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CHEMISTRY · Stage 7 · Chapter 5: Acids and Alkalis

Chemistry 5.5 Acid rain — Environmental Causes, Effects and Soil Liming

5 min read 6 parts Lesson 5 of 7

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5.5 Acid rain — Environmental Causes, Effects and Soil Liming — Alhussan International Academy — Chemistry (Stages 7–9) notes: Explain how acid rain forms, its effects on buildings, trees and lakes, and how its impact can be reduced.Normal rain — it is slightly acidic (about pH 5…

  • Cars and lorries
  • Burning rubbish
  • Power stations
  • Sulfur dioxide
  • Nitrogen oxides
  • Buildings and statues

Section 1 of 6What acid rain is

  • Normal rain
    • it is slightly acidic (about pH 5.6) because carbon dioxide dissolves in it to form carbonic acid.
  • Acid rain = rain with a pH below about 5.0, caused mainly by sulfur dioxide and nitrogen oxides dissolving in water droplets in clouds.
    • They form sulfuric and nitric acids.
  • Spread
    • winds carry the pollution hundreds of kilometres, so acid rain can fall in different countries from where it was produced.

Section 2 of 6Sources of acidic gases

  • Cars and lorries
    • they burn fossil fuels and make acidic gases.
  • Factories
    • some make glass, process leather or make products from oil and produce acidic waste gases.
  • Burning rubbish
    • it makes acidic gases.
  • Power stations
    • those that burn coal make waste products including acidic gases, and some neutralise the gases while others send them into the air.
  • Sulfur dioxide
    • it forms when fossil fuels containing sulfur are burned and when sulfur-containing metal ores are roasted, and volcanoes release it.
    • sulfur + oxygen → sulfur dioxide; sulfur dioxide + water + oxygen → sulfuric acid.
  • Nitrogen oxides
    • they form when nitrogen and oxygen in the air react at the high temperatures in car engines and furnaces, and during lightning.

Section 3 of 6Effects of acid rain

  • Buildings and statues
    • limestone and marble, which are calcium carbonate, are eroded.
    • Nitric acid + calcium carbonate → calcium nitrate + water + carbon dioxide.
    • Sulfuric acid + calcium carbonate → calcium sulfate + water + carbon dioxide.
    • The marble is damaged for ever.
  • Lakes and rivers
    • they become acidic, so some plants and animals cannot live in them.
    • Aluminium is released from soil into water and is toxic.
  • Trees
    • acid rain damages leaves and washes nutrients such as magnesium and calcium out of the soil.
  • Rock
    • acid rain damages some types of rock, and scientists think it may have caused a landslide in China.
  • Metal structures
    • they corrode faster.

Section 4 of 6Case study: the Taj Mahal

  • Problem
    • the white marble of the Taj Mahal in India is turning yellow because acid rain reacts with it.
    • Cars, lorries, factories and burning rubbish near it make the acidic gases.
  • Possible actions
    • factories can neutralise their acidic waste, and people can drive electric or hydrogen cars, walk or cycle.
    • Some methods are easier, more effective or cheaper than others.
  • Decision
    • the government used science to evaluate methods and also considered the jobs factories provide.
    • It told factories to neutralise waste or move away and banned petrol and diesel vehicles in the area.
    • Some people obeyed and others did not, and in 2021 acid rain continued to damage the Taj Mahal.
  • Lesson
    • every possible action has advantages and disadvantages, and governments need to evaluate them before deciding.

Section 5 of 6Reducing the problem

  • Prevention
    • it is better than cure.
    • Remove sulfur from fuel before burning.
    • Use flue gas desulfurisation in power stations, where calcium carbonate or calcium oxide reacts with sulfur dioxide.
    • Fit catalytic converters to cars to change nitrogen oxides into nitrogen.
    • Use cleaner methods to make electricity, such as wind turbines or solar cells, and use public transport.
  • International limits
    • governments agree limits on emissions.
  • Repairing damage
    • liming adds calcium hydroxide (slaked lime) or calcium carbonate to acidic lakes and soils to neutralise the acid and raise the pH.
    • It is a short-term fix and costly, so reducing emissions is the long-term solution.

Section 6 of 6Data and decisions

  • Monitoring
    • scientists collect rainwater and measure its pH at different places and times.
  • pH change
    • if the average pH falls from 5.6 to 4.2, the rain has become about 25 times more acidic.
  • Regulation
    • after laws lowered emissions in Europe and North America the pH of rain in many areas rose.
    • Recovery is slow because soils and lake sediments hold acid for years.
  • Cost
    • desulfurisation raises electricity prices but reduces damage to health, buildings and farming.
  • Natural sources
    • volcanoes also contribute to acid rain.

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