Complete and incomplete combustion of fuels, the formation of carbon dioxide, carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates, and the effects of these pollutants.

A C6.2 structured question. Reward: the word equation for complete combustion is hydrocarbon plus oxygen produces carbon dioxide plus water (for example $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$). Complete combustion needs plenty of oxygen. Carbon monoxide is produced by incomplete combustion, which happens when there is not enough oxygen (a limited oxygen supply), as in an engine: the carbon in the fuel is only partially oxidised, forming carbon monoxide ($\text{CO}$) instead of carbon dioxide (and carbon, as soot or particulates, may also form). Markers credit the correct complete-combustion word (or symbol) equation giving carbon dioxide and water, and the explanation that carbon monoxide forms by incomplete combustion when oxygen is limited (the carbon is only partly oxidised). A common error is to write carbon monoxide as a product of complete combustion.

A Higher tier question on the effects of pollutants. Reward: carbon monoxide is a toxic gas: it is taken up by the blood (binding to haemoglobin) in place of oxygen, so it reduces the amount of oxygen the blood can carry, which can cause unconsciousness or death; it is colourless and odourless so it cannot be detected easily. Sulfur dioxide is harmful because it causes respiratory (breathing) problems and, with oxides of nitrogen, it causes acid rain: sulfur dioxide is released when fuels containing sulfur impurities are burned, it dissolves in water in the atmosphere (clouds) and is oxidised to form an acidic solution (sulfuric acid), which falls as acid rain, damaging buildings and statues (especially limestone), harming trees, and making lakes and rivers acidic so that fish and other organisms die. Markers credit carbon monoxide being toxic by reducing the oxygen-carrying capacity of the blood, sulfur dioxide causing breathing problems, and the acid rain mechanism (sulfur dioxide dissolving in water to form an acidic solution that falls as rain and damages the environment). A common error is to link carbon monoxide to acid rain.