Chemistry · Unit 17

Environmental Chemistry

Chemistry and the planet

This unit applies the rest of the subject to the planet, which is where chemistry stops being neutral and starts having consequences people argue about.

It covers the atmosphere's composition, air pollution and its sources, greenhouse gases and the mechanism of climate change, water treatment, and the principles of green chemistry.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

The Atmosphere
Air Pollution
Greenhouse Gases and Climate
Water Treatment
Green Chemistry

Where this fits

Best late in the track. Overlaps with Geography's Environment and Sustainability.

Where people slip

The greenhouse effect and ozone depletion are separate problems with different causes and different chemistry. They get conflated constantly and share almost nothing but the atmosphere.

How the unit is structured

Environmental Chemistry runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Air Pollution

Multiple choiceLevel 1
1. Which toxic gas is formed by the incomplete combustion of fuels?
- Carbon monoxidecorrect
- Carbon dioxide
- Oxygen
- Water vapour
Incomplete combustion, where there is too little oxygen, produces poisonous carbon monoxide (CO).
Choose all that applyLevel 2
2. Which of these gases are harmful air pollutants released when fossil fuels are burned? (Select all that apply.)
- Carbon monoxidecorrect
- Sulfur dioxidecorrect
- Nitrogen dioxidecorrect
- Nitrogen (N2)
Carbon monoxide, sulfur dioxide and nitrogen dioxide are all pollutants, whereas unreactive nitrogen gas makes up most of clean air.
Fill the blankLevel 2
3. Oxides of nitrogen form inside car engines because the ____ there is very high.
- temperaturecorrect
- pressure
- humidity
- sulfur level
The very high temperature in an engine makes nitrogen and oxygen from the air react to form nitrogen oxides.
Odd one outLevel 2
4. Which gas is NOT a pollutant that a three-way catalytic converter is designed to remove?
- Carbon dioxidecorrect
- Carbon monoxide
- Unburnt hydrocarbons
- Nitrogen oxides
A converter removes CO, unburnt hydrocarbons and nitrogen oxides; carbon dioxide is one of the products, not something removed.
Put in orderLevel 2
5. Put the stages of acid rain formation from sulfur dioxide in the correct order.
Answer: Sulfur dioxide (SO2) is released when fossil fuels burn -> SO2 is oxidised to sulfur trioxide (SO3) -> SO3 dissolves in rain water to form sulfuric acid (H2SO4) -> Acidic rain falls and lowers the pH of lakes and soils
SO2 is released by burning fuels, oxidised to SO3, then dissolves in rain to form sulfuric acid that acidifies the land.
Sort into groupsLevel 2
6. Sort each gas by whether it contributes to acid rain.
Answer: Sulfur dioxide = Causes acid rain; Nitrogen dioxide = Causes acid rain; Carbon monoxide = Does not cause acid rain; Oxygen = Does not cause acid rain
Sulfur dioxide and nitrogen dioxide dissolve to form acids in rain, while carbon monoxide and oxygen do not.

Green Chemistry

Multiple choiceLevel 1
7. Reduce, reuse and ____ are the 'three Rs'. Which word completes them?
- Recyclecorrect
- Remove
- React
- Refine
The three Rs are reduce, reuse and recycle, which cut down waste and save resources.
Build the sentenceLevel 2
8. Arrange the words to describe why a biofuel is called carbon neutral.
Answer: a carbon-neutral fuel returns recently absorbed carbon dioxide
A carbon-neutral fuel returns recently absorbed carbon dioxide, unlike fossil fuels which release ancient stored carbon.
Choose all that applyLevel 2
9. Which of these are aims of green chemistry? (Select all that apply.)
- Producing less wastecorrect
- Using less energycorrect
- Using renewable raw materialscorrect
- Using as many hazardous solvents as possible
Green chemistry aims to cut waste, save energy and use renewable materials while avoiding hazardous chemicals.
Fact or fibLevel 2
10. PLA (poly(lactic acid)) is a bioplastic that can be made from plant starch and is compostable.
Answer: True
PLA is made from fermented plant sugars and breaks down under industrial composting, unlike ordinary polythene.
Fill the blankLevel 2
11. The percentage of reactant atoms that end up in the useful product is called the ____ economy.
- atomcorrect
- mass
- energy
- percentage
Atom economy measures how much of the starting atoms become useful product; a high value means less waste.
Guess the numberLevel 3
12. A reaction gives 40 g of useful product out of 50 g of total products. What is its atom economy (%)?
Answer: 80 %
Atom economy = (40 / 50) x 100 = 80%, meaning one-fifth of the product mass is unwanted by-product.

Greenhouse Gases and Climate

Choose all that applyLevel 1
13. Which of these are greenhouse gases? (Select all that apply.)
- Carbon dioxidecorrect
- Methanecorrect
- Water vapourcorrect
- Nitrogen
Carbon dioxide, methane and water vapour all trap heat, while nitrogen is not a greenhouse gas.
Build the sentenceLevel 2
14. Complete the term: the total greenhouse gases caused by a person or product is its ____ ____.
Answer: carbon footprint
A carbon footprint is the total amount of greenhouse gases produced by an activity, product or person.
Fact or fibLevel 2
15. Cutting down large areas of forest increases the amount of carbon dioxide in the atmosphere.
Answer: True
Trees absorb CO2, so removing forests means less is taken in and more stays in the air.
Fill the blankLevel 2
16. Greenhouse gases warm the Earth by absorbing the ____ radiation that the planet gives off.
- infraredcorrect
- ultraviolet
- visible light
- radio
Greenhouse gases let sunlight in but absorb the infrared (heat) radiation the Earth emits, keeping the surface warm.
Guess the numberLevel 2
17. Atmospheric CO2 has risen from about 280 ppm before the industrial revolution to roughly what value today?
Answer: 420 ppm
CO2 has climbed to around 420 ppm, the highest level in hundreds of thousands of years.
Match the pairsLevel 2
18. Match each greenhouse gas to a major source of it.
Answer: Carbon dioxide = Burning fossil fuels; Methane = Cattle and rice paddies; Water vapour = Evaporation from oceans; CFCs = Old aerosols and fridges
Different greenhouse gases come from different sources, from fossil fuels to farming and old refrigerants.

The Atmosphere

Guess the numberLevel 1
19. Roughly what percentage of the air is oxygen?
Answer: 21 %
Oxygen makes up about 21%, roughly one fifth, of the atmosphere.
Multiple choiceLevel 1
20. About what proportion of clean, dry air is nitrogen?
- About 78%correct
- About 21%
- About 1%
- About 0.04%
Nitrogen makes up roughly 78% of the atmosphere, making it by far the most abundant gas in air.
Fact or fibLevel 2
21. Earth's early atmosphere is thought to have formed mainly from gases released by volcanoes.
Answer: True
Intense early volcanic activity released carbon dioxide, water vapour and other gases that formed the first atmosphere.
Fill the blankLevel 2
22. Oxygen built up in Earth's atmosphere once early ____ began to photosynthesise.
- algae and plantscorrect
- volcanoes
- bacteria releasing methane
- meteorites
Photosynthesis by early algae and plants gradually released the oxygen that now makes up about a fifth of the air.
Match the pairsLevel 2
23. Match each gas to its approximate proportion in clean, dry air.
Answer: Nitrogen = 78%; Oxygen = 21%; Argon = 0.9%; Carbon dioxide = 0.04%
Air is roughly 78% nitrogen, 21% oxygen, 0.9% argon and only about 0.04% carbon dioxide.
Odd one outLevel 2
24. Which gas is the odd one out because it is NOT a normal component of clean, dry air?
- Hydrogencorrect
- Nitrogen
- Oxygen
- Argon
Nitrogen, oxygen and argon are all major parts of air, but free hydrogen is not.

Water Treatment

Build the sentenceLevel 2
25. Arrange the words to describe what nitrogen-fixing bacteria do in the soil.
Answer: they turn nitrogen gas into useful nitrogen compounds
Nitrogen-fixing bacteria turn unreactive nitrogen gas into nitrogen compounds that plants can use.
Fact or fibLevel 2
26. Potable water can still contain dissolved salts, so it is not the same as pure water.
Answer: True
Potable (drinkable) water often contains dissolved minerals, whereas pure water is only H2O.
Fill the blankLevel 2
27. Removing dissolved salt from seawater to make it drinkable is called ____.
- desalinationcorrect
- chlorination
- filtration
- condensation
Desalination removes salt from seawater, for example by distillation or reverse osmosis.
Match the pairsLevel 3
28. Match each stage of the nitrogen cycle to what it does.
Answer: Nitrogen fixation = N2 gas turned into ammonia/ammonium; Nitrification = Ammonium turned into nitrate; Denitrification = Nitrate turned back into N2 gas; Assimilation = Plants absorb nitrate to build proteins
Fixation makes ammonia, nitrification makes nitrate, denitrification returns N2 to the air, and assimilation lets plants build proteins.
Multiple choiceLevel 3
29. Why is desalination not used everywhere to make drinking water, even though seawater is plentiful?
- It uses a large amount of energy, making it expensive
- Seawater contains no salt to remove
- It produces poisonous chlorine gas
- It only works on rainwater
Desalination needs a lot of energy to boil water or force it through membranes, so it is costly and used mainly where fresh water is scarce.
Odd one outLevel 3
30. Which of these is NOT a genuine way to soften hard water?
- Adding common table saltcorrect
- Ion exchange with a resin
- Adding washing soda (sodium carbonate)
- Boiling temporary hard water
Adding common table salt (sodium chloride) does not remove Ca2+ or Mg2+ ions, so it cannot soften the water.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Environmental Chemistry

120 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about Environmental Chemistry

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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