Carbon monoxide is a toxic gas produced by the combustion of fossil fuels. Explain why carbon monoxide, rather than carbon dioxide, is formed during the burning of some fossil fuels.

Fossil fuels often contain sulfur impurities. Explain how removing these sulfur impurities before burning the fuel reduces the erosion of historic monuments made of marble (calcium carbonate).

Nitrogen oxides (${NO}_x$) are atmospheric pollutants also produced when cars burn fuel. Explain why nitrogen oxides are formed in car engines, even though nitrogen is a very unreactive gas.