ChemistryICSE Class 9 · Atmospheric and Environmental Chemistry

Acid Rain and Global Warming: Causes, Equations, Effects and Remedies (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Acid rain (pH below 5.6) forms when sulphur dioxide (from burning sulphur-containing fuels) and oxides of nitrogen (from high-temperature combustion and engines) dissolve in rain to give sulphuric and nitric acids. It leaches soil nutrients, acidifies lakes and corrodes marble and metals. Global warming is caused by greenhouse gases such as carbon dioxide and methane trapping infrared heat.

Understand Acid Rain and Global Warming: Causes, Equations, Effects and Remedies

Acid rain. Normal rain has a pH of about 5.6 (dissolved CO₂). Rain with a pH below 5.6 is acid rain, mainly because of sulphuric acid and nitric acid.

*Oxides of sulphur* (coal and petroleum contain sulphur):
$S + O_{2} \to S O_{2}$
$2 S O_{2} + O_{2} \to 2 S O_{3}$ (slowly, helped by sunlight and dust)
$S O_{3} + H_{2} O \to H_{2} S O_{4}$ ; $S O_{2} + H_{2} O \to H_{2} S O_{3}$

*Oxides of nitrogen* (furnaces, power stations, vehicle engines and lightning, at very high temperatures):
$N_{2} + O_{2} \to 2 N O$ ; $2 N O + O_{2} \to 2 N O_{2}$ ; $4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$

Effects.

*Soil:* washes out nutrients (Ca, Mg, K); releases toxic aluminium; makes soil acidic and harms soil organisms and crops.
*Water:* acidic lakes; fish eggs fail to hatch; aluminium harms gills.
*Buildings and metals:* marble and limestone are eaten away, $C a C O_{3} + H_{2} S O_{4} \to C a S O_{4} + H_{2} O + C O_{2}$ ; iron and steel corrode faster.

Reduction. Low-sulphur fuels; removing SO₂ from chimney gases (lime or limestone scrubbers: $C a O + S O_{2} \to C a S O_{3}$ ; $C a C O_{3} + S O_{2} \to C a S O_{3} + C O_{2}$ ); catalytic converters in vehicles ( $2 N O \to N_{2} + O_{2}$ ); public transport, CNG and electric vehicles; liming of lakes as a short-term fix.

Greenhouse effect and global warming. Sunlight warms the Earth; the Earth gives off infrared radiation, part of which is absorbed and re-emitted by greenhouse gases. More gas traps more heat and raises the average temperature.

Gas	Sources	Reduction
Carbon dioxide	burning fuels, deforestation	renewable energy, efficiency, planting trees
Methane	paddy fields, cattle, landfills, marshes, coal mines	biogas capture, landfill management, composting
Oxides of nitrogen (N₂O)	nitrogen fertilisers, engines	careful fertiliser use, catalytic converters
Water vapour	evaporation of oceans	follows temperature

*Effects:* melting ice and rising sea level, more extreme weather, loss of species, spread of disease.

Mass questions. Use the balanced equations: for example, 32 kg of sulphur gives 64 kg of SO₂, and 32 kg of sulphur could form 98 kg of H₂SO₄ if all the SO₂ became acid.

Acid Rain and Global Warming: Causes, Equations, Effects and Remedies: formulas and key facts

Sulphur route: $S \to S O_{2} \to S O_{3} \to H_{2} S O_{4}$; S + O₂; 2SO₂ + O₂; SO₃ + H₂O.
Nitrogen route: $N_{2} \to 2 N O \to 2 N O_{2} \to 4 H N O_{3}$; High temperature in engines and lightning.
Marble attacked: $C a C O_{3} + H_{2} S O_{4} \to C a S O_{4} + H_{2} O + C O_{2}$; 100 g marble per 98 g acid.
Scrubbing SO₂: $C a O + S O_{2} \to C a S O_{3}$; 56 g CaO per 64 g SO₂.

How to solve Acid Rain and Global Warming: Causes, Equations, Effects and Remedies questions

Follow the gas from the source to the acid.

1Write the sequence of balanced equations from fuel to acid.
2For masses use the relative masses with coefficients (S = 32, O = 16, H = 1, Ca = 40, C = 12).
3Convert percentage of sulphur in coal to a mass first.
4For remedies match each to its target gas.
5For global warming give gas, source and reduction in a table.

Acid Rain and Global Warming: Causes, Equations, Effects and Remedies in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

A coal-fired plant

Suppose a plant burns 800 kg of coal containing 3% sulphur (S = 32, O = 16, H = 1).

Find the mass of sulphur dioxide released and the mass of sulphuric acid that could form from it.

Try it first, then show the solutionSolution

Sulphur $= 0.03 \times 800 = 24$ kg.
$S \to S O_{2}$ : $24 \times \frac{64}{32} = 48$ kg.
$S \to H_{2} S O_{4}$ : $24 \times \frac{98}{32} = 73.5$ kg.

Answer: 48 kg SO₂; up to 73.5 kg H₂SO₄.

Memory hook: Percentage to mass, then ratio.

Tricks to remember Acid Rain and Global Warming: Causes, Equations, Effects and Remedies

Short memory aids. Say each one aloud once, then use it on a question the same day.

Below 5.6
Acid rain has pH below 5.6.
Use it when: Definition.
Two engines, two acids
Sulphur gives H₂SO₄; nitrogen oxides give HNO₃.
Use it when: Composition.
Gas, source, remedy
Learn each greenhouse gas as a trio.
Use it when: Table questions.

Acid Rain and Global Warming: Causes, Equations, Effects and Remedies: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: pH of acid rain · 1 mark · MCQBoard level

Rain is considered acid rain when its pH is below:
(a) 7.0
(b) 5.6
(c) 3.0
(d) 8.4

Try it first, then show the solutionSolution

Normal rain has pH about 5.6 because of dissolved carbon dioxide.

Answer: (b) 5.6

Example 2: From sulphur to acid · 2 marks · Short answerBoard level

Write the balanced equations for the formation of sulphuric acid in the atmosphere starting from sulphur in a fuel.

Try it first, then show the solutionSolution

$S + O_{2} \to S O_{2}$ and $2 S O_{2} + O_{2} \to 2 S O_{3}$ . (1)
$S O_{3} + H_{2} O \to H_{2} S O_{4}$ . (1)

Answer: See working

Example 3: Coal and sulphur dioxide · 3 marks · Short answerHarder board level

800 kg of coal contains 3% sulphur (S = 32, O = 16, H = 1). Find the mass of sulphur, the mass of sulphur dioxide formed on burning and the maximum mass of sulphuric acid that could result.

Try it first, then show the solutionSolution

Sulphur $= 24$ kg. (1)
$S O_{2} = 24 \times \frac{64}{32} = 48$ kg. (1)
$H_{2} S O_{4} = 24 \times \frac{98}{32} = 73.5$ kg. (1)

Answer: 24 kg S; 48 kg SO₂; 73.5 kg H₂SO₄

Example 4: Marble statue · 3 marks · Short answerHarder board level

4.9 kg of sulphuric acid in acid rain falls on a marble statue (Ca = 40, C = 12, O = 16, S = 32, H = 1). Write the equation and find the mass of marble (taken as CaCO₃) that dissolves and the mass of carbon dioxide released.

Try it first, then show the solutionSolution

$C a C O_{3} + H_{2} S O_{4} \to C a S O_{4} + H_{2} O + C O_{2}$ . (1)
Marble $= 4.9 \times \frac{100}{98} = 5$ kg. (1)
Carbon dioxide $= 4.9 \times \frac{44}{98} = 2.2$ kg. (1)

Answer: 5 kg CaCO₃; 2.2 kg CO₂

Example 5: A town with a power station · 4 marks · Case-basedHOTS / competency

A town’s coal-fired power station releases sulphur dioxide and oxides of nitrogen.
(a) Write the equations by which NO is formed and then turned into nitric acid in rain.
(b) A scrubber uses quicklime: write the equation and find the mass of CaO needed for 6.4 kg of SO₂ (Ca = 40, S = 32, O = 16).
(c) Why does a catalytic converter help?
(d) State one effect of the acid rain on a lake.

Try it first, then show the solutionSolution

(a) $N_{2} + O_{2} \to 2 N O$ ; $2 N O + O_{2} \to 2 N O_{2}$ ; $4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$ . (1½)
(b) $C a O + S O_{2} \to C a S O_{3}$ ; $6.4 \times \frac{56}{64} = 5.6$ kg CaO. (1½)
(c) It converts NO into harmless nitrogen and oxygen so that less NO₂ and HNO₃ form. (½)
(d) Fish eggs do not hatch and fish die; aluminium poisons gills. (½)

Answer: (b) 5.6 kg (c) NO → N₂ + O₂ (d) fish die

Where students lose marks in Acid Rain and Global Warming: Causes, Equations, Effects and Remedies

The slips examiners see most often, and what to write instead.

Mistake: Acid rain explained only by carbon dioxide
CO₂ gives normal rain pH 5.6; acid rain is due to SO₂ and NOx.
Wrong working
Acid rain is caused by CO₂.
Right working
Acid rain is caused by oxides of sulphur and nitrogen (pH below 5.6).
Do this: Name H₂SO₄ and HNO₃.
Usually costs: 1 mark.
Mistake: Equations not balanced
The nitrogen route has O₂ and H₂O in the last step.
Wrong working
NO₂ + H₂O → HNO₃
Right working
4NO₂ + 2H₂O + O₂ → 4HNO₃
Do this: Count atoms.
Usually costs: 1 mark.
Mistake: Percentage not converted to mass first
Mass of S is 3% of the coal.
Wrong working
SO₂ from 800 kg coal as 800 × 2.
Right working
Sulphur 24 kg, then SO₂ 48 kg.
Do this: Find the element’s mass first.
Usually costs: The answer.

Exam tips

Always write the sequence of equations from fuel to acid.
Give two effects each on soil, water and buildings.
Use a table for greenhouse gases: source and remedy.
State pH 5.6 as the boundary for acid rain.

Practice questions on Acid Rain and Global Warming: Causes, Equations, Effects and Remedies

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The oxide of nitrogen formed when nitrogen and oxygen combine at the high temperature of a vehicle engine is:
(a) N₂O
(b) NO
(c) NO₂
(d) N₂O₅
Hint
The first oxide in the nitrogen route.
Answer
(b) NO
Q2 · 1 mark · Very short answerBoard level
Name a greenhouse gas released from paddy fields.
Hint
Also from cattle.
Answer
Methane.
Q3 · 2 marks · Short answerHarder board level
Write balanced equations for NO → NO₂ and NO₂ → HNO₃ in the atmosphere.
Hint
Oxygen and water are needed.
Answer
$2 N O + O_{2} \to 2 N O_{2}$ ; $4 N O_{2} + 2 H_{2} O + O_{2} \to 4 H N O_{3}$ .
Q4 · 3 marks · Short answerHarder board level
A scrubber removes 12.8 kg of sulphur dioxide using limestone: $C a C O_{3} + S O_{2} \to C a S O_{3} + C O_{2}$ (Ca = 40, C = 12, S = 32, O = 16). Find the mass of limestone used and the carbon dioxide released.
Hint
64 kg SO₂ needs 100 kg CaCO₃.
Answer
Limestone $= 12.8 \times \frac{100}{64} = 20$ kg; CO₂ $= 12.8 \times \frac{44}{64} = 8.8$ kg.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Catalytic converters reduce the formation of acid rain.
Reason (R): They convert nitric oxide from vehicle exhausts into nitrogen.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Less NO means less HNO₃.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
A town wants to cut acid rain. Compare low-sulphur fuel and catalytic converters: which gas does each target, and which acid is reduced?
Hint
Different sources, different gases.
Answer
Low-sulphur fuel (and scrubbers) reduces SO₂ and so sulphuric acid in rain. Catalytic converters convert NO from engine exhausts into N₂ and reduce HNO₃. Both are needed because the two acids come from different gases and sources.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.pH of acid rain?Show answer
Below 5.6.
2.Acids in acid rain?Show answer
Sulphuric and nitric (also sulphurous and nitrous).
3.Source of NO in engines?Show answer
Nitrogen and oxygen of air combine at very high temperature.
4.Greenhouse gas from cattle?Show answer
Methane.
5.What does acid rain do to marble?Show answer
Dissolves calcium carbonate: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.

Acid Rain and Global Warming: Causes, Equations, Effects and Remedies: frequently asked questions

How is acid rain formed?

Sulphur dioxide and oxides of nitrogen from burning fuels dissolve in rainwater to form sulphuric and nitric acids.

What are the effects of acid rain?

It leaches soil nutrients and releases toxic aluminium, acidifies lakes killing fish, and corrodes marble, limestone and metals.

How can acid rain be reduced?

Use low-sulphur fuels, scrub SO₂ from chimney gases, fit catalytic converters, and use cleaner transport.