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Chemistry 5070 · Chapter 10

Chemistry of the Environment

Water and its tests, pollutants in natural water and the treatment of the domestic water supply, fertilisers, the composition of air, air pollutants and their effects, the greenhouse effect, acid rain, catalytic converters and photosynthesis.

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10.1 Water

Tests for water

Water is detected by a colour change in an anhydrous compound:

TestResult with water
anhydrous copper(II) sulfate (white powder)turns blue
anhydrous cobalt(II) chloride (blue, often as paper)turns pink

CuSOX4(s)+5 HX2O(l)→CuSOX4 ⋅ 5 HX2O(s)\ce{CuSO4(s) + 5H2O(l) -> CuSO4.5H2O(s)}

These tests show that water is present, but they do not show that it is pure water.

Testing the purity of water

A pure substance has a sharp, fixed melting point and boiling point. Pure water boils at 100 ∘C100\,^\circ\text{C} and melts (freezes) at 0 ∘C0\,^\circ\text{C} at normal atmospheric pressure. Impurities make the boiling point higher and the melting point lower, and the change of state happens over a range of temperatures.

Distilled water, not tap water, is used in practical chemistry because it contains fewer chemical impurities (dissolved salts and gases) that could interfere with the reactions or the tests.

Substances in natural water

Water from rivers, lakes and the ground can contain several other substances:

SubstanceSource or effect
dissolved oxygenbeneficial: aquatic animals and plants need it to live
metal compoundssome are beneficial (they provide essential minerals for life, such as calcium and magnesium), but some are toxic, for example compounds of lead and mercury
plasticsharm aquatic life, as animals can eat them or be trapped in them
sewagecontains harmful microbes that cause disease, such as cholera and typhoid
nitrates (from fertilisers) and phosphates (from fertilisers and detergents)cause plants and algae to grow quickly, and when they die and decay, the microbes use up the oxygen. The water is deoxygenated and fish and other aquatic life die

Treating the domestic water supply

The stages in treating water for homes: screening, sedimentation, filtration, carbon filter and chlorination

The stages in treating water for homes: screening, sedimentation, filtration, carbon filter and chlorination
StageWhat it does
sedimentationthe water stands in a tank so the larger solid particles settle at the bottom. A chemical can be added to make small particles clump together and settle
filtrationpasses through beds of sand and gravel to remove the remaining small solids
carbon (activated charcoal)removes tastes and odours from the water
chlorinationchlorine is added to kill microbes (bacteria and viruses)

10.2 Fertilisers

Plants need the elements nitrogen, phosphorus and potassium to grow well. They take them from the soil as ions, and crops remove them, so fertilisers replace them.

  • Ammonium salts and nitrates are used as fertilisers to supply nitrogen, for example ammonium nitrate, NHX4NOX3\ce{NH4NO3}, ammonium sulfate, (NHX4)X2SOX4\ce{(NH4)2SO4}, and potassium nitrate, KNOX3\ce{KNO3}.
  • NPK fertilisers contain compounds of all three elements: Nitrogen, Phosphorus and K(potassium). They give improved plant growth and a higher crop yield.

NHX3(aq)+HNOX3(aq)→NHX4NOX3(aq)\ce{NH3(aq) + HNO3(aq) -> NH4NO3(aq)}

2 NHX3(aq)+HX2SOX4(aq)→(NHX4)X2SOX4(aq)\ce{2NH3(aq) + H2SO4(aq) -> (NH4)2SO4(aq)}

10.3 Air quality and climate

Composition of clean, dry air

GasPercentage by volume
nitrogen, NX2\ce{N2}about 78%78\%
oxygen, OX2\ce{O2}about 21%21\%
noble gases (mostly argon) and carbon dioxide, COX2\ce{CO2}the remaining 1%1\% or so

Air pollutants

PollutantSourceAdverse effect
carbon dioxide, COX2\ce{CO2}complete combustion of carbon-containing fuelsa greenhouse gas: higher levels lead to global warming and climate change
carbon monoxide, CO\ce{CO}incomplete combustion of carbon-containing fuelstoxic: it stops the blood carrying oxygen
particulates (carbon)incomplete combustion of carbon-containing fuelsincreased risk of respiratory problems and cancer
methane, CHX4\ce{CH4}decomposition of vegetation and waste, and gases from the digestion in animals (e.g. cattle)a greenhouse gas that causes global warming and climate change
oxides of nitrogen, NOXx\ce{NO_x}car enginesacid rain, photochemical smog and respiratory problems
sulfur dioxide, SOX2\ce{SO2}combustion of fossil fuels that contain sulfur compoundsacid rain

C(s)+OX2(g)→COX2(g)(complete combustion)\ce{C(s) + O2(g) -> CO2(g)} \quad \text{(complete combustion)}

2 C(s)+OX2(g)→2 CO(g)(incomplete combustion, with too little oxygen)\ce{2C(s) + O2(g) -> 2CO(g)} \quad \text{(incomplete combustion, with too little oxygen)}

The greenhouse effect

The greenhouse effect: radiation from the Sun reaches the Earth, the Earth emits thermal radiation, and greenhouse gases absorb some and send it back

The greenhouse effect: radiation from the Sun reaches the Earth, the Earth emits thermal radiation, and greenhouse gases absorb some and send it back
  • The Sun's radiation passes through the atmosphere and warms the Earth's surface.
  • The warm Earth gives out thermal (infrared) energy.
  • The greenhouse gases, carbon dioxide and methane, absorb some of this thermal energy and emit (and reflect) it in all directions, including back towards the Earth.
  • This reduces the thermal energy lost to space, so the Earth's temperature increases. More greenhouse gas in the air means more warming: this is global warming, which leads to climate change.

Reducing the effects

ProblemStrategies
climate changeplant trees (they remove COX2\ce{CO2} by photosynthesis), reduce livestock farming (less methane), decrease the use of fossil fuels, and use more hydrogen and renewable energy (wind, solar)
acid rainuse catalytic converters in vehicles, burn low-sulfur fuels, and use flue gas desulfurisation with calcium oxide to remove SOX2\ce{SO2} from power station gases

In flue gas desulfurisation the acidic sulfur dioxide reacts with basic calcium oxide:

CaO(s)+SOX2(g)→CaSOX3(s)\ce{CaO(s) + SO2(g) -> CaSO3(s)}

Oxides of nitrogen and catalytic converters

Oxides of nitrogen form in car engines because the high temperature (and the spark) makes nitrogen and oxygen from the air react:

NX2(g)+OX2(g)→2 NO(g)\ce{N2(g) + O2(g) -> 2NO(g)}

A catalytic converter in the exhaust (with platinum and rhodium catalysts) converts the harmful gases into less harmful ones: for example, carbon monoxide and nitrogen oxide react to give carbon dioxide and nitrogen:

2 CO(g)+2 NO(g)→2 COX2(g)+NX2(g)\ce{2CO(g) + 2NO(g) -> 2CO2(g) + N2(g)}

Photosynthesis

carbon dioxide+water→chlorophylllightglucose+oxygen\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light}} \text{glucose} + \text{oxygen}

6 COX2(g)+6 HX2O(l)→CX6HX12OX6(aq)+6 OX2(g)\ce{6CO2(g) + 6H2O(l) -> C6H12O6(aq) + 6O2(g)}

Photosynthesis removes carbon dioxide from the atmosphere and is endothermic, using energy from sunlight.

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