Chemical tests for water

Two reactions detect the presence of water. The colour change happens because the anhydrous solid takes up water to become hydrated:

• Anhydrous copper(II) sulfate turns from white → blue.
• Anhydrous cobalt(II) chloride turns from blue → pink.

Watch out: these reactions only test for the presence of water — they do not tell you the water is pure. A muddy puddle would still turn copper(II) sulfate blue. To check purity you measure its boiling/melting point (next screen).

Spotting water

Testing for pure water

You test purity using fixed boiling and melting points. Pure water:

• boils at exactly 100 °C (at standard pressure)
• melts/freezes at exactly 0 °C

Dissolved impurities raise the boiling point and lower the melting point, and the change happens over a range of temperatures rather than at a sharp value.

In the lab we use distilled water, not tap water, because it contains fewer chemical impurities (dissolved salts) that could interfere with reactions.

What natural water contains

Water from rivers, lakes and the sea is never pure. It may carry helpful and harmful substances:

• Beneficial: dissolved oxygen (for aquatic life); some metal compounds (essential minerals).
• Potentially harmful: some toxic metal compounds; plastics (harm aquatic life); sewage (harmful microbes that cause disease); nitrates and phosphates from fertilisers and detergents.

Why nitrates & phosphates matter: they cause deoxygenation of the water and damage aquatic life. (You only need this outcome — the detailed eutrophication process is not required.)

Treating the domestic water supply

To make water safe to drink, the supply is treated in stages:

Why each stage?

Tap the correct purpose for each water-treatment step.

How does river water become safe to drink?

NPK fertilisers

Plants need three key mineral elements from the soil to grow well. Ammonium salts and nitrates are used as fertilisers because they supply nitrogen, and NPK fertilisers supply all three:

Tip: the symbol for potassium is K (from kalium) — that's where the K in NPK comes from.

Which elements?

Where does the nitrogen in exhaust come from?

The composition of clean, dry air

Clean, dry air is approximately:

Remember: "clean, dry" air — water vapour is excluded, and CO₂ is only about 0.04% of the whole.

Your turn — composition of air

The common air pollutants

You need the source and adverse effect of each pollutant:

Watch out: carbon monoxide (CO) is the toxic gas, and it comes from incomplete combustion. Don't confuse it with CO₂ from complete combustion. SO₂ and NOₓ both cause acid rain.

Pollutant → effect

Tap a pollutant on the left, then its adverse effect on the right.

Where does it come from?

Reducing the pollutants

Strategies you must know, matched to the problem:

• Acid rain: use catalytic converters in vehicles; cut SO₂ by using low-sulfur fuels and flue-gas desulfurisation with calcium oxide.
• Climate change: planting trees, reducing livestock farming, decreasing use of fossil fuels, and increasing the use of hydrogen and renewable energy (e.g. wind, solar).

Inside a catalytic converter

Oxides of nitrogen (NOₓ) form in car engines because the very high temperature makes the nitrogen and oxygen in the air react together. A catalytic converter removes both this and carbon monoxide in one reaction:

Two harmful gases — toxic CO and acid-rain-causing NO — are converted into much less harmful CO₂ and harmless N₂.

Supplement (Extended only): you should be able to state this equation and explain that NOₓ forms in the engine and is removed by the catalytic converter.

The converter reaction

Where does the nitrogen in exhaust come from?

What stops some energy escaping to space?

Greenhouse gases & the greenhouse effect

The two greenhouse gases you must know are carbon dioxide (CO₂) and methane (CH₄).

Watch out: the greenhouse effect is natural and necessary — without it Earth would be far too cold for life. The concern is that higher levels of CO₂ and methane increase global warming, which leads to climate change.

Which is a greenhouse gas?

What stops some energy escaping to space?

Photosynthesis removes CO₂

Plants take CO₂ back out of the air. Photosynthesis is the reaction between carbon dioxide and water to make glucose and oxygen, in the presence of chlorophyll and using energy from light:

Supplement: you need the balanced symbol equation as well as the word equation. This is why planting trees is a climate-change strategy — trees take in CO₂.

Reduce the effects

Tap whether each action mainly tackles acid rain or climate change.

The key facts to know

You've covered the key ideas across the three parts of Cambridge IGCSE Topic 10 — water, fertilisers, and air quality & climate. Press Finish to see your score.