ILC Chemistry Ordinary Level Chapter 9 - Complete Environmental Chemistry (Water)

Chapter 9 Complete Coverage

This complete file combines all four parts of Chapter 9 into one structured Ordinary Level chapter and adds the missing syllabus points so the full chapter is covered in one place.

1. Learning Objectives

Define hard water and explain its cause.
Identify calcium and magnesium ions as the main ions responsible for hardness.
Distinguish between temporary and permanent hardness.
Explain how boiling removes temporary hardness.
Describe methods used to soften water, including ion exchange and deionisation.
Explain the effect of hard water on soap and limescale formation.
Answer OL exam-style questions on hardness in water.

2. What is Hard Water?

Hard water is water that does not easily form a lather with soap. It usually contains dissolved calcium ions, Ca²⁺, and magnesium ions, Mg²⁺.

Hard water contains Ca²⁺ and/or Mg²⁺ ions.

These ions react with soap to form a grey-white solid called scum. This wastes soap and makes cleaning less effective.

Water Type	Soap Test Result	Reason
Soft water	Forms lather easily	Few Ca²⁺/Mg²⁺ ions present.
Hard water	Forms scum first, then lather slowly	Ca²⁺/Mg²⁺ ions react with soap.

Hard water wastes soap because scum forms before lather.

Examiner Tip: If asked what causes hard water, write calcium and magnesium ions, not just “minerals”.

3. How Water Becomes Hard

Rainwater is naturally soft. It becomes hard when it passes through rocks containing calcium and magnesium compounds, especially limestone.

Hardness comes from dissolved calcium and magnesium compounds.

Carbon dioxide in rainwater forms weak carbonic acid. This helps dissolve calcium carbonate in limestone, forming soluble calcium hydrogencarbonate.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

Examiner Trap: Limestone itself is not usually “floating in water”. Dissolved calcium compounds cause the hardness.

4. Temporary and Permanent Hardness

Type of Hardness	Cause	Can boiling remove it?
Temporary hardness	Calcium or magnesium hydrogencarbonates	Yes
Permanent hardness	Calcium or magnesium sulfates/chlorides	No

Temporary Hardness:
Ca(HCO₃)₂ in water. Removed by boiling.

Permanent Hardness:
CaSO₄ or MgSO₄ in water. Not removed by boiling.

Common Mistake: Saying all hard water can be softened by boiling. Only temporary hardness is removed by boiling.

5. Removing Temporary Hardness by Boiling

Temporary hardness is removed by boiling because calcium hydrogencarbonate decomposes to form insoluble calcium carbonate, which appears as limescale.

Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O

The calcium carbonate formed is insoluble and can be removed by filtration or may deposit as scale inside kettles and boilers.

Boiling converts soluble hydrogencarbonate into insoluble carbonate.

Examiner Tip: In equations, remember temporary hardness contains hydrogencarbonate; boiling forms carbonate.

6. Problems Caused by Hard Water

Problem	Explanation
Wastes soap	Calcium/magnesium ions react with soap to form scum.
Forms limescale	Calcium carbonate deposits in kettles, boilers and pipes.
Reduces heating efficiency	Limescale acts as an insulator, so more energy is needed.
Blocks pipes/appliances	Scale buildup can narrow pipes and damage appliances.

Examiner Trap: Limescale is mainly calcium carbonate, CaCO₃, not calcium hydrogencarbonate.

7. Benefits of Hard Water

Hard water is not always bad. It can have some benefits.

It may provide calcium ions, which are useful for bones and teeth.
It can have a pleasant taste compared with very soft water.
It may reduce the dissolving of harmful metals from old pipes by forming a protective layer.

Examiner Tip: If asked for advantages and disadvantages, give balanced points. Hard water is not automatically “bad water”.

8. Removing Hardness: Ion Exchange

Ion exchange is used in water softeners. Calcium and magnesium ions are exchanged for sodium ions.

Ca²⁺ / Mg²⁺ ions are removed and replaced by Na⁺ ions.

Ion exchange removes hardness ions from water.

Common Mistake: Saying ion exchange removes all ions. It mainly swaps hardness ions for sodium ions.

9. Removing Hardness: Deionisation

Deionisation removes ions from water using ion exchange resins. It can produce very pure water.

Method	What it removes	Result
Simple water softener	Ca²⁺ and Mg²⁺	Softened water
Deionisation	Many positive and negative ions	Very pure/deionised water

Examiner Tip: Ion exchange softening and deionisation are related, but deionisation is more complete because it removes many ions, not just hardness ions.

10. Interactive Simulators and Virtual Labs

Simulator 1: ChemCollective Virtual Lab

Use for: Exploring water samples, ions and solution testing.

Student Task: Compare a hard water sample with a soft water sample and predict the soap test result.

Open Virtual Lab

Simulator 2: PhET – Concentration

Use for: Understanding dissolved particles/ions in solution.

Student Task: Link dissolved ions with water hardness.

Open Simulator

Simulator 3: PhET – Sugar and Salt Solutions

Use for: Visualising ions in solution.

Student Task: Compare ionic and molecular substances dissolving in water.

Open Simulator

11. Worked Examples

Example 1:
Question: What ions cause hardness in water?
Answer: Calcium ions, Ca²⁺, and magnesium ions, Mg²⁺.

Example 2:
Question: How can temporary hardness be removed?
Answer: By boiling. Calcium hydrogencarbonate decomposes to calcium carbonate, carbon dioxide and water.

Example 3:
Question: Why does hard water waste soap?
Answer: Calcium and magnesium ions react with soap to form scum before lather can form.

12. Examiner Secrets, Mistakes and Traps

Examiner Secret: Hardness questions usually test three things: the ions responsible, temporary vs permanent hardness, and how hardness is removed.

Examiner Tip: For temporary hardness, always connect boiling with formation of insoluble CaCO₃.

Common Mistake: Saying hard water contains “calcium metal”. It contains calcium ions, Ca²⁺.

Common Mistake: Saying permanent hardness is removed by boiling. It is not.

Examiner Trap: Scum is not lather. Scum forms first in hard water and prevents easy lathering.

13. Exam Practice Questions

Q1. Define hard water. [2 marks]

Q2. Name the two ions mainly responsible for hardness in water. [2 marks]

Q3. Explain the difference between temporary and permanent hardness. [4 marks]

Q4. Explain how boiling removes temporary hardness. [3 marks]

Q5. Give two disadvantages of hard water. [2 marks]

Q6. Describe how ion exchange softens water. [3 marks]

MCQs with Explanations

1. Hard water is mainly caused by: A. sodium ions B. calcium and magnesium ions C. oxygen gas D. chloride only

Answer: B. Ca²⁺ and Mg²⁺ ions cause hardness.

2. Temporary hardness can be removed by: A. boiling B. freezing C. adding oil D. filtration only

Answer: A. Boiling decomposes hydrogencarbonates.

3. The scale formed when temporary hard water is boiled is mainly: A. sodium chloride B. calcium carbonate C. sulfur dioxide D. magnesium metal

Answer: B. CaCO₃ forms as limescale.

4. Ion exchange softeners replace calcium/magnesium ions with: A. sodium ions B. oxygen molecules C. chlorine gas D. carbon atoms

Answer: A. Many domestic softeners exchange hardness ions for Na⁺ ions.

14. Last-Minute Revision Sheet

Hard water does not easily form lather with soap.
Hardness is caused by Ca²⁺ and Mg²⁺ ions.
Hard water forms scum with soap.
Temporary hardness is caused by hydrogencarbonates.
Permanent hardness is caused by sulfates/chlorides.
Boiling removes temporary hardness only.
Boiling forms insoluble calcium carbonate, CaCO₃.
Limescale reduces heating efficiency.
Ion exchange removes Ca²⁺/Mg²⁺ and replaces them with Na⁺.
Deionisation removes many ions and produces very pure water.

15. Self-Assessment Checklist

I can define hard water.
I can name the ions causing hardness.
I can explain the soap test for hard water.
I can distinguish temporary and permanent hardness.
I can explain how boiling removes temporary hardness.
I can describe problems caused by hard water.
I can explain ion exchange softening.
I can explain deionisation in simple OL language.

16. Mark Scheme

Q1. Water that does not easily form lather with soap [1]; due to dissolved calcium/magnesium ions [1].

Q2. Calcium ions / Ca²⁺ [1]; magnesium ions / Mg²⁺ [1].

Q3. Temporary hardness is caused by hydrogencarbonates [1] and can be removed by boiling [1]; permanent hardness is caused by sulfates/chlorides [1] and cannot be removed by boiling [1].

Q4. Boiling decomposes calcium hydrogencarbonate [1]; insoluble calcium carbonate forms [1]; carbon dioxide and water are also produced / hardness ions removed from solution [1].

Q5. Wastes soap / forms scum / forms limescale / blocks pipes / reduces heating efficiency. Any two [2].

Q6. Water passes through ion exchange resin [1]; Ca²⁺/Mg²⁺ ions are removed [1]; replaced by Na⁺ ions [1].

2. Why Water Must Be Treated

Natural water from rivers, lakes and reservoirs may contain mud, suspended solids, dissolved substances and microorganisms. It must be treated before it is safe to drink.

Water treatment removes solids, reduces impurities and kills harmful microorganisms.

Possible Problem in Water	Why it matters	Treatment Link
Suspended solids	Makes water cloudy and dirty.	Sedimentation and filtration.
Microorganisms	May cause disease.	Chlorination.
Fine particles	Too small to settle quickly alone.	Flocculation.
Low fluoride level	May increase tooth decay risk.	Fluoridation, where used.

Examiner Tip: Do not just say “water is cleaned”. Name the process and what it removes.

3. Municipal Water Treatment Stages

Municipal water treatment prepares water for public supply. The order can vary slightly, but the main stages are usually:

Screening → Flocculation → Sedimentation → Filtration → Chlorination → Fluoridation

Each stage has a different job in making water safe and acceptable.

4. Flocculation and Sedimentation

Flocculation is when chemicals are added to make tiny particles clump together into larger particles called flocs.

Sedimentation is when these larger particles settle to the bottom due to gravity.

Process	What happens?	Why it is useful
Flocculation	Small particles clump together.	Makes fine particles easier to remove.
Sedimentation	Flocs sink to the bottom.	Removes suspended solids before filtration.

Flocculation makes small particles easier to remove by settling.

Examiner Trap: Flocculation is not the same as filtration. Flocculation clumps particles; filtration traps particles.

5. Filtration

Filtration removes remaining suspended solids by passing water through layers such as sand and gravel.

Filtration removes particles that did not settle earlier.

Common Mistake: Filtration removes suspended solids, but it does not kill microorganisms. Chlorination is used for that.

6. Chlorination

Chlorination is the addition of chlorine to water to kill harmful microorganisms such as bacteria.

Chlorination kills harmful microorganisms.

Chlorine is used because it is effective and can remain in the water for some time, helping prevent contamination during distribution.

Advantage	Concern
Kills harmful microorganisms.	Too much chlorine can affect taste and smell.
Helps keep water safe in pipes.	Must be carefully controlled.

Examiner Tip: Chlorination is about killing microorganisms, not removing hardness.

7. Fluoridation

Fluoridation is the addition of fluoride compounds to water, where used, to help reduce tooth decay.

Fluoridation helps protect teeth from decay.

Fluoridation is not mainly a purification step. It is added for dental health reasons.

Examiner Trap: Fluoridation does not kill bacteria. Chlorination kills bacteria. Tiny word swap, huge exam faceplant.

8. Sewage Treatment

Sewage treatment treats wastewater from homes, schools, industries and other buildings before it is released safely back into the environment.

Sewage treatment removes solids, reduces organic waste and lowers pollution risk.

Stage	Main Purpose	What happens?
Primary treatment	Remove large solids and suspended solids.	Screening and settling.
Secondary treatment	Break down organic matter biologically.	Microorganisms digest organic waste.
Tertiary treatment	Further purification.	May remove nutrients, pathogens or remaining impurities.

Sewage treatment is not the same as drinking-water treatment.

9. Primary Sewage Treatment

Primary treatment uses physical methods to remove large and suspended solids.

Large objects are removed by screens.
Grit and sand settle out.
Suspended solids settle as sludge.
Floating material such as grease may be skimmed off.

Examiner Tip: Primary treatment is mostly physical separation, not biological digestion.

10. Secondary Sewage Treatment

Secondary treatment uses microorganisms to break down organic matter in sewage.

Microorganisms need oxygen to respire and break down organic waste. This lowers the amount of biodegradable material in the water.

Microorganisms + oxygen help break down organic waste.

Common Mistake: Saying secondary treatment means “adding chlorine only”. Secondary treatment is mainly biological.

11. Tertiary Sewage Treatment

Tertiary treatment is extra treatment after primary and secondary treatment. It may remove nutrients such as nitrates and phosphates, remaining pathogens, or fine impurities.

Possible Target	Why remove it?
Nitrates/phosphates	Can cause eutrophication in rivers and lakes.
Pathogens	Reduce disease risk.
Fine particles	Improve water clarity and quality.

Examiner Trap: Tertiary treatment is not always identical everywhere. It depends on the final water quality needed.

12. Interactive Simulators and Virtual Labs

Simulator 1: ChemCollective Virtual Lab

Use for: Water-quality and solution-testing context.

Student Task: Compare untreated and treated water samples and list possible impurities.

Open Virtual Lab

Simulator 2: PhET – Concentration

Use for: Understanding dissolved substances and concentration.

Student Task: Link concentration to dissolved impurities in water.

Open Simulator

Simulator 3: PhET – Sugar and Salt Solutions

Use for: Visualising ions in water.

Student Task: Explain why filtration cannot remove dissolved ions.

Open Simulator

13. Worked Examples

Example 1:
Question: What is the purpose of chlorination?
Answer: Chlorination kills harmful microorganisms in water.

Example 2:
Question: Why is flocculation used before sedimentation?
Answer: It makes tiny particles clump together into larger flocs, which settle more easily.

Example 3:
Question: What happens in secondary sewage treatment?
Answer: Microorganisms break down organic waste, usually with oxygen supplied.

14. Examiner Secrets, Mistakes and Traps

Examiner Secret: Water treatment questions love asking “purpose of each stage”. Learn the job, not just the name.

Examiner Tip: Use “microorganisms” for secondary sewage treatment and “chlorine kills microorganisms” for drinking-water treatment.

Common Mistake: Saying filtration kills bacteria. Filtration removes particles; chlorination kills microorganisms.

Common Mistake: Confusing flocculation with fluoridation. Flocculation clumps particles; fluoridation protects teeth.

Examiner Trap: Drinking-water treatment and sewage treatment are related to water quality, but they are not the same process.

15. Exam Practice Questions

Q1. Give two reasons why water must be treated before drinking. [2 marks]

Q2. Explain the purpose of flocculation. [2 marks]

Q3. What happens during sedimentation? [2 marks]

Q4. State the purpose of chlorination. [2 marks]

Q5. Explain the difference between chlorination and fluoridation. [4 marks]

Q6. Name the three main stages of sewage treatment and give one purpose of each. [6 marks]

MCQs with Explanations

1. Chlorination is mainly used to: A. remove hardness B. kill microorganisms C. add calcium D. remove sand

Answer: B. Chlorine kills harmful microorganisms.

2. Flocculation causes small particles to: A. evaporate B. turn into gas C. clump together D. become fluoride

Answer: C. Flocculation makes small particles form larger flocs.

3. Secondary sewage treatment mainly uses: A. microorganisms B. gold C. dry ice D. sand only

Answer: A. Microorganisms break down organic waste.

4. Fluoridation is mainly linked with: A. tooth decay prevention B. killing all bacteria C. removing gravel D. softening water

Answer: A. Fluoride helps reduce tooth decay where fluoridation is used.

16. Last-Minute Revision Sheet

Water treatment removes solids, reduces impurities and kills harmful microorganisms.
Flocculation makes tiny particles clump into flocs.
Sedimentation lets heavier particles settle.
Filtration removes remaining suspended solids.
Chlorination kills harmful microorganisms.
Fluoridation helps reduce tooth decay.
Sewage treatment treats wastewater before release.
Primary sewage treatment removes solids physically.
Secondary treatment uses microorganisms to break down organic waste.
Tertiary treatment gives further purification, often removing nutrients/pathogens.

17. Self-Assessment Checklist

I can describe municipal water treatment stages.
I can explain flocculation and sedimentation.
I can state the purpose of filtration.
I can explain chlorination.
I can explain fluoridation.
I can distinguish drinking-water treatment from sewage treatment.
I can describe primary, secondary and tertiary sewage treatment.

18. Mark Scheme

Q1. To remove solids/impurities [1]; to kill harmful microorganisms/prevent disease [1].

Q2. Chemicals are added to make small particles clump together [1]; forming larger flocs that settle more easily [1].

Q3. Suspended particles/flocs settle to the bottom [1]; due to gravity, forming sediment/sludge [1].

Q4. Chlorine is added [1] to kill harmful microorganisms/bacteria [1].

Q5. Chlorination kills microorganisms [1] using chlorine [1]; fluoridation adds fluoride compounds [1] to help reduce tooth decay [1].

Q6. Primary removes large/suspended solids [2]; secondary uses microorganisms to break down organic waste [2]; tertiary gives further treatment/removes nutrients/pathogens/fine impurities [2].

2. Water Quality

Water quality describes how suitable water is for living organisms, drinking, farming, industry or recreation.

Good water quality means suitable dissolved oxygen, low pollution, safe pH and low harmful contaminants.

Factor	Why it matters
Dissolved oxygen	Needed by fish and aquatic organisms for respiration.
BOD	Shows how much oxygen is needed to break down organic waste.
Nitrates/phosphates	Can cause eutrophication.
Heavy metals	Toxic and may build up in food chains.
pH	Affects aquatic life and reactions in water.

Examiner Tip: Link each pollutant to its effect on oxygen level or organisms.

3. Dissolved Oxygen (DO)

Dissolved oxygen is oxygen gas dissolved in water. Fish, insects and many aquatic organisms need dissolved oxygen for respiration.

High dissolved oxygen usually indicates healthier water.

Low dissolved oxygen can damage aquatic ecosystems.

Examiner Trap: Oxygen in H₂O is not the same as dissolved oxygen gas. Aquatic organisms need dissolved O₂.

4. Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand is the amount of dissolved oxygen needed by microorganisms to break down organic matter in water.

High BOD = lots of organic waste = oxygen gets used up

BOD Level	Meaning	Effect
Low BOD	Little organic waste	More oxygen remains for aquatic life.
High BOD	Lots of organic waste	Microorganisms use oxygen, lowering DO.

Common Mistake: High BOD is usually bad because it means high oxygen demand.

5. Organic Pollution and Oxygen Sag

Organic pollution includes sewage, animal waste and food-processing waste. Microorganisms multiply and use oxygen to break it down.

Organic waste enters water.
Microorganisms feed on the waste and multiply.
Microorganisms use dissolved oxygen for respiration.
Dissolved oxygen decreases.
Fish and other aquatic organisms may die.

Examiner Tip: Use the chain: organic waste → microorganisms increase → oxygen used up → fish die.

6. Eutrophication

Eutrophication is the enrichment of water with nutrients such as nitrates and phosphates, causing excessive plant and algal growth.

Nitrates/phosphates → algal bloom → dead algae → bacteria increase → oxygen falls → fish die

Pollutant	Common Sources
Nitrates	Fertilisers, animal waste, sewage.
Phosphates	Detergents, fertilisers, sewage.

The major damage happens when decay uses dissolved oxygen.

Examiner Trap: The big oxygen drop happens when dead algae are decomposed by microorganisms.

7. Heavy Metal Pollution

Heavy metals such as lead, mercury and cadmium can pollute water. They are toxic and can build up in organisms.

Heavy Metal	Possible Source	Risk
Lead	Old pipes, industrial waste.	Can damage nervous system.
Mercury	Industrial discharge, mining.	Can build up in food chains.
Cadmium	Batteries, industry, mining.	Toxic to kidneys and organisms.

Bioaccumulation means a toxic substance builds up in an organism over time. Biomagnification means concentration increases higher up the food chain.

Examiner Tip: Heavy metal pollution is serious because metals are toxic, persistent and can build up in food chains.

8. pH and Water Quality

pH measures how acidic or alkaline a solution is.

pH 7 = neutral | pH below 7 = acidic | pH above 7 = alkaline

pH	Description	Example
0–6	Acidic	Acid rain / acidic water
7	Neutral	Pure water
8–14	Alkaline	Alkaline solution

Common Mistake: pH 7 is neutral, not “no pH”.

Examiner Trap: Strong acid does not mean concentrated acid. Strength and concentration are different ideas.

9. Interactive Simulators and Virtual Labs

Simulator 1: PhET – pH Scale

Use for: Comparing acidic, neutral and alkaline solutions.

Open Simulator

Simulator 2: PhET – Concentration

Use for: Understanding dissolved substances and pollution concentration.

Open Simulator

Simulator 3: ChemCollective Virtual Lab

Use for: Water-quality testing context.

Open Virtual Lab

10. Worked Examples

Example 1: Untreated sewage contains organic waste. Microorganisms break it down and use dissolved oxygen, so fish may die.

Example 2: High BOD indicates lots of organic waste and high oxygen demand by microorganisms.

Example 3: Nitrates cause algal blooms. Dead algae are decomposed by microorganisms, which use oxygen and reduce dissolved oxygen.

11. Examiner Secrets, Mistakes and Traps

Examiner Secret: For pollution questions, trace pollutant → oxygen level → effect on aquatic life.

Examiner Tip: BOD and DO are linked: high BOD usually means dissolved oxygen falls.

Common Mistake: Saying high BOD means high oxygen available. It means high oxygen demand.

Examiner Trap: Heavy metals are dangerous even in small amounts because they can accumulate.

12. Exam Practice Questions

Q1. What is dissolved oxygen and why is it important? [3 marks]

Q2. Define biochemical oxygen demand. [2 marks]

Q3. Explain how untreated sewage can lower dissolved oxygen in a river. [4 marks]

Q4. Describe eutrophication. [5 marks]

Q5. Give two sources of nitrates or phosphates in water. [2 marks]

Q6. Explain why heavy metals are dangerous pollutants. [3 marks]

Q7. State the pH of a neutral solution and identify whether pH 3 is acidic or alkaline. [2 marks]

MCQs with Explanations

1. High BOD usually means: A. very clean water B. high oxygen demand C. no organic waste D. pH 7 always

Answer: B. BOD is oxygen needed by microorganisms to break down organic matter.

2. Eutrophication is mainly caused by excess: A. nitrates and phosphates B. gold C. oxygen only D. sand

Answer: A. These nutrients cause algal blooms.

3. pH 7 is: A. acidic B. alkaline C. neutral D. toxic metal

Answer: C. pH 7 is neutral.

13. Last-Minute Revision Sheet

Dissolved oxygen is O₂ dissolved in water.
Fish and aquatic organisms need dissolved oxygen for respiration.
BOD is the oxygen needed by microorganisms to break down organic waste.
High BOD usually means polluted water and low oxygen risk.
Organic pollution increases microorganisms and reduces dissolved oxygen.
Eutrophication is caused by excess nitrates/phosphates.
Eutrophication causes algal blooms, decay, oxygen depletion and fish deaths.
Heavy metals are toxic and can build up in food chains.
pH below 7 is acidic, pH 7 neutral, pH above 7 alkaline.

14. Self-Assessment Checklist

I can explain dissolved oxygen.
I can define BOD.
I can link organic waste to oxygen depletion.
I can describe eutrophication in order.
I can name sources of nitrate and phosphate pollution.
I can explain heavy metal pollution.
I can use the pH scale correctly.

15. Mark Scheme

Q1. Oxygen gas dissolved in water [1]; needed by aquatic organisms/fish [1]; for respiration/survival [1].

Q2. Amount of dissolved oxygen [1] needed by microorganisms to break down organic matter [1].

Q3. Sewage contains organic waste [1]; microorganisms break it down and multiply [1]; microorganisms use dissolved oxygen in respiration [1]; dissolved oxygen falls and fish may die [1].

Q4. Nitrates/phosphates enter water [1]; algae grow rapidly/algal bloom [1]; algae die [1]; microorganisms decompose dead algae using oxygen [1]; dissolved oxygen falls and aquatic life may die [1].

Q5. Fertilisers / sewage / detergents / animal waste. Any two [2].

Q6. Heavy metals are toxic [1]; persistent/do not break down easily [1]; bioaccumulate/biomagnify in food chains [1].

Q7. Neutral pH = 7 [1]; pH 3 is acidic [1].

2. Chapter 9 Big Picture

Environmental Chemistry of Water connects chemistry with drinking-water treatment, sewage treatment, pollution control and water-quality testing.

Water quality combines several chemistry ideas from the full chapter.

3. pH and Water Analysis

pH measures how acidic or alkaline a solution is.

pH = -log₁₀[H⁺]

At Ordinary Level, focus on interpreting the pH scale and linking abnormal pH to water-quality problems.

pH Range	Description	Water Quality Meaning
Below 7	Acidic	May harm aquatic organisms if too acidic.
7	Neutral	Pure water is neutral.
Above 7	Alkaline	May harm organisms if too alkaline.

Examiner Trap: pH 7 is neutral. It does not mean “no pH”.

4. pH Scale Diagram

Water outside a safe pH range can damage aquatic ecosystems.

5. Instrumental Analysis at OL Level

Instrumental analysis uses instruments to test substances accurately. In this chapter, it is linked to water-quality testing.

Instrument / Method	What it tests	Why useful
pH meter	pH of water	More precise than indicator paper.
Colorimeter	Concentration of coloured substances	Can compare water samples.
Ion testing kits	Nitrates, phosphates, hardness ions	Helps identify pollution or hardness.
Dissolved oxygen probe	Dissolved oxygen level	Shows if aquatic life can be supported.

Examiner Tip: Know what each instrument measures and why that measurement matters.

6. Full Chapter Summary Table

Topic	Key Idea	Exam Phrase
Hard water	Contains Ca²⁺/Mg²⁺ ions.	Forms scum with soap.
Temporary hardness	Ca/Mg hydrogencarbonates.	Removed by boiling.
Water treatment	Flocculation, sedimentation, filtration, chlorination.	Each stage has a job.
Sewage treatment	Primary, secondary, tertiary.	Physical, biological, further purification.
Dissolved oxygen	O₂ dissolved in water.	Needed by aquatic life.
BOD	Oxygen needed to break down organic waste.	High BOD usually means pollution.
Eutrophication	Nutrients cause algal bloom and oxygen depletion.	Nitrates/phosphates → algae → decay → low O₂.
Heavy metals	Toxic pollutants.	Bioaccumulation and biomagnification.
pH	Acidic, neutral or alkaline.	pH 7 is neutral.

7. Practical Applications

Soap Test for Hardness

Hard water forms scum and needs more soap to form lather. Soft water forms lather easily.

Testing pH

Use universal indicator or a pH meter to measure acidity or alkalinity.

Testing Water Pollution

Water quality can be investigated using dissolved oxygen tests, pH measurement, nitrate/phosphate tests and observations of living organisms.

Example: A river with high nitrate level, high BOD and low dissolved oxygen is likely polluted and may suffer from eutrophication.

8. Interactive Simulators and Virtual Labs

Simulator 1: PhET – pH Scale

Use for: Testing acidic, neutral and alkaline solutions.

Open Simulator

Simulator 2: PhET – Concentration

Use for: Understanding pollutant concentration in water.

Open Simulator

Simulator 3: ChemCollective Virtual Lab

Use for: Virtual water-quality testing and solution analysis.

Open Virtual Lab

9. Worked Examples

Example 1: A water sample forms scum with soap and little lather. It is hard water and likely contains Ca²⁺ or Mg²⁺ ions.

Example 2: A lake has excess phosphates, algal bloom and dead fish. This is eutrophication due to oxygen depletion after algae decay.

Example 3: A water sample has pH 4. It is acidic and may harm aquatic organisms if released untreated.

10. Examiner Secrets, Mistakes and Traps

Examiner Secret: Chapter 9 exam answers are strongest when they follow a chain of cause and effect.

Examiner Tip: Always name the exact process: chlorination kills microorganisms, filtration removes solids, flocculation clumps particles.

Common Mistake: Saying hard water is unsafe to drink. Hard water is not necessarily unsafe; it just contains hardness ions.

Common Mistake: Saying high BOD means lots of oxygen. It means lots of oxygen is needed.

Examiner Trap: Fluoridation protects teeth; chlorination kills microorganisms. Do not swap them.

11. Final Chapter 9 Exam Practice

Q1. Name the ions responsible for hardness in water. [2 marks]

Q2. Explain how boiling removes temporary hardness. [3 marks]

Q3. Describe the purpose of flocculation, filtration and chlorination in water treatment. [6 marks]

Q4. Explain secondary sewage treatment. [3 marks]

Q5. Define BOD and explain what high BOD indicates. [4 marks]

Q6. Describe eutrophication. [5 marks]

Q7. Explain why heavy metals are dangerous water pollutants. [3 marks]

Q8. A water sample has pH 10. Is it acidic, neutral or alkaline? [1 mark]

MCQs with Explanations

1. Hardness is mainly caused by: A. Ca²⁺/Mg²⁺ B. oxygen C. nitrogen gas D. sugar

Answer: A. Calcium and magnesium ions cause hardness.

2. Chlorination is used to: A. kill microorganisms B. form limescale C. increase BOD D. add scum

Answer: A. Chlorine kills harmful microorganisms.

3. High BOD means: A. low pollution B. high oxygen demand C. no microorganisms D. pH 14

Answer: B. BOD is oxygen needed by microorganisms.

4. pH 10 is: A. acidic B. neutral C. alkaline D. impossible

Answer: C. pH above 7 is alkaline.

12. Full Chapter 9 Last-Minute Revision Sheet

Hard water contains Ca²⁺ and Mg²⁺ ions.
Hard water forms scum with soap.
Temporary hardness is removed by boiling.
Boiling forms insoluble calcium carbonate.
Ion exchange removes Ca²⁺/Mg²⁺ and replaces them with Na⁺.
Flocculation clumps small particles.
Sedimentation lets particles settle.
Filtration removes suspended solids.
Chlorination kills microorganisms.
Fluoridation helps reduce tooth decay.
Secondary sewage treatment uses microorganisms.
Dissolved oxygen is needed by aquatic life.
High BOD means high oxygen demand.
Eutrophication is caused by excess nitrates/phosphates.
Heavy metals can bioaccumulate and biomagnify.
pH below 7 acidic, pH 7 neutral, pH above 7 alkaline.

13. Self-Assessment Checklist

I can explain hardness in water.
I can distinguish temporary and permanent hardness.
I can describe municipal water treatment.
I can describe sewage treatment stages.
I can define dissolved oxygen and BOD.
I can explain eutrophication.
I can explain heavy metal pollution.
I can use pH to classify water samples.
I can answer mixed Chapter 9 exam questions.

14. Mark Scheme

Q1. Calcium ions / Ca²⁺ [1]; magnesium ions / Mg²⁺ [1].

Q2. Calcium hydrogencarbonate decomposes [1]; insoluble calcium carbonate forms [1]; carbon dioxide and water are also produced / hardness is removed [1].

Q3. Flocculation clumps fine particles [2]; filtration removes suspended solids [2]; chlorination kills microorganisms [2].

Q4. Microorganisms are used [1] to break down organic waste [1], usually with oxygen/aeration supplied [1].

Q5. BOD is the dissolved oxygen needed by microorganisms [1] to break down organic matter [1]; high BOD indicates lots of organic waste/pollution [1] and can reduce dissolved oxygen [1].

Q6. Nitrates/phosphates enter water [1]; algal bloom occurs [1]; algae die [1]; microorganisms decompose them using oxygen [1]; dissolved oxygen falls and fish may die [1].

Q7. Heavy metals are toxic [1]; persistent/do not break down easily [1]; can bioaccumulate/biomagnify in food chains [1].

Q8. Alkaline [1].

Additional Syllabus Coverage Added for Complete Chapter 9

This final combined version includes the extra Ordinary Level points that needed to be made fully explicit for the Chapter 9 syllabus.

Free Chlorine, Total Suspended Solids and Total Dissolved Solids

Term	Meaning	Why it matters
Free chlorine	The small amount of chlorine left in treated water after disinfection.	Shows that the water still has some protection against microorganisms.
Total suspended solids (TSS)	Solid particles floating in water.	High TSS can make water cloudy and reduce quality.
Total dissolved solids (TDS)	Substances dissolved in water, including salts and minerals.	High TDS can affect taste and suitability for use.

Examiner Tip: Suspended solids are particles you can filter out. Dissolved solids are not removed by simple filtration.

EDTA and Hardness Testing

EDTA is used in some hardness tests because it reacts with metal ions such as Ca²⁺ and Mg²⁺. This helps show how hard the water is.

EDTA + Ca²⁺/Mg²⁺ → complex / titration end-point

Why it matters: Hardness tests help decide whether water needs softening or treatment before use.

pH Scale, Universal Indicator and Limitations

Universal indicator is a mixture of indicators that gives a range of colours across the pH scale. It gives an approximate pH, but it is less precise than a pH meter. This means indicator paper is useful for quick testing, while a pH meter is better when an accurate value is needed.

Method	Use	Limitation / Advantage
Universal indicator	Quick estimate of pH	Colour matching is approximate
pH paper	Simple classroom test	Less accurate than a meter
pH meter	Precise measurement of pH	Needs correct calibration and careful use

Examiner Tip: If the question asks for the best method for accurate pH measurement, write pH meter, not universal indicator.

pH of Strong Acids and Strong Bases

At Ordinary Level, remember the general pattern:

Strong acids have very low pH values. Strong bases have very high pH values.

Type	Typical pH idea	Example
Strong acid	Often around pH 0–2	Hydrochloric acid
Neutral substance	pH 7	Pure water
Strong base	Often around pH 12–14	Sodium hydroxide

pH Adjustment in Water Treatment

Water treatment may include pH adjustment so that water is not too acidic or too alkaline. This helps protect pipes, improves taste, and keeps water suitable for consumers and aquatic environments.

pH adjustment keeps treated water within a safe and suitable range.

Water Analysis: Colorimetry and Anion Tests

Colorimetry can be used to estimate the concentration of coloured substances in water samples. It is useful when comparing samples or checking how concentration changes after treatment.

Simple anion tests may also be used in water analysis:

Ion	Simple test idea	Observation
Chloride	Add silver nitrate solution after acidifying	White precipitate
Sulfate	Add barium chloride solution after acidifying	White precipitate
Nitrate	Use a suitable nitrate test procedure	Positive nitrate result

Chlorine Estimation

The amount of chlorine present in treated water must be carefully controlled. Too little chlorine may not kill enough microorganisms, while too much chlorine may affect taste and smell. In practice, chlorine levels can be measured by suitable water-testing methods.

Suspended Solids and Dissolved Solids

Type	Meaning	Removal / Significance
Suspended solids	Particles floating in the water	Removed by sedimentation and filtration
Dissolved solids	Substances dissolved in the water	Not removed by ordinary filtration; may affect water quality

Water Standards and Safe Limits

Water supplied for drinking must meet legal and safety standards. In exam answers, it is enough to say that treatment aims to keep harmful substances and microorganisms below accepted safe limits.

Examiner Trap: Filtration removes suspended solids, but it does not remove every dissolved substance or automatically disinfect the water.

PREMIUM REVISION NOTES

Chapter 9 Complete Coverage

1. Learning Objectives

2. What is Hard Water?

3. How Water Becomes Hard

4. Temporary and Permanent Hardness

5. Removing Temporary Hardness by Boiling

6. Problems Caused by Hard Water

7. Benefits of Hard Water

8. Removing Hardness: Ion Exchange

9. Removing Hardness: Deionisation

10. Interactive Simulators and Virtual Labs

Simulator 1: ChemCollective Virtual Lab

Simulator 2: PhET – Concentration

Simulator 3: PhET – Sugar and Salt Solutions

11. Worked Examples

12. Examiner Secrets, Mistakes and Traps

13. Exam Practice Questions

MCQs with Explanations

14. Last-Minute Revision Sheet

15. Self-Assessment Checklist

16. Mark Scheme

2. Why Water Must Be Treated

3. Municipal Water Treatment Stages

4. Flocculation and Sedimentation

5. Filtration

6. Chlorination

7. Fluoridation

8. Sewage Treatment

9. Primary Sewage Treatment

10. Secondary Sewage Treatment

11. Tertiary Sewage Treatment

12. Interactive Simulators and Virtual Labs

Simulator 1: ChemCollective Virtual Lab

Simulator 2: PhET – Concentration

Simulator 3: PhET – Sugar and Salt Solutions

13. Worked Examples

14. Examiner Secrets, Mistakes and Traps

15. Exam Practice Questions

MCQs with Explanations

16. Last-Minute Revision Sheet

17. Self-Assessment Checklist

18. Mark Scheme

2. Water Quality

3. Dissolved Oxygen (DO)

4. Biochemical Oxygen Demand (BOD)

5. Organic Pollution and Oxygen Sag

6. Eutrophication

7. Heavy Metal Pollution

8. pH and Water Quality

9. Interactive Simulators and Virtual Labs

Simulator 1: PhET – pH Scale

Simulator 2: PhET – Concentration

Simulator 3: ChemCollective Virtual Lab

10. Worked Examples

11. Examiner Secrets, Mistakes and Traps

12. Exam Practice Questions

MCQs with Explanations

13. Last-Minute Revision Sheet

14. Self-Assessment Checklist

15. Mark Scheme

2. Chapter 9 Big Picture

3. pH and Water Analysis

4. pH Scale Diagram

5. Instrumental Analysis at OL Level

6. Full Chapter Summary Table

7. Practical Applications

Soap Test for Hardness

Testing pH

Testing Water Pollution

8. Interactive Simulators and Virtual Labs

Simulator 1: PhET – pH Scale

Simulator 2: PhET – Concentration

Simulator 3: ChemCollective Virtual Lab

9. Worked Examples

10. Examiner Secrets, Mistakes and Traps

11. Final Chapter 9 Exam Practice

MCQs with Explanations

12. Full Chapter 9 Last-Minute Revision Sheet

13. Self-Assessment Checklist