Chemistry of the Environment: Water, Fertilisers and Air
Save the Planet, Pass the Paper 🌍
Introduction
1. Introduction
Okay, this is the chapter where chemistry meets the real world: the water you drink, the food on your plate, and the air that's slowly cooking the planet. And here's the good bit, it's basically a facts chapter. The marks come from knowing exact colours, exact numbers, exact sources and effects, and one word equation (photosynthesis).
So this is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Big rule for the whole chapter, keep two things separate, present vs pure (a test for water isn't a test for clean water) and natural vs pollutant (CO₂ is normal air, SO₂ is not). Let's save the planet and pass the paper. 🌍
So this is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Big rule for the whole chapter, keep two things separate, present vs pure (a test for water isn't a test for clean water) and natural vs pollutant (CO₂ is normal air, SO₂ is not). Let's save the planet and pass the paper. 🌍
2. Testing for Water
Two different jobs here. To show water is present, use a chemical colour test: anhydrous copper(II) sulfate goes white → blue, and anhydrous cobalt(II) chloride goes blue → pink. To show water is pure, you can't use colour, you check its boiling point: pure water boils sharply at exactly 100 °C (and freezes at 0 °C). So "is it water?" and "is it pure water?" are two separate questions with two separate tests.
Key idea🔑 Key idea: Copper(II) sulfate white → blue and cobalt(II) chloride blue → pink prove water is present. A sharp 100 °C boiling proves it's pure. 💧
Worked example
Worked Example: Spotting Water
White to Blue 🔵
Which test and colour change shows the presence of water?
- 1The test for water uses anhydrous copper(II) sulfate, which is white.
- 2Add water and it turns blue.
So the change is white → blue (and it shows water is present, not that it's pure). ✅
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3. Is the Water Pure?
Colour tests prove water is there, but for pure you check a temperature. Pure water boils at a sharp, fixed 100 °C (and melts at 0 °C). If something's dissolved in it, the boiling point climbs above 100 °C and the change smears over a range instead of one clean value. So: sharp and exact = pure; high and spread out = impure.
Key idea🔑 Key idea: Pure water boils sharply at 100 °C. An impurity raises the boiling point and spreads it over a range. 🌡️
Worked example
Worked Example: Pure or Not?
Sharp at 100 ❄️
Liquid X boils at exactly 100 °C. Liquid Y boils at 102 °C and the temperature keeps rising. Which is pure water?
- 1Pure water boils at a single fixed 100 °C and stays there.
- 2Liquid X does this; Y boils too high and over a range (so it's impure).
So the answer is X. ✅
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4. Cleaning Water for Drinking
Natural water can contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, and nitrates and phosphates from fertilisers and detergents. Some of these are beneficial: dissolved oxygen for aquatic life, and some metal compounds provide essential minerals. Others are harmful: toxic metal compounds, plastics harming aquatic life, sewage carrying disease-causing microbes, and nitrates and phosphates causing deoxygenation of water. Turning it into tap water: sedimentation and filtration remove solids (big bits settle, then sand and gravel trap smaller bits), carbon removes tastes and odours, and chlorination kills microbes. One trap: this makes water safe, not pure, because dissolved salts stay in.
Key idea🔑 Key idea: Water treatment = sedimentation and filtration (solids) → carbon (tastes and odours) → chlorination (microbes). It makes water safe, not pure. Beneficial: dissolved oxygen, essential minerals. Harmful: toxic metal compounds, plastics, sewage, nitrates and phosphates. 🚰
Worked example
Worked Example: Clean It in Order
Settle → Filter → Chlorinate 🧪
Muddy water with stones, fine clay and germs is cleaned by sedimentation, filtration and chlorination. What's the correct order?
- 1Sedimentation settles the big stones, then filtration removes the fine clay.
- 2Chlorination kills the germs last.
So the order is sedimentation → filtration → chlorination. ✅
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5. NPK Fertilisers
Plants take elements from the soil, and harvesting removes them, so farmers add fertilisers to put them back. Ammonium salts and nitrates are used as fertilisers. An NPK fertiliser provides the elements nitrogen, phosphorus and potassium for improved plant growth, and since one compound rarely has all three, it's usually a mixture. The trick in the exam is reading the ions: NH₄⁺ or NO₃⁻ gives N, PO₄³⁻ gives P, K⁺ gives K.
Key idea🔑 Key idea: Ammonium salts and nitrates are fertilisers. NPK = nitrogen, phosphorus, potassium for improved plant growth. Read the ions: NH₄/NO₃ → N, PO₄ → P, K → K. 🌱
Worked example
Worked Example: The Third NPK Element
Don't Forget the K ✔️
Phosphorus is one element in NPK fertilisers. Which is another: potassium, krypton, nickel or platinum?
- 1NPK stands for nitrogen, phosphorus and potassium.
- 2Of the options, only potassium is one of those three.
So the answer is potassium. ✅
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6. What's in Clean Air
Clean, dry air is a fixed mixture, and these numbers are pure free marks: nitrogen 78%, oxygen 21%, argon about 1%, carbon dioxide about 0.04%. Those four are the normal gases. Anything else, sulfur dioxide, carbon monoxide, oxides of nitrogen, is a pollutant and only shows up in polluted air. Classic trap: argon is in clean air, but carbon monoxide is not.
Key idea🔑 Key idea: Clean dry air = N₂ 78%, O₂ 21%, Ar ~1%, CO₂ ~0.04%. Everything else is a pollutant. 🌬️
Worked example
Worked Example: What's Clean Air Made Of?
78 and 21 🔢
Which row shows clean, dry air: (A) nitrogen 78%, oxygen 21%, a little carbon dioxide; (B) nitrogen 21%, oxygen 78%, a little carbon dioxide; (C) nitrogen 21%, oxygen 78%, a little carbon monoxide; (D) nitrogen 78%, oxygen 21%, a little carbon monoxide?
- 1Clean air is mostly nitrogen (78%), then oxygen (21%), with a little carbon dioxide.
- 2Carbon monoxide is a pollutant, so (C) and (D) are out.
So the answer is (A). ✅
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7. Acid Rain and Toxic Gases
Each pollutant has its own source and its own harm. Carbon monoxide (CO) is toxic, reducing the oxygen your blood can carry; it comes from incomplete combustion of carbon-containing fuels, and so do particulates, which increase the risk of respiratory problems and cancer. Sulfur dioxide (SO₂) comes from burning fossil fuels that contain sulfur compounds; it dissolves in rain to make acid rain that damages buildings, trees and lakes. Oxides of nitrogen come from car engines and cause acid rain, photochemical smog and respiratory problems. Keep them separate: acid rain ≠ global warming.
Key idea🔑 Key idea: CO = toxic. Particulates = respiratory problems and cancer. SO₂ and oxides of nitrogen = acid rain (oxides of nitrogen also photochemical smog). CO₂/CH₄ = global warming. Don't swap them. 🌧️
Worked example
Worked Example: Which Gas Causes Acid Rain?
Blame the Sulfur ⚗️
What is a harmful effect of sulfur dioxide in the atmosphere: acid rain, global warming, less oxygen, or ozone damage?
- 1Sulfur dioxide is acidic and dissolves in rain to make it acidic.
- 2That makes acid rain (not global warming, which is CO₂ and methane).
So the answer is acid rain. ✅
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8. Cleaning Up the Air
The syllabus names three strategies against acid rain. Catalytic converters in vehicles change harmful gases (oxides of nitrogen, and carbon monoxide) into less harmful ones. Low-sulfur fuels mean less sulfur dioxide forms when the fuel burns. Flue gas desulfurisation at power stations removes sulfur dioxide by reacting the acidic gas with calcium oxide, a base. Different problem, different fix.
Key idea🔑 Key idea: Acid rain strategies: catalytic converters (oxides of nitrogen → less harmful), low-sulfur fuels (less SO₂ made), flue gas desulfurisation with calcium oxide (base removes acidic SO₂). 🚗
Worked example
Worked Example: What's a Catalytic Converter For?
Harmful In, Harmless Out 🔧
What is the main purpose of a catalytic converter in a car exhaust?
- 1Exhaust gases contain harmful carbon monoxide and oxides of nitrogen.
- 2The converter changes these into less harmful gases.
So its purpose is to change harmful exhaust gases into less harmful ones. ✅
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9. Greenhouse Gases
The two gases to name are carbon dioxide (CO₂), from the complete combustion of carbon-containing fuels, and methane (CH₄), from decomposing vegetation and waste gases from digestion in animals. Higher levels of both cause increased global warming (a rise in the Earth's average temperature), which leads to climate change. At Core you state the effect; explaining the mechanism is Extended work. The strategies to learn, each with its reason: planting trees (they take CO₂ out of the air by photosynthesis), reducing livestock farming (less methane), decreasing fossil-fuel use (less CO₂), and using more hydrogen and renewable energy such as wind and solar (no CO₂ from burning fuel).
Key idea🔑 Key idea: CO₂ and CH₄ → increased global warming → climate change. Strategies: plant trees, less livestock farming, fewer fossil fuels, more hydrogen and renewable energy. 🌡️
Worked example
Worked Example: What Rising Greenhouse Gases Do
Hotter Planet ♨️
What is the main consequence of rising carbon dioxide and methane: a rise in average global temperature, a fall in temperature, more acid rain, or less oxygen?
- 1Carbon dioxide and methane are the gases whose higher levels cause increased global warming.
- 2Global warming is a rise in the average global temperature, and it leads to climate change.
So the answer is a rise in the average global temperature. ✅
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10. Photosynthesis
Green plants pull carbon dioxide out of the air, reacting it with water in the presence of chlorophyll using energy from light. Learn the word equation: carbon dioxide + water → glucose + oxygen. Big picture: forests are a carbon sink that lowers CO₂, so growing more forests helps limit the rise of carbon dioxide, and chopping them down makes the global-warming problem worse.
Key idea🔑 Key idea: carbon dioxide + water → glucose + oxygen. Photosynthesis removes CO₂ from the air. 🌳
Worked example
Worked Example: Photosynthesis Word Equation
Carbon Dioxide In, Oxygen Out 🍃
Which word equation correctly represents photosynthesis?
- 1Plants take in carbon dioxide + water.
- 2Using light they make glucose + oxygen.
So it's carbon dioxide + water → glucose + oxygen. ✅
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