humans and animals. To treat this issue, cars are now fitted with devices called catalytic converters which  eliminate nitrogen oxides.
Lead Compounds: Compounds of lead are waste  products of fuel burning in cars. They are considered pollutants because they  are poisonous and they are said to cause mental disabilities to young children.  To treat this problem, gas stations now provide unleaded fuel.
 
 Catalytic Converters: Car fuels contain carbon; so carbon  monoxide gas is released by cars as waste fumes, as well as nitrogen oxides.  These are pollutant gases. To prevent these gases from polluting air, a device  called catalytic converter is fitted at the end of the exhaust. This device contains a catalyst which catalyses the reaction between these two gases producing two harmless gases, nitrogen and carbon dioxide:
2NO + 2CO → 2CO2 + N2
 2NO2 + 4CO → 4CO2 + N2
The catalyst of the device works best at  temperature around 200°C.
 
 The Carbon Cycle:  The carbon cycle is a natural global cycle  of the element carbon. It is what maintains a constant level of carbon dioxide  in air (0.03%). The cycle goes as follows:
 
  • Plants absorb carbon dioxide from air and  undergo photosynthesis reaction which turns it into glucose and produces
     oxygen: 6CO2 + 6H2O → C6H12O6 + 6O2
  • The carbon is now stored in plants as  glucose. One of two things happen, either the plants get eaten by animals or  humans, or the plant dies and decays.
  • If the plant is eaten by animals or humans,  glucose in the plant is used by them in a process called respiration to release  energy for their body.
     C6H12O6 + 6O2 → 6CO2 + 6H2O
     Respiration is the opposite of  photosynthesis. Carbon dioxide is one of the products of it, which is released  by the humans through breathing into the air. Thus carbon dioxide returns to  the atmosphere.
  • If the plant dies. It is buried underground  and by time it decays forming coal and other fossil fuels. These substances  contain the carbon which was made and stored by the plants and they are then taken by power stations which put them to use.
  • Power stations burn carbon-containing fuels  that were obtained as coal or fossil fuels formed by dead plants. This is a combustion reaction.
     C + O2 → CO2
     Carbon dioxide is result of these  reactions. Carbon dioxide produced is released to the air through chimneys of  power stations. Thus the cycle is completed and all carbon dioxide returns to  the atmosphere.
 
 Green House Gases:  The sun sends energy to the earth in two  forms, light and heat. Some of the heat energy reflects back to the space, some  however are trapped inside the Earth. This is caused by some gases and it is  called the greenhouse effect. The main greenhouse gases are carbon dioxide and  methane.
Carbon dioxide is formed in many ways. It  is formed on a large scale in power stations by the combustion of carbon  containing fuels. Carbon dioxide is also caused by respiration of living  organisms. The gas can also be produced by a reaction between an acid and a  carbonate, like that of the corrosion limestone.
Methane, the other greenhouse gas is formed  by animals. When animals eat and digest their food, methane gas is one of the  waste products of this process. It is released to the atmosphere by animals. When plants die and decompose over many years, methane gas is also produced.
The greenhouse effect poses a threat to the  world now days. This is because greenhouse gases, especially carbon dioxide,  have increased in amounts in the atmosphere due to activity of humans. Lots of  fuel combustion is taking place around the world, increasing the levels of CO2,  while trees are being chopped off to made use of instead of leaving to replace  CO2 with oxygen. These activities cause an increase of the levels of CO2 in the  atmosphere, which leads to more heat trapping in earth. This rises the global temperature  of the earth causing what’s called global warming.
 Global warming is the increase of the  temperature of the earth due to the increase of levels of greenhouse gases.  Global warming has effects on the earth. To start with, it north and south poles,  which are made of ice, will start to melt raising sea levels. The sea temperature will also rise causing death to marine lives. This is also accompanied by other natural disasters such as hurricanes and heavy rains.
Humans could prevent this by reducing  combustion of fossil fuels and leaving forests to live.
 
Rusting:
 Rusting is the corrosion of iron as a  result of reaction with oxygen from air and water. If iron objects are left  uncovered and exposed to air & water, iron will react with oxygen forming hydrated iron oxide (also known as rust). Rust is a reddish brown flaky solid  which will fall of the object making it thinner and loses it its shape. Iron  must come in contact with air and water in order for rusting to happen. The  formula of rust is Fe2O3. xH2O. Steel can also rust since it is made up of  mostly iron.
Rusting can become very dangerous in some  cases. For example, bridges that cross rivers stand on columns that are made of  iron. The conditions of rusting are present in this case (Water from the river and oxygen from the air). There is a risk that the columns will rust and collapse with the whole bridge. In another case, ships are made of iron. Again,  the conditions of rusting are present (water from the sea and oxygen from the  air). In fact, this situation is more critical because sea water contains  minerals that act as a catalyst to speed up the reaction of rusting.
There some available methods to prevent  rusting. These methods are based on covering the iron object with another  substance to create a barrier between iron and oxygen and water so that rusting  does not take place:
Painting: The iron or steel object is  painted all over. The paint creates the desired barrier to prevent iron or  steel coming in contact with air and water. This method is used in car bodies  and bridges.
Electroplating: The iron or steel object  gets electroplated with another metal that doesn’t corrode. The object is  usually electroplated with tin or chromium since they are very unreactive. This  method is used in food cans and car bumpers.
Sacrificial Protection: This method is  based on the idea that metals that are higher than iron in the reactivity  series will react in preference to it and thus that metal is corroded and the  iron is protected. Metals usually used as protectors in this method are zinc  and magnesium since they are higher than iron in the reactivity series. In  ships for example, zinc or magnesium bars are attached to the iron base of the  ship which is in contact with water and oxygen from air. But rusting doesn’t  take place since zinc or magnesium is the one that gets corroded. These bars  must be replaced from time to time because once they all get corroded, iron becomes unprotected and rusts. This method is usually used in ships or bridge  columns. The zinc or magnesium bars do not have to completely cover the iron or  steel because as long as they are attached to each other the zinc or magnesium  bars get corroded and not the iron.
 Galvanisation: Galvanisation is a very  reliable method for preventing rusting. It is basically covering the whole  object by a protective layer of zinc. This can be done either by electroplating  the object with zinc or dipping it into molten zinc. The zinc layer provides a  barrier that prevents iron or steel from coming in contact with air and water.  The zinc gets corroded instead iron thus protecting it. If the a part of the  zinc coat falls off and the iron or steel gets exposed to air and water, the  bare part still doesn’t get corroded since it is protected by sacrificial protection now.

 

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