Air, Water and Everyday Chemistry: Unit Review

Introduction

This unit has taken you from the air you breathe to the water you drink and the waste you throw away. The ideas are closely linked: the same burning fuels that change the composition of air produce acid rain that falls into the water cycle; the rocks that make water hard also help clean chimney gases. This review pulls the main ideas together so that you can see the connections and check that you are ready for exam questions on the whole unit.

Core explanation

1. Air. Dry air is a mixture , not a compound: about 78% nitrogen, 21% oxygen, 0.9% argon and about 0.04% carbon dioxide, plus variable water vapour. Oxygen is needed for respiration and combustion; nitrogen is unreactive; argon is a noble gas. The gases are separated by the fractional distillation of liquid air , which works because they have different boiling points. Their uses depend on their properties: oxygen in medicine and steelmaking, nitrogen in food packaging, argon in lamps and welding. Oxygen relights a glowing splint; carbon dioxide turns limewater milky. The carbon cycle balances CO₂ through photosynthesis, respiration, combustion and ocean exchange.

2. Water and the water cycle. Most of Earth's water is salty seawater; only a tiny fraction is accessible fresh water. The Sun drives the water cycle: evaporation and transpiration add water vapour to air, condensation forms clouds, precipitation returns water to the ground, and run-off and groundwater carry it back to the sea. Natural water contains dissolved gases and ions. Anhydrous copper(II) sulfate turns from white to blue with water, and pure water boils at 100 °C at standard pressure. To make water safe it is filtered and chlorinated ; desalination produces fresh water from seawater but uses a lot of energy.

3. Hard water. Hardness is caused by dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺) , picked up as rainwater flows through rocks such as limestone and gypsum. Hard water forms scum with soap and leaves limescale in kettles and pipes.

Type Caused by Removed by boiling? --- --- --- Temporary hardness Calcium hydrogencarbonate Yes — it decomposes to calcium carbonate (limescale) Permanent hardness Calcium and magnesium sulfates and chlorides No

Both types are removed by washing soda (sodium carbonate), which precipitates calcium carbonate, and by ion-exchange resins, which swap Ca²⁺ and Mg²⁺ for Na⁺. Hard water has benefits (taste, calcium for teeth and bones) and drawbacks (wasted soap, scale, higher energy bills).

4. Pollution.

Pollutant Source Effect Solution --- --- --- --- Carbon monoxide Incomplete combustion Toxic; binds to haemoglobin Good ventilation, servicing, catalytic converters Particulates (soot) Incomplete combustion, diesel Lung and heart disease, global dimming Filters, cleaner fuels Sulfur dioxide Sulfur in fuels Acid rain Low-sulfur fuels, flue gas desulfurisation Nitrogen oxides N₂ and O₂ in hot engines Acid rain, smog Catalytic converters Carbon dioxide, methane Fossil fuels, farming, landfill Enhanced greenhouse effect Renewables, efficiency, forests CFCs Old fridges and aerosols Ozone layer depletion Montreal Protocol ban Nitrates, phosphates Fertiliser run-off, sewage Eutrophication Careful fertiliser use, buffer strips Plastics Litter, packaging Harm to wildlife, microplastics Reduce, reuse, recycle

Step-by-step reasoning

To tackle a long answer that links topics:

1. Identify the substance and where it comes from. 2. Describe the chemical change, with an equation if possible. 3. State the effect on air, water or living things. 4. Suggest a solution and explain how it works. 5. Evaluate: mention any drawback or new product formed.

Visual explanation

Picture a single landscape: a power station and road on one side, clouds above, rain falling on limestone hills, a river flowing past farmland to a lake, and a town with a water treatment works. Labels show air composition, the water cycle arrows, hard water forming in the hills, and pollutants entering at each point.

Real-world analogy

The Earth's air and water systems are like a shared kitchen used by billions of people. Everyone breathes the same air and drinks from the same water cycle, so what one household pours down the sink or sends up the chimney eventually reaches everyone else.

Real-world example

Limestone links several parts of this unit. It dissolves slowly in rainwater containing CO₂, making water hard; it is eroded by acid rain; it is used to neutralise acidic lakes; and powdered limestone scrubs sulfur dioxide from power station chimneys.

Why?

Why does one fuel-burning process link to so many topics in this unit? Burning fuels uses oxygen from air, releases CO₂ that affects the carbon cycle and climate, produces SO₂ and NOₓ that form acid rain in the water cycle, and may produce CO and soot through incomplete combustion.

Common misconception

"Pollution problems are all the same." The enhanced greenhouse effect, ozone depletion and acid rain have different causes, different chemicals and different solutions. Mixing them up is one of the most common errors in this unit.

Worked example

Question: Water from a chalk area is boiled and becomes softer, but not completely soft. Explain.

Reasoning: Boiling decomposes calcium hydrogencarbonate into insoluble calcium carbonate, removing temporary hardness. Any calcium sulfate present is not affected by boiling, so permanent hardness remains.

Answer: The water contains both temporary and permanent hardness; boiling removes only the temporary part.

Quick check

1. Which gas makes up about 78% of dry air? Answer: Nitrogen.

Exam focus

Expect questions that link topics: acid rain and limestone, the carbon cycle and the greenhouse effect, fertilisers and dissolved oxygen. Learn key tests (glowing splint, limewater, anhydrous copper(II) sulfate) and always pair a pollutant with its source, its effect and a method of reducing it.

Advanced insight

Many environmental problems in this unit were tackled successfully once the chemistry was understood: sulfur emissions have fallen sharply, CFCs were phased out and leaded petrol was removed. These successes show that careful measurement, scientific explanation and international agreement together can reverse damage, a model now being applied to greenhouse gases.

Summary

Air is a mixture of nitrogen, oxygen, argon and small amounts of CO₂ and water vapour, separated by fractional distillation. The Sun drives the water cycle, and water is treated to make it safe. Calcium and magnesium ions cause hardness, temporary or permanent, removed by boiling, washing soda or ion exchange. Pollutants from fuels, farming and waste harm air and water, and chemistry provides ways to reduce them.

Practice questions

1. Explain why air is described as a mixture rather than a compound. Answer: Its gases are not chemically combined, are present in variable proportions, and can be separated by physical means such as fractional distillation. 2. State the difference between temporary and permanent hardness. Answer: Temporary hardness, caused by calcium hydrogencarbonate, is removed by boiling; permanent hardness, caused by sulfates and chlorides, is not. 3. Name the pollutant responsible for each: acid rain, ozone depletion, eutrophication. Answer: Sulfur dioxide (and nitrogen oxides); CFCs; nitrates and phosphates from fertilisers. 4. Describe two stages in making river water safe to drink. Answer: Filtration removes solid particles, and chlorination kills harmful microorganisms. 5. Give a test for carbon dioxide and its result. Answer: Bubble the gas through limewater; the limewater turns milky (cloudy).