Higher Level Revision Notes

Leaving Cert Higher Level Chemistry

Leaving Cert Higher Level Chemistry Chapter 9: Environmental Chemistry: Water

You learn how water is tested, treated, cleaned, softened, and monitored so you can explain real-world chemistry clearly.

What this page doesTurns a difficult chapter into clear notes, diagrams, and one simple interactive tool.

Student promisePlain English first, exam language second.

Curriculum

Irish Leaving Certificate (ILC)

Level

Higher Level

Subject

Chemistry

Chapter

Chapter 9 — Environmental Chemistry: Water

Simple English Summary

You learn how water is tested, treated, cleaned, softened, and monitored so you can explain real-world chemistry clearly.

Teacher voice: Read the ideas first, then use the detailed notes and diagrams to lock in the exam wording.

What To Focus On

Describe water hardness and treatment.
Explain sewage and drinking-water treatment.
Use water quality tests in simple language.
Connect chemistry to environmental care.

DisclaimerThis publication is an independent educational resource developed by ExamsLogic and compiled for student revision. It is based on publicly available official curricula and is not endorsed by any examination board.

Open Simulators Jump To Notes

Quick Simpler Start

In one sentence

You learn how water is tested, treated, cleaned, softened, and monitored so you can explain real-world chemistry clearly.

Exam habit

When you revise this chapter, ask yourself: "Can I explain this to a friend in one easy paragraph?" If yes, you are in a good place.

Interactive Simulator

This small tool gives you a quick visual check before you move deeper into the chapter notes.

Simulator

Water Test Explorer

Pick a water-analysis test and the tool explains what it measures.

Water test

Choose a test to see what it tells you.

1. Learning Objectives

Explain hard water and identify calcium/magnesium ions as the main cause.
Distinguish temporary and permanent hardness.
Describe methods of water softening including ion exchange, deionisation and EDTA hardness testing.
Describe municipal water treatment and sewage treatment stages.
Explain DO, BOD, eutrophication and heavy metal pollution.
Calculate pH for strong acids and strong bases.
HL Use Ka and Kb expressions for weak acids and weak bases.
HL Explain indicator theory, colorimetry, AAS and instrumental water analysis.

2. Hardness in Water

Hard water contains dissolved calcium ions, Ca²⁺, and magnesium ions, Mg²⁺. These ions react with soap to form scum and reduce lather.

Type of hardness	Cause	Removal
Temporary hardness	Calcium/magnesium hydrogencarbonates	Boiling removes it.
Permanent hardness	Calcium/magnesium sulfates or chlorides	Ion exchange or deionisation.

Examiner Tip: Temporary hardness can be removed by boiling. Permanent hardness cannot be removed by boiling.

Examiner Trap: Hard water is not usually unsafe to drink. The main issues are soap wastage, scale formation and lower heating efficiency.

3. Removing Hardness and Water Softening

Ion Exchange

Ion exchange replaces calcium and magnesium ions with sodium ions.

Ca²⁺ / Mg²⁺ removed → Na⁺ enters water

Deionisation

Deionisation removes both positive and negative ions using ion-exchange resins, producing much purer water than ordinary softening.

EDTA Hardness Estimation HL Practical

Total hardness can be estimated using EDTA. EDTA forms a stable complex with calcium and magnesium ions.

M²⁺ + EDTA⁴⁻ → [M–EDTA]²⁻

HL Note: For the hardness experiment, the important exam point is that one mole of EDTA reacts with one mole of M²⁺ ions.

4. Municipal Water Treatment

Drinking water treatment makes water safe, clear and suitable for use.

Stage	Purpose
Flocculation	Small particles join to form larger flocs.
Sedimentation	Larger particles settle.
Filtration	Sand/gravel remove suspended solids.
Chlorination	Kills harmful microorganisms.
Fluoridation	Used for dental health in water supplies where applied.
pH adjustment	Keeps water in a suitable pH range and protects pipes.

Examiner Tip: Chlorination kills microorganisms; it does not remove hardness.

5. Sewage Treatment

Sewage treatment removes solids, organic matter and harmful substances before water is released back into the environment.

Stage	Main process	Purpose
Primary treatment	Screening and settlement	Removes large solids and sludge.
Secondary treatment	Microorganisms break down organic matter	Reduces BOD.
Tertiary treatment	Further chemical/physical treatment	Removes nutrients, pathogens or specific pollutants.

Examiner Tip: Secondary treatment is biological. Microorganisms break down organic waste and reduce BOD.

6. Water Quality: DO, BOD and Eutrophication

Dissolved oxygen (DO) is the oxygen dissolved in water. Aquatic organisms need it to survive.

Biochemical oxygen demand (BOD) is the amount of oxygen required by microorganisms to break down organic matter in water.

High BOD usually means polluted water and can lead to low DO.

Examiner Trap: High BOD is bad. High DO is usually good.

Eutrophication

Eutrophication happens when excess nitrates and phosphates enter water, causing algal blooms. When algae die, microorganisms decompose them and use up dissolved oxygen.

Nutrients → algal bloom → decay → BOD rises → DO falls

7. Heavy Metal Pollution

Heavy metals such as lead and cadmium are dangerous because they are toxic and can build up in food chains.

Pollutant	Problem
Lead	Toxic; can affect nervous system and contaminate drinking water.
Cadmium	Toxic; may come from industrial sources and accumulates in ecosystems.

Applied chemistry link: Heavy-metal analysis is an important use of instrumental methods such as AAS.

8. pH Basics

pH measures acidity or alkalinity.

pH = -log₁₀[H⁺]

pH value	Meaning
Below 7	Acidic
7	Neutral
Above 7	Alkaline

Strong Acids and Strong Bases

Strong acids and strong bases dissociate almost completely in water, so their pH can be found from ion concentration.

For strong acids: pH = -log[H⁺]

For strong bases: pOH = -log[OH⁻] and pH = 14 - pOH

Example 1: 0.010 mol L^-1 HCl gives [H⁺] = 0.010, so pH = 2.

Example 2: 0.0010 mol L^-1 NaOH gives [OH⁻] = 0.0010, so pOH = 3 and pH = 11.

9. Weak Acids and Weak Bases HL

Weak acids and weak bases ionise only partially in water.

Weak acids

HA ⇌ H⁺ + A⁻

K_a = [H⁺][A⁻] / [HA]

Weak bases

B + H₂O ⇌ BH⁺ + OH⁻

K_b = [BH⁺][OH⁻] / [B]

HL Note: A stronger weak acid has a larger K_a. A stronger weak base has a larger K_b.

Common Mistake: Do not confuse concentration with strength. A dilute strong acid can still be stronger than a concentrated weak acid.

10. Theory of Indicators HL

An indicator is usually a weak acid or weak base whose acid and conjugate-base forms have different colours.

HIn ⇌ H⁺ + In⁻

Changing pH changes the equilibrium position, so the colour changes.

Condition	Effect on indicator equilibrium
Acidic solution	Favours HIn form
Alkaline solution	Favours In⁻ form

Examiner Tip: Indicator colour change is an equilibrium shift, not “magic colour switching”.

11. Instrumental Analysis and Water Testing HL

pH Meter

A pH meter gives a more accurate pH value than indicator paper.

Colorimetry

Colorimetry estimates the concentration of a coloured substance by measuring how much light is absorbed or transmitted.

More colour intensity usually means higher concentration.

AAS (Atomic Absorption Spectroscopy)

AAS is used to detect and estimate metals such as lead or cadmium in water.

Method	Main use
pH meter	Accurate pH measurement
Colorimetry	Estimating concentration of coloured substances, e.g. free chlorine
AAS	Analysis of heavy metals such as lead and cadmium

12. Water Analysis Applications

Free chlorine estimation Mandatory Experiment 9.1

Colorimetry or a comparator can be used to estimate free chlorine in swimming-pool water or bleach.

Total suspended and dissolved solids Mandatory Experiment 9.2

Type	Meaning	How measured
Suspended solids	Particles floating in water	Filtration and weighing residue
Dissolved solids	Substances dissolved in water	Evaporation of filtrate and weighing residue

Determination of pH Mandatory Experiment 9.2

pH may be measured with indicator paper or, more accurately, by pH meter.

Estimation of total hardness using EDTA Mandatory Experiment 9.3

This gives a practical estimate of total hardness in a water sample.

Estimation of dissolved oxygen by redox titration Mandatory Experiment 9.4

Dissolved oxygen can be estimated by redox titration, linking Chapter 9 with volumetric analysis and redox chemistry.

13. Applied Analysis Examples

Analysis	Why important
Lead in water	Checks for toxic contamination of drinking water
Cadmium in water	Detects industrial pollution risk
River and lake water testing	Assesses pollution, eutrophication and biological safety
Fertilisers	Can be analysed for nutrient or metal content

HL Link: The syllabus expects applications such as analysis of river/lake water and analysis of heavy metals in water, especially lead and cadmium. fileciteturn48file1turn48file18

14. Examiner Secrets, Mistakes and Traps

Secret 1: High BOD usually means low water quality.

Secret 2: Use the correct instrument name when asked about analysis.

Secret 3: For HL pH questions, know when to use Ka/Kb and when to use the strong acid/base formulas.

Common Mistake: Confusing hardness with acidity. They are different issues.

Common Mistake: Saying filtration removes dissolved solids. It removes suspended solids only.

Examiner Trap: High chlorine is not always “good” — too much affects taste and may be unsafe.

15. Last-Minute Revision Sheet

Hard water is caused mainly by Ca²⁺ and Mg²⁺.
Temporary hardness can be removed by boiling.
Ion exchange softens water; deionisation removes nearly all ions.
Municipal water treatment includes flocculation, sedimentation, filtration, chlorination and often fluoridation.
High BOD means microorganisms are using a lot of oxygen.
Eutrophication is driven by nitrates and phosphates.
Strong acid/base pH comes from direct ion concentration.
Weak acids and bases use Ka and Kb.
Indicators work by equilibrium colour change.
Colorimetry, pH meter and AAS are key Chapter 9 instruments.
Know the four mandatory practicals 9.1–9.4.

16. Self-Assessment Checklist

I can distinguish temporary and permanent hardness.
I can explain ion exchange and deionisation.
I can describe municipal and sewage treatment.
I can explain DO, BOD and eutrophication.
I can calculate pH for strong acids and strong bases.
I can write Ka and Kb expressions.
I can explain indicator theory.
I can identify when to use colorimetry, pH meter and AAS.
I know the named Chapter 9 HL practicals.