Chemistry · Chemistry of the elements · Grade 7

Air: composition and pollution

What is in the air over Tashkent, why does fog thicken in winter, and why is rain acidic?

The situation

On a cold, windless winter day a grey haze hangs over the city: the distant mountains disappear from view, and air-quality maps warn “unhealthy”. In summer, even when the sky is clear, dust from the desert sometimes covers the city. What is the air we breathe made of, and what harmful substances get into it? Where do they come from, why does some rain eat away marble statues, and how can the air be cleaned?

Topic

Composition of air, pollution and acid rain. Air over Tashkent: composition, winter smog (PM2.5) and acid rain — traffic, heating and factory smoke

Key ideas

Composition of air
PM2.5 and gases
Acid rain

Formulas

\begin{matrix} φ (N_{2}) = 78.08 % & φ (O_{2}) = 20.95 % \\ φ (Ar) = 0.93 % & φ (CO_{2}) \approx 0.042 % \end{matrix}

The composition of dry air (by volume). Average molar mass ≈ 29 g/mol.

S + O_{2} \overset{}{\to} SO_{2}

When the sulfur in coal and fuel oil burns, sulfur(IV) oxide is formed.

2 SO_{2} + O_{2} \overset{}{\to} 2 SO_{3}

In the atmosphere SO₂ is oxidised and then, in raindrops, turns into sulfuric acid: $SO_{3} + H_{2} O \overset{}{\to} H_{2} SO_{4}$ .

4 NO_{2} + O_{2} + 2 H_{2} O \overset{}{\to} 4 HNO_{3}

Nitric acid from the nitrogen oxides formed in engines.

2 CO + 2 NO \overset{}{\to} 2 CO_{2} + N_{2}

In a car's catalytic converter the toxic CO and NO are turned into less harmful gases.

Theory

Air is a mixture of gases

Dry air by volume: nitrogen 78.08 %, oxygen 20.95 %, argon 0.93 %, carbon dioxide ≈ 0.042 % (2024, Mauna Loa: 424.6 ppm; in 1959 it was ≈ 316 ppm), and the rest — neon, helium, methane, krypton and others. By mass: N₂ 75.5 %, O₂ 23.1 %, Ar 1.29 %. The average molar mass is $\overline{M} = \sum φ_{i} M_{i} \approx 28.97$ g/mol. In addition, air contains water vapour in variable amounts (from 0 to ≈ 4 %) and dust. In the 1770s A. Lavoisier heated mercury in a closed vessel for 12 days and showed that about one fifth of the air was absorbed and that the remaining gas supported neither combustion nor breathing — he called this gas “azote” (“lifeless”), nitrogen.

Laboratory: the proportion of oxygen in air

The rusting method: damp steel wool is placed at the closed end of a measuring tube, and the tube is stood upside down in a vessel of water. The iron rusts and binds the oxygen; within a day almost all the O₂ is used up and water rises to take its place: ≈ 21 mL in 100 mL of air. Read the level with it equalised to the water level in the vessel. The candle method gives a wrong result. Under a bell jar a candle goes out when the oxygen has fallen to about 15–16 %, CO₂ is formed and partly replaces the oxygen — for chemical reasons the volume of gas falls by even less than 2 %. The water rises mainly for a physical reason: the flame heats the air, part of it escapes as bubbles, and when the candle goes out the remaining air cools and contracts. According to measurements, the water rises by about 10–13 % of the bell jar's volume for physical reasons and by less than 2 % for chemical reasons — so the conclusion “the water rose by 1/5, therefore O₂ is 21 %” is wrong.

What pollutes the air

Particulate matter PM2.5 and PM10 (diameter smaller than 2.5 and 10 µm) penetrates deep into the lungs; its sources are fuel combustion, transport and dust. Carbon monoxide CO is a product of incomplete combustion and blocks the transport of oxygen in the blood. Nitrogen oxides NOₓ form from the nitrogen and oxygen of the air at the high temperature inside an engine: $N_{2} + O_{2} \overset{}{\to} 2 NO$ , $2 NO + O_{2} \overset{}{\to} 2 NO_{2}$ . Sulfur(IV) oxide SO₂ comes from the sulfur in coal and fuel oil. In sunlight NOₓ and volatile compounds form ground-level ozone. WHO guidelines (2021): PM2.5 — daily 15, annual 5; PM10 — 45; NO₂ — 25; SO₂ — 40 µg/m³; CO — 4 mg/m³ (daily). According to World Bank estimates, the annual mean of PM2.5 in Tashkent is more than six times above the WHO guideline; the shares of the sources: dust blown in from across the border 36 %, heating 28 %, transport 16 %, industry 13 %. Pollution is highest in winter (heating), and dust predominates in summer.

Temperature inversion

Normally air gets colder with height: warm polluted air rises and mixes. On calm winter nights the ground and the air next to it cool quickly, while a warmer layer remains above — this is a temperature inversion. The warm layer acts like a “lid”: smoke and dust cannot rise above 100–200 m and collect near the ground. In the model the pollutant concentration is $C \propto \frac{E}{u \cdot H}$ — the greater the emission $E$ , and the smaller the wind speed $u$ and mixing height $H$ , the dirtier the air. In places ringed by mountains with little wind this is especially dangerous.

Acid rain

Even clean rain is slightly acidic: CO₂ from the air dissolves in water to form carbonic acid, giving a pH of ≈ 5.6. Acid rain has a pH below 5.6: it contains sulfuric and nitric acids formed from SO₂ and NO₂. It eats away limestone and marble: $CaCO_{3} + H_{2} SO_{4} \overset{}{\to} CaSO_{4} + H_{2} O + CO_{2} ↑$ , damages leaves, washes nutrients out of the soil and acidifies lakes — according to the US Environmental Protection Agency, at pH 5 the eggs of most fish do not hatch. The laboratory model: sulfur is burned in a vessel of oxygen, then water with universal indicator is poured in and shaken — SO₂ dissolves to form sulfurous acid ( $SO_{2} + H_{2} O \overset{}{⇌} H_{2} SO_{3}$ ) and the indicator turns red; a piece of marble dissolves, releasing CO₂, and a flower petal is bleached by SO₂. Safety: SO₂ is toxic and burns the airways — carry out the experiment only in a fume cupboard, wearing safety goggles; do not smell burning sulfur.

Protective measures and the model

Protecting the atmosphere: catalytic converters in cars (reducing CO, NOₓ and hydrocarbons by up to 90 %), public transport and the metro, heating homes with gas or a central system instead of coal, dust-trapping filters and SO₂ capture at factories, trees and green areas, no open burning of waste. Protecting the hydrosphere: treating waste water and reducing acid rain also protect lakes and rivers. The model: city air is a single-“box” model: the contribution of each source is proportional to $1 / (u H)$ ; the numbers were chosen to match the general picture published for Tashkent and do not replace real monitoring; dust is assumed to come from outside the city. The time of the rusting experiment is sped up (1 s = 1 h); the candle is paraffin C₂₅H₅₂, and it goes out at 16 % O₂; SO₂ is taken to dissolve completely in water, and the solutions are ideal.

Did you know?

Argon was discovered in 1894 because of the density of “atmospheric nitrogen”: Rayleigh found that 1 litre of nitrogen taken from air weighs 1.2572 g, while nitrogen made chemically weighs 1.2505 g. The difference was due to a heavier “impurity” — argon.
The amount of CO₂ measured at the Mauna Loa (Hawaii) Observatory since 1958 averaged ≈ 316 ppm in 1959 and reached 424.6 ppm in 2024 (NOAA data).

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Oxygen and ozone — Grade 7
Hydrogen — the lightest gas — Grade 7
Water: composition and properties — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Non-metals and halogens — Grade 8
Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8