The same chemistry can feed you or poison the air. What decides it is whether the reaction is one we wanted.
Rusting is an oxidation reaction that needs both water and oxygen. It occurs rapidly in humid climates, so it is commonly observed in Singapore. It weakens iron and steel structures — in 2018 a highway bridge in Italy collapsed, killing a number of motorists, and rusting of the cables holding up the bridge was found to have contributed.
Every prevention method works the same way: keep water and oxygen away from the iron.
Combustion of fuels can produce air pollutants such as carbon monoxide, sulfur dioxide and oxides of nitrogen.
| Air pollutant | Source | Effects |
| Carbon monoxide | Burning of fuels in factories and motor vehicles in the presence of insufficient oxygen | A colourless, odourless and highly toxic gas. Reduces the amount of oxygen taken up by red blood cells, leading to headaches and eventually death due to suffocation. |
| Sulfur dioxide | Burning of sulfur-containing fuels such as coal and petroleum in power stations and factories; volcanic eruptions | Irritates the eyes, nose and lungs; worsens asthma, heart disease and respiratory problems. Dissolves in rainwater to form acid rain, which damages buildings. |
| Oxides of nitrogen | Burning of fuels at high temperatures in motor vehicles, factories and power stations; reaction of nitrogen and oxygen in the air during lightning strikes | Dissolve in rainwater to form acid rain, even at low levels — harms aquatic life and damages buildings. |
Burning fuels also releases carbon dioxide. Removing forests makes this worse, because fewer trees are left to absorb carbon dioxide by photosynthesis. The extra carbon dioxide traps heat within the atmosphere, resulting in global warming.
When large amounts of carbon dioxide are released into the atmosphere, some dissolves in seawater and forms carbonic acid. The seawater becomes more acidic over time — this is known as the acidification of oceans.
The shells and skeletons of small marine animals such as oysters and corals contain calcium carbonate. You already know what acid does to a carbonate — so these shells are broken down as they react with the acidic seawater.
The two anchor points on that slider come straight from your revision guide: in 1875 atmospheric carbon dioxide was about 280 mg/l and seawater pH was 8.2, with thicker shells. By 2011 it was 400 mg/l and pH 8.1, with measurably thinner shells.
Organisms such as fish and plants may die as they are unable to survive under conditions of a lower pH, which in turn affects other organisms that depend on them. Mangroves, marshes and seagrass meadows store large amounts of carbon; when they are destroyed, they release that carbon back into the atmosphere as carbon dioxide.