When sulphur burns, or when compounds that contain sulphur burn such as coal and oil, sulphur dioxide gas is released. SO2 is a choking, foul smelling and acidic gas which contributes to acid rain. It is released naturally as a decay product of dead plants and animals and, more significantly, when volcanoes erupt. The sulphur dioxide released into the atmosphere during eruptions tends to shield the Earth from some of the Sun’s energy causing cooling. One suggested solution to the effects of climate change caused by greenhouse gas emission would be geoengineering; intentionally loading the atmosphere (above cloud height) with sulphur dioxide to lower the temperature.
Sulphur is in the same column of the periodic table as oxygen, so it reacts easily to form sulphur dioxide, which is also released if you heat iron or zinc sulphide in air. Sulphur dioxide can be converted into sulphur trioxide and then into sulphuric acid. Sulphuric acid is one of the world’s most important industrial chemicals so tonnes of it are produced around the world every year. Sulphur dioxide has a number of uses but a common one is as a preservative. It is present in wine (in tiny amounts) to stop bacteria from spoiling the wine. It can also be used to preserve dried fruit such as apricots or figs. When it is used as a preservative it is listed in the ingredients as E220.
‘E numbers’ are the code designations for food additives approved for use by the European Union – the E stands for Europe. The term is often used insultingly, “oh, that is just full of E numbers”, but there is no particular reason to be alarmed by E numbers. For example, anything with added vitamin C has added E300 because ascorbic acid (vitamin C) is food additive E300. Tocopherol (vitamin E) is food additive E306; acetic acid (vinegar) is E260; citric acid (which gives lemons their zing) is E330; sodium bicarbonate (baking powder) is E500; argon (used as a packaging gas) is E938.
Sulphur dioxide released into the atmosphere often results in the formation of acid rain. All rainwater is slightly acidic (pH5.7) because carbon dioxide dissolves in it to form carbonic acid. Other molecules in the air can lower the pH further. Some of these molecules may be naturally occurring, such as nitrogen oxide produced during thunderstorms or sulphur dioxide released by volcanoes. However, higher levels of sulphur dioxide released by industrial processes such as the burning of coal can lead to ‘super acid rain’ with a pH as low as 3.5. This acidity reacts with limestone (chalk/marble) to release carbon dioxide and damage stonework. It may also leach toxic elements such as lead, arsenic or cadmium out of the soil and into waterways where they can accumulate and poison wildlife. Forests at high altitudes may experience ‘acid fog’ and be directly damaged. Also coniferous trees can have their pines damaged by acid rain making them more vulnerable in cold weather.
Sulphur dioxide emissions from factories can be reduced by burning low sulphur fuels in the first place or by scrubbing the waste gases with the alkali lime (calcium hydroxide) or soda lime (sodium hydroxide). These processes are more expensive but they have become standard practice in many parts of the world.
Questions…
- Name a gas other than carbon dioxide that is a greenhouse gas and strongly contributing to climate change?
- Apart from sulphur dioxide, name a gas that might be released by the burning of fossil fuels?
- Name a food naturally rich in ascorbic acid.
- What is the chemical name for limestone (chalk/marble)?
- How might the reaction between iron and sulphur differ from the reaction between zinc and sulphur?
- What would the products be in each case?
- Arsenic is neither a metal nor a non-metal element. What word is used to describe arsenic?
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