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, and one word equation the examiner asks for every single year.
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
Turning river water into tap water is three steps, and the order matters. First sedimentation (big bits settle out), then filtration (sand and gravel trap the smaller bits), then chlorination (a little chlorine to kill the harmful microbes). Solids out first, germs killed last. One trap: this makes water safe, not pure, the dissolved ions stay in. Oh, and the harmful microbes come from sewage.
Key idea🔑 Key idea: Water treatment order = sedimentation → filtration → chlorination. It makes water safe, not pure. Microbes come from sewage. 🚰
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 need three big elements from the soil: N (nitrogen) for leaves, P (phosphorus) for roots, K (potassium) for flowers and fruit. Crops use them up, so farmers add fertilisers to put them back. An NPK fertiliser supplies all three, and since one compound rarely has everything, you often mix compounds. The trick in the exam is reading the ions: NH₄⁺ or NO₃⁻ gives N, PO₄³⁻ gives P, K⁺ gives K.
Key idea🔑 Key idea: N (leaves), P (roots), K (flowers/fruit). NPK = all three. 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 harm. Carbon monoxide (CO) is toxic, it stops your blood carrying oxygen (it comes from incomplete combustion). Sulfur dioxide (SO₂) is acidic, it dissolves in rain to make acid rain that damages buildings, trees and lakes (it comes from sulfur in the fuel). Oxides of nitrogen also cause acid rain plus breathing problems. Keep them separate: acid rain ≠ global warming.
Key idea🔑 Key idea: CO = toxic. SO₂ (and NOₓ) = acid rain. 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
We have ways to cut each pollutant. Catalytic converters in car exhausts change harmful gases (carbon monoxide and oxides of nitrogen) into less harmful ones. Flue gas desulfurisation at power stations removes sulfur dioxide by neutralising the acidic gas with a base like calcium oxide. And particulate filters trap the tiny solid bits from diesel engines. Different problem, different fix.
Key idea🔑 Key idea: Converter = harmful gases → less harmful. Flue gas desulfurisation = base neutralises acidic SO₂. Filters = trap particulates. 🚗
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 greenhouse gases to name are carbon dioxide (CO₂) and methane (CH₄). They trap heat in the atmosphere, so as their levels rise (mostly from burning fossil fuels, plus cattle and rotting waste for methane), the average global temperature goes up, that's global warming, and it drives climate change. CO₂ is only 0.04% of air, but even a small rise traps a lot more heat.
Key idea🔑 Key idea: Greenhouse gases (CO₂ and CH₄) trap heat → average global temperature rises → global warming. 🌡️
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?
- 1Greenhouse gases trap heat in the atmosphere.
- 2More trapped heat means the average global temperature rises, causing 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 using light energy. 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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