10 Chemistry of the environment | IGCSE Chemistry 0620

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🎯What you need to be able to do

Test for water and for pure water; explain why distilled water is used in practical work.
State what natural water may contain, which substances are beneficial or harmful, and how domestic water is treated.
State that ammonium salts and nitrates are fertilisers, and describe NPK fertilisers.
State the composition of clean, dry air, and the sources and effects of CO₂, CO, particulates, methane, oxides of nitrogen and SO₂.
Explain strategies against climate change and acid rain; describe photosynthesis.
Explain the greenhouse effect, how oxides of nitrogen form and are removed by catalytic converters, and give the photosynthesis equation EXTENDED.

📚The chemistry

Water

Testing for water: it turns anhydrous copper(II) sulfate from white to blue, and anhydrous cobalt(II) chloride from blue to pink. These show that water is present. Testing for purity: pure water melts at exactly 0 °C and boils at exactly 100 °C (at normal pressure); impurities change these values. Distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities that could interfere with results.

Water from natural sources can contain:

beneficial: dissolved oxygen (for aquatic life); some metal compounds (essential minerals)

harmful: toxic metal compounds; plastics (harm aquatic life); sewage (microbes that cause disease); nitrates and phosphates from fertilisers and detergents (lead to deoxygenation of the water and damage to aquatic life)

A flow chart of domestic water treatment. River or reservoir water goes through sedimentation, where solids settle out; filtration through sand and gravel; carbon, which removes tastes and odours; chlorination, which kills microbes; and then to homes as clean water. — Treatment of the domestic water supply: sedimentation and filtration remove solids, carbon removes tastes and odours, chlorine kills microbes.

Fertilisers

Plants need nitrogen, phosphorus and potassium for good growth. NPK fertilisers supply all three. Ammonium salts and nitrates (for example ammonium nitrate, NH₄NO₃) supply nitrogen; potassium comes from potassium salts and phosphorus from phosphates.

Air

A pie chart of clean, dry air: nitrogen 78 percent, oxygen 21 percent and other gases 1 percent. The remaining 1 percent is noble gases, mostly argon at 0.9 percent, and carbon dioxide at about 0.04 percent. — Clean, dry air: about 78% nitrogen, 21% oxygen, and the rest noble gases and carbon dioxide.

The pollutants on the syllabus, with their sources and effects:

Carbon dioxide — complete combustion of carbon-containing fuels. More CO₂ increases global warming, which leads to climate change.
Carbon monoxide and particulates — incomplete combustion of carbon-containing fuels. CO is a toxic gas; particulates increase the risk of respiratory problems and cancer.
Methane — decomposition of vegetation and waste gases from digestion in animals. It increases global warming.
Oxides of nitrogen — car engines. They cause acid rain, photochemical smog and respiratory problems.
Sulfur dioxide — burning fossil fuels that contain sulfur compounds. It causes acid rain.

Reducing the damage

Climate change: planting trees (they take in CO₂), less livestock farming (less methane), burning fewer fossil fuels, and using more hydrogen and renewable energy such as wind and solar.
Acid rain: catalytic converters in vehicles; low-sulfur fuels; and flue gas desulfurisation, in which calcium oxide reacts with the sulfur dioxide in power-station waste gases.

The greenhouse effect EXTENDED

Energy from the Sun passes through the atmosphere to the Earth's surface; some is reflected. The Earth emits thermal, infrared, energy. Some escapes to space, but some is absorbed by carbon dioxide and methane in the atmosphere and re-emitted, some of it back towards Earth. — Greenhouse gases absorb thermal energy emitted by the Earth and re-emit it in all directions, some back towards the surface, which reduces the thermal energy lost to space. EXTENDED

Catalytic converters EXTENDED

In a car engine the temperature is high enough for nitrogen and oxygen from the air to react: \( \mathrm{N_2 + O_2 \rightarrow 2NO} \). The catalytic converter turns the harmful gases in the exhaust into less harmful ones. Carbon monoxide is oxidised and nitrogen monoxide is reduced:

A catalytic converter in an exhaust pipe: a box containing a honeycomb coated with catalyst, giving a large surface area. Gases from the engine, carbon monoxide, nitrogen monoxide and unburnt fuel, go in; carbon dioxide, nitrogen and water come out. Equation: 2CO + 2NO gives 2CO2 + N2. — The honeycomb gives the catalyst a large surface area. EXTENDED

Photosynthesis

Plants make glucose from carbon dioxide and water, using energy from light, in the presence of chlorophyll:

carbon dioxide + water → glucose + oxygen

\[ \mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2} \]

EXTENDED The symbol equation is Supplement. Photosynthesis removes CO₂ from the air, which is why planting trees is a climate strategy.

✏️Worked example

A power station burns coal that contains sulfur compounds. (a) Name the gas formed from the sulfur and state its environmental effect. [2] (b) Name the substance used to remove this gas from the waste gases, and suggest why it works. [2] (c) EXTENDED Explain how carbon dioxide from the power station contributes to global warming. [3]

(a) Sulfur dioxide. It causes acid rain.

(b) Calcium oxide (flue gas desulfurisation). Sulfur dioxide is an acidic oxide and calcium oxide is a basic oxide, so they react together.

(c) CO₂ absorbs thermal energy emitted by the Earth’s surface and re-emits it, some back towards the Earth. This reduces the thermal energy lost to space, so the atmosphere warms.

Check it. (b) links topics: acidic oxide + basic oxide (topic 7). That is the reasoning an examiner wants for “suggest why”.

Mixing up the pollutants. CO₂ and methane cause global warming, not acid rain; SO₂ and oxides of nitrogen cause acid rain, not global warming. And “the ozone layer” is not on this syllabus.

📝Practise

In the style of the multiple-choice and theory papers. EXTENDED marks Supplement content.

1. (Multiple choice.) Approximately what percentage of clean, dry air is oxygen? A: 1%. B: 21%. C: 78%. D: 79%.

B. Nitrogen is 78%; the remaining 1% is mostly argon.

2. (Theory.) In the treatment of drinking water, state the purpose of (a) carbon, (b) chlorine. [2]

(a) Removes tastes and odours. (b) Kills microbes.

3. (Theory.) A colourless liquid turns anhydrous copper(II) sulfate blue. Does this show that the liquid is pure water? How could you check? [2]

No: it only shows that water is present. Measure its boiling point: pure water boils at exactly 100 °C (or melts at 0 °C).

4. (Theory.) A farmer uses potassium nitrate and ammonium phosphate. Which of the elements N, P and K does each supply? [2]

Potassium nitrate: K and N. Ammonium phosphate: N and P. Together they supply all three.

5. (Theory.) A gas heater burns methane in a poorly ventilated room. Name the toxic gas that can form, and explain why it forms. [2]

Carbon monoxide. There is not enough oxygen for complete combustion (incomplete combustion).

6. (Theory.) Give two strategies for reducing climate change, and explain how one of them works. [3]

Any two: plant trees; reduce livestock farming; burn fewer fossil fuels; use hydrogen or renewable energy (wind, solar). E.g. trees take in carbon dioxide for photosynthesis.

7. (Theory.) Fertiliser washes from a field into a river. Describe the effect on the river. [2]

The nitrates and phosphates lead to deoxygenation of the water (the dissolved oxygen falls), which damages or kills aquatic life.

8. (Theory.) EXTENDED Explain why oxides of nitrogen form in a car engine, and write an equation for their removal in a catalytic converter. [3]

The high temperature in the engine makes nitrogen and oxygen from the air react. \( \mathrm{2CO + 2NO \rightarrow 2CO_2 + N_2} \).

9. (Theory.) EXTENDED Write the balanced symbol equation for photosynthesis, and name the two conditions it needs. [3]

\( \mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2} \). Light (energy) and chlorophyll.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

PhET “The Greenhouse Effect” — add greenhouse gases and watch the energy balance change
NASA Climate — the carbon dioxide record, with live data