Water

  • Tests for water: anhydrous copper(II) sulfate turns from white to blue, and anhydrous cobalt(II) chloride turns from blue to pink.
  • These tests show water is present but not that it is pure. To test purity, measure the melting point and boiling point. Pure water melts at 0 °C and boils at 100 °C, while impurities lower the melting point and raise the boiling point.
  • Distilled water is used in practical chemistry instead of tap water because it contains fewer chemical impurities.
  • Water from natural sources may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers (phosphates also from detergents).
  • Some are beneficial: dissolved oxygen supports aquatic life and some metal compounds provide essential minerals. Others are harmful: some metal compounds are toxic, some plastics harm aquatic life, and sewage contains microbes that cause disease.
  • Nitrates and phosphates deoxygenate the water and damage aquatic life.
  • Treatment of the domestic water supply: sedimentation and filtration to remove solids, carbon to remove tastes and odours, then chlorination to kill microbes.

Fertilisers

  • Ammonium salts and nitrates are used as fertilisers.
  • NPK fertilisers provide the elements nitrogen, phosphorus and potassium, which improve plant growth.

Air quality

Clean, dry air is approximately 78% nitrogen (N₂), 21% oxygen (O₂) and the remainder a mixture of noble gases and carbon dioxide (CO₂).

  • Carbon dioxide comes from the complete combustion of carbon-containing fuels. More CO₂ increases global warming, which leads to climate change.
  • Carbon monoxide and particulates come from incomplete combustion of carbon-containing fuels. Carbon monoxide is a toxic gas. Particulates increase the risk of respiratory problems and cancer.
  • Methane comes from the decomposition of vegetation and waste gases from digestion in animals. Like CO₂, it increases global warming.
  • Oxides of nitrogen come from car engines. They cause acid rain, photochemical smog and respiratory problems.
  • Sulfur dioxide comes from burning fossil fuels that contain sulfur compounds. It causes acid rain.
  • ExtendedOxides of nitrogen form in car engines because the high temperature makes nitrogen and oxygen from the air react. Catalytic converters remove them, for example 2CO + 2NO → 2CO₂ + N₂.

Climate change and acid rain strategies

  • Reduce climate change by planting trees, reducing livestock farming, decreasing the use of fossil fuels, and increasing the use of hydrogen and renewable energy such as wind and solar.
  • Reduce acid rain by fitting catalytic converters to vehicles, using low-sulfur fuels, and removing sulfur dioxide from flue gases with calcium oxide (flue gas desulfurisation).
  • ExtendedCarbon dioxide and methane cause global warming by absorbing, reflecting and emitting thermal energy, which reduces thermal energy loss to space.

Photosynthesis

  • Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll and using energy from light.
  • Word equation: carbon dioxide + water → glucose + oxygen.
  • ExtendedSymbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

Exam tips

  • Link every pollutant to a source and a harmful effect. Mark schemes usually want both.
  • For water pollution, say that nitrates and phosphates deoxygenate the water and so damage aquatic life.
  • When explaining the greenhouse effect, say that gases absorb infrared (thermal) radiation and keep it in the atmosphere.
  • Give the colour change for both water tests, including the starting colour.
  • Equation questions on catalytic converters expect balanced symbols: check the atoms on both sides.

Common mistakes

  • Saying a blue colour with copper(II) sulfate proves the water is pure. It only proves water is present.
  • Saying oxides of nitrogen cause global warming. They cause acid rain, smog and respiratory problems.
  • Saying carbon monoxide causes global warming or acid rain. It is toxic and comes from incomplete combustion.
  • Writing that fertilisers 'feed' plants. They supply the essential elements N, P and K.
  • Blaming sulfur dioxide on car engines alone. Its main source is burning sulfur-containing fuels such as coal, while oxides of nitrogen come from hot engines.