Acid rain

The sulphur route. Fossil fuels such as coal, petrol, and diesel contain sulphur as an impurity. When these fuels burn, the sulphur burns alongside the carbon:

where: $S$ is the sulphur impurity in the fuel, $O_2$ is atmospheric oxygen supplying the combustion, and $SO_2$ is sulphur dioxide, the primary sulphur pollutant released. Some of this sulphur dioxide is further oxidised in the air to sulphur trioxide:

The nitrogen route. Separately, the very high temperatures inside furnaces and internal-combustion engines are hot enough to force the otherwise unreactive nitrogen and oxygen of the air to combine directly — a reaction that simply does not happen at ordinary temperatures:

where: $N_2$ and $O_2$ are the two major gases of the air itself, $NO$ is nitric oxide formed first at high engine or furnace temperature, and $NO_2$ is nitrogen dioxide formed by its further oxidation in air. This route needs no sulphur at all — it is a pure high-temperature-combustion effect, which is why even a low-sulphur petrol engine still contributes to acid rain.

Turning oxides into acids. These four gases — $SO_2$, $SO_3$, $NO$, $NO_2$ — are collectively the oxides of sulphur and nitrogen. None of them is itself an acid; each becomes one only on dissolving in the water droplets of clouds:

where: $H_2SO_3$ is sulphurous acid, $H_2SO_4$ is sulphuric acid (the more significant and stronger of the two sulphur acids), and $HNO_3$ is nitric acid, formed as nitrogen dioxide disproportionates in atmospheric moisture. The rain, snow, fog, or dew that falls carrying these dissolved acids is acid rain. The figure below traces both routes from the burning fuel to the acid that finally falls as rain.

Acid rain is entirely a story of what dissolves in atmospheric water and what does not. Ordinary rain is mildly acidic because atmospheric carbon dioxide dissolves in cloud moisture to form weak carbonic acid — a background acidity every raindrop already carries. What makes acid rain different is not a new mechanism but a far stronger one: sulphur and nitrogen oxides dissolve in exactly the same droplets, but the acids they form — sulphurous, sulphuric, and nitric acid — are markedly stronger than carbonic acid, so even small quantities of these gases sharply raise the acidity of the rain that eventually falls. The gas itself is never the acid; $SO_2$ and $NO_2$ are acidic oxides that must first react with water before an acid molecule exists, which is why a coal plant's smoke is not acid rain until it meets cloud moisture. The two sources differ in what triggers them: the sulphur route needs sulphur impurity in the fuel and simple combustion, while the nitrogen route needs no sulphur at all — only the extreme heat inside an engine or furnace, hot enough to break the strong triple bond of atmospheric $N_2$ and force it to react with $O_2$. This is why vehicle exhaust remains a major contributor to acid rain even when burning clean, low-sulphur petrol. Once formed, the acids do not stay where they are released — winds carry the gases and the rain that eventually forms may fall hundreds of kilometres downwind, which is why acid rain damage often shows up far from any obvious smokestack. On reaching the ground, the acids attack calcium carbonate directly, which is why marble and limestone monuments corrode, and they alter soil chemistry by leaching away essential plant nutrients while releasing toxic metal ions such as aluminium that harm plant roots. In water bodies, the added acidity lowers the pH of lakes and rivers, reducing their capacity to support fish and other aquatic life, sometimes fatally.

Question: A coal-burning power plant releases sulphur dioxide into the air, while diesel trucks nearby run hot engines. Trace, with equations, how both sources contribute to the acid rain that eventually falls, and name the acids formed.

Step 1. Identify the sulphur source. Coal contains sulphur as an impurity, so combustion releases sulphur dioxide directly: $S + O_2 \rightarrow SO_2$.

Step 2. Follow the sulphur oxide further. Atmospheric oxygen, often aided by dust or sunlight, oxidises some of this $SO_2$ to $SO_3$: $2SO_2 + O_2 \rightarrow 2SO_3$.

Step 3. Identify the nitrogen source. The trucks' hot diesel engines force atmospheric nitrogen and oxygen to combine, a reaction that needs no sulphur at all: $N_2 + O_2 \rightarrow 2NO$, followed by $2NO + O_2 \rightarrow 2NO_2$.

Step 4. Dissolve every oxide in cloud moisture. $SO_2 + H_2O \rightarrow H_2SO_3$, $SO_3 + H_2O \rightarrow H_2SO_4$, and $3NO_2 + H_2O \rightarrow 2HNO_3 + NO$. Answer: the power plant's sulphur dioxide and sulphur trioxide give sulphurous acid and sulphuric acid, while the trucks' nitrogen oxides give nitric acid, and the rain that falls carries all three, making it acid rain.

• Thinking acid rain is caused only by sulphur compounds — nitrogen oxides from high-temperature combustion are an equally important cause and need no sulphur in the fuel at all.
• Confusing normal, mildly acidic rain (from dissolved atmospheric $CO_2$ forming weak carbonic acid) with acid rain, which is markedly more acidic because of $SO_2$, $SO_3$, $NO$, and $NO_2$.
• Forgetting that the oxide must first dissolve in water to form the acid — $SO_2$ gas itself is not an acid; only the reaction $SO_2 + H_2O \rightarrow H_2SO_3$ produces one.
• Assuming acid rain only damages buildings — its effects on soil chemistry (nutrient loss, toxic metal release) and on aquatic ecosystems in lakes and rivers are equally examinable.

Ask the tutor: Want to walk through the full sulphur-to-acid-rain or nitrogen-to-acid-rain equation chain again, or see how both routes combine in a single mixed exam question? Ask the AI tutor to take you through it step by step.