Atmospheric pollutants from fuels: Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur.

The idea

Fuels are substances that release energy when burned, and their composition explains which products form. Most fuels are based on carbon and/or hydrogen: coal is mainly carbon with variable impurities, while natural gas is largely methane, CH₄, so it is rich in both elements. Many fuels also contain sulfur compounds as impurities, especially coal and crude-oil fractions. When a fuel burns, its carbon atoms can form carbon dioxide, CO₂, or carbon monoxide, CO, its hydrogen atoms form water vapour, H₂O, and any sulfur forms sulfur dioxide, SO₂. Knowing the elements present therefore lets you predict the combustion products and the pollutants released.

For example: Burning sulfur-containing coal produces SO₂, which can contribute to acid rain, whereas burning pure methane cannot produce SO₂ because no sulfur is present.

What earns the marks

• Most fuels contain carbon and/or hydrogen as their main combustible elements.
• Coal is mainly carbon but also contains variable impurities, including sulfur compounds.
• Hydrocarbon fuels such as methane contain hydrogen as well as carbon.
• Sulfur is present in some fuels as an impurity rather than as the main fuel component.
• The elements present in a fuel determine which combustion products can form, for example sulfur in a fuel can lead to sulfur dioxide.
• Combustion products include carbon dioxide and/or carbon monoxide from carbon, and water vapour from hydrogen.

Common mistakes and how to put them right

• Stating that all fuels contain sulfur.

  Put it right: Sulfur is only present in some fuels, such as coal and heavier oil fractions, as an impurity.

• Claiming that coal is a pure element.

  Put it right: Coal is mainly carbon but contains variable impurities, including sulfur compounds.

• Assuming water vapour forms only if hydrogen gas is burned.

  Put it right: Any fuel containing hydrogen, such as methane, produces water vapour when burned.

Exam tips

• Link each element in a fuel to the product it can form: carbon to CO₂ or CO, hydrogen to H₂O, sulfur to SO₂.
• Use the phrase 'may also contain some sulfur' when describing fuels, because sulfur content varies between fuels.
• When asked to predict pollutants, first identify which elements the fuel contains, then name the corresponding oxides.