Who this is for: Cambridge IGCSE Chemistry (0620) students who want water — treatment, purity tests and hardness — to become precise syllabus answers instead of a vague description of “cleaning water”.
What query it owns: how to understand and revise water in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the water revision-guide angle, while Tutopiya’s Water subtopic page owns the learning resource and the free Water quiz owns the practice.

Water is a core topic in the Chemistry Of The Environment unit of Cambridge IGCSE Chemistry (0620). Examiners expect you to describe water treatment stages, state tests for purity, and explain hard water versus soft water including advantages and disadvantages. This guide organises each idea with the exam phrasing that signals it.

Key takeaways

• Natural water contains dissolved substances and insoluble particles — it must be treated before use.
• Treatment stages include sedimentation, filtration and chlorination (or sterilisation).
• Pure water boils at 100 °C, freezes at 0 °C and has a neutral pH — the anhydrous copper(II) sulfate test shows no water present.
• Hard water contains dissolved calcium and magnesium ions; it does not lather easily with soap.
• Temporary hardness is removed by boiling; permanent hardness needs ion exchange or addition of sodium carbonate.

What is water in Cambridge IGCSE Chemistry?

Water chemistry at IGCSE covers the composition of natural water, the stages of treatment to make it potable, tests to confirm purity, and the difference between hard and soft water. You must also link water to environmental issues such as pollution and the water cycle.

Read the full notes on Tutopiya’s Water subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Water treatment	Sedimentation → filtration → chlorination	”Describe how water is made fit to drink.”
Purity test	Anhydrous CuSO₄ turns blue; boiling/freezing points	”Describe a test for pure water.”
Hard water	Ca²⁺ and Mg²⁺ ions dissolved	”Explain why hard water does not lather.”
Soft water	No Ca²⁺/Mg²⁺ (or very low)	“Compare hard and soft water.”
Removing hardness	Boiling (temporary), ion exchange, Na₂CO₃	”Suggest how to soften hard water.”

How to describe water treatment — step by step

1. Sedimentation — large particles settle to the bottom.
2. Filtration — remaining insoluble particles removed through sand/gravel filters.
3. Chlorination — chlorine kills harmful bacteria (sterilisation).
4. Optional extra steps — activated carbon removes taste/odour (if mentioned in question).
5. State purpose of each stage — do not just name them.

Test yourself with the free Water quiz.

Hard vs soft water: which does the question want?

Situation	What to write	Typical signal words
Hard water disadvantage	Wastes soap, forms scum, furring in kettles	”does not lather”, “kettle fur”
Hard water advantage	Provides Ca²⁺ for bones/teeth (health benefit)	“health benefit of hard water”
Temporary hardness	Removed by boiling — forms limescale	”boiling removes hardness”
Permanent hardness	Needs ion exchange or sodium carbonate	”ion exchange resin”

Water in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical water stem
Describe	Treatment stages or test procedure	”Describe how river water is treated.”
Explain	Why hard water behaves differently	”Explain why hard water wastes soap.”
State	Named ion or treatment step	”State two ions that cause hardness.”
Suggest	Method to soften or test	”Suggest how to remove permanent hardness.”
Compare	Hard vs soft water	”Compare the action of soap in hard and soft water.”

Worked exam-style stems (how to answer the wording)

1. “Describe how water from a reservoir is made fit to drink.” Sedimentation allows large particles to settle. Filtration removes remaining insoluble material. Chlorination kills bacteria. Reward: three stages with purposes.
2. “Explain why hard water does not lather easily with soap.” Ca²⁺ and Mg²⁺ ions react with soap to form scum (insoluble salt), using up soap before lather forms. Reward: ions named + scum formed.
3. “Describe a chemical test to show that a liquid is pure water.” Add anhydrous copper(II) sulfate — it turns from white to blue if water is present. Pure water gives no colour change when the liquid is already water (or use boiling point 100 °C / pH 7). Reward: correct reagent + observation.

When you can recognise the wording instantly, work through the Water quiz and Chemistry Of The Environment topical past paper questions.

How water connects to the rest of the environment unit

Water links to Air (dissolved gases), Fertilisers (nitrate pollution in water) and Carbonates (removing hardness with sodium carbonate). The Cambridge IGCSE Chemistry resource hub links every environment subtopic.

Common mistakes students make

• Listing treatment steps without stating the purpose of each.
• Confusing anhydrous and hydrated copper(II) sulfate in the purity test.
• Saying hard water is “bad” without mentioning the health benefit of calcium ions.
• Thinking distillation is the same as water treatment at a reservoir scale.
• Forgetting that chlorination kills bacteria — it does not remove insoluble solids.

When you need more support

If water treatment or hardness questions keep losing marks, work through the Water quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What are the main stages of water treatment? Sedimentation, filtration and chlorination. Each removes a different type of impurity.

What causes hard water? Dissolved calcium and magnesium ions, usually from limestone or chalk in the ground.

How is temporary hardness removed? By boiling — hydrogencarbonate ions decompose and calcium carbonate precipitates.

How do I revise water effectively? Learn treatment stages with purposes, practise hard/soft comparisons, then take the Water quiz.

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