What you'll learn
Safe water is essential for health, agriculture and industry, and providing it is a major challenge for Caribbean nations, many of which rely on limited rainfall, groundwater or desalination. Water chemistry explains how natural water becomes contaminated, how it is treated to make it safe to drink, and how its quality is analysed. This guide covers the water cycle and sources of water, the unique properties of water, impurities in natural water, the stages of municipal water treatment — screening, coagulation and flocculation, sedimentation, filtration and disinfection — hardness of water and methods of softening, desalination by reverse osmosis and distillation, sewage treatment, and methods of water analysis including pH, dissolved oxygen, biochemical oxygen demand (BOD), and tests for ions.
CAPE questions ask you to describe and explain treatment stages with equations, explain the causes and removal of hardness, and interpret water-quality data. Linking each stage to the impurity it removes is the key to clear answers.
Key terms and definitions
Potable water — Water that is safe to drink.
Suspended solids — Insoluble particles carried in water.
Coagulation — Adding a chemical that neutralises the charges on fine particles so they can clump together.
Flocculation — Gentle mixing that allows coagulated particles to form larger clumps (flocs).
Sedimentation — Allowing flocs and particles to settle out under gravity.
Disinfection — Killing pathogenic microorganisms, usually by chlorination.
Hard water — Water containing dissolved calcium and magnesium ions, which form scum with soap.
Temporary hardness — Hardness caused by hydrogencarbonates, removable by boiling.
Permanent hardness — Hardness caused by sulfates and chlorides of calcium and magnesium, not removed by boiling.
Desalination — The removal of dissolved salts from seawater or brackish water.
Reverse osmosis — Forcing water through a semi-permeable membrane against the osmotic gradient, leaving salts behind.
Biochemical oxygen demand (BOD) — The amount of dissolved oxygen used by microorganisms to decompose organic matter in a water sample over five days at 20 °C.
Core concepts
Sources of water in the Caribbean
- Surface water — rivers, streams and reservoirs, important in Jamaica, Trinidad and Guyana.
- Groundwater — aquifers in limestone and other rocks, especially important on islands such as Barbados, where most water comes from underground sources.
- Rainwater harvesting — collecting roof run-off in tanks, common in the Grenadines and the Virgin Islands.
- Desalination — producing fresh water from seawater, widely used in Antigua, the Bahamas, the Cayman Islands, Aruba and other islands with limited fresh water.
Impurities in natural water
- Suspended solids — silt, clay and organic matter, which make water cloudy (turbid).
- Dissolved substances — salts (calcium, magnesium, sodium, chloride, sulfate, hydrogencarbonate), nitrates and phosphates from agriculture, and sometimes heavy metals.
- Dissolved gases — oxygen, carbon dioxide.
- Microorganisms — bacteria, viruses and parasites, some of which cause diseases such as cholera, typhoid and gastroenteritis.
Municipal water treatment
- Screening — removes large debris (leaves, plastic, fish) using metal screens.
- Pre-treatment / aeration — water may be aerated to remove dissolved gases and oxidise iron and manganese ions so they precipitate.
- Coagulation and flocculation — aluminium sulfate (alum) or iron(III) salts are added. The highly charged Al³⁺ ions neutralise the negative charges on tiny clay and organic particles, so they can come together. Aluminium hydroxide forms a gelatinous precipitate that traps particles: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s) Gentle stirring encourages the particles to form larger flocs.
- Sedimentation — the water flows slowly through settling tanks, where the heavy flocs sink and are removed as sludge.
- Filtration — water passes through beds of sand and gravel (sometimes with activated carbon), removing remaining fine particles and some microorganisms. Activated carbon also adsorbs organic compounds that cause unpleasant tastes and odours.
- Disinfection — chlorine is added to kill pathogens: Cl₂ + H₂O ⇌ HCl + HClO Chloric(I) acid (HClO) is the active disinfectant. A small residual of chlorine remains to protect water in pipes. Alternatives include ozone and ultraviolet light, which leave no residual.
- pH adjustment — lime or sodium carbonate may be added to raise the pH and reduce corrosion of pipes.
- Fluoridation (in some places) — adding fluoride to reduce tooth decay.
Hard water
Hardness is caused by dissolved Ca²⁺ and Mg²⁺ ions, picked up when rainwater containing dissolved CO₂ flows over limestone (CaCO₃) and dolomite. This is common in limestone islands such as Barbados and parts of Jamaica.
CaCO₃(s) + H₂O(l) + CO₂(aq) → Ca(HCO₃)₂(aq)
Problems of hard water:
- forms scum with soap: Ca²⁺ + 2C₁₇H₃₅COO⁻ → (C₁₇H₃₅COO)₂Ca(s), wasting soap;
- forms scale (limescale) in kettles, boilers and pipes, reducing efficiency and blocking pipes.
Benefits: calcium is good for teeth and bones; hard water tastes better to some people and is less likely to dissolve lead from old pipes.
Softening water
Temporary hardness is removed by boiling, which decomposes calcium hydrogencarbonate: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g) (This is also how limescale forms in kettles.)
Permanent hardness is not removed by boiling and requires:
- Adding washing soda (sodium carbonate), which precipitates calcium carbonate: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)
- Ion exchange — water passes through a resin that exchanges Ca²⁺ and Mg²⁺ for Na⁺ (or H⁺) ions. The resin is regenerated by flushing with concentrated sodium chloride solution.
- Distillation — boils and condenses the water, leaving all dissolved salts behind (expensive).
Lime softening (adding calcium hydroxide) is used in large treatment works to remove temporary hardness: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃ + 2H₂O.
Desalination
Reverse osmosis: seawater is pumped at high pressure against a semi-permeable membrane. Water molecules pass through, but most dissolved ions are left behind. It is now the most common method in the Caribbean, but it requires significant energy, pre-treatment to protect the membranes, and disposal of concentrated brine, which can harm marine life near outflows.
Distillation (thermal desalination): seawater is heated and the vapour condensed, leaving salts behind. It produces very pure water but uses large amounts of energy unless waste heat is available.
Sewage treatment
Untreated sewage contaminates water with pathogens and nutrients, harming health, coral reefs and tourism. Treatment stages:
- Preliminary: screening and grit removal.
- Primary: sedimentation of solids as sludge.
- Secondary: aerobic bacteria break down dissolved organic matter (activated sludge or trickling filters), reducing BOD.
- Tertiary: removal of nitrates and phosphates (for example, precipitating phosphate with iron or aluminium salts), and disinfection before discharge.
Water analysis
- pH — measured with a pH meter or indicators; drinking water is typically about pH 6.5–8.5.
- Dissolved oxygen (DO) — essential for aquatic life; measured with an oxygen meter or by the Winkler titration.
- BOD — high BOD indicates heavy organic pollution, such as sewage or food waste.
- Turbidity — cloudiness, measured with a turbidity meter.
- Ions — chloride by titration with silver nitrate; sulfate by gravimetric analysis with barium chloride; nitrate and phosphate by colorimetry; iron by colorimetry after forming a coloured complex; heavy metals by atomic absorption spectroscopy.
- Hardness — measured by titration with EDTA, using an indicator such as Eriochrome Black T.
- Microbiological tests — counting coliform bacteria as an indicator of faecal contamination.
Worked examples
Example 1: Coagulation
Explain why aluminium sulfate is added during water treatment. (3 marks)
Fine suspended clay particles carry negative charges and repel each other, so they stay suspended (1). Highly charged Al³⁺ ions neutralise these charges, allowing the particles to clump together (1). Aluminium hydroxide precipitate also traps particles, forming flocs that settle out during sedimentation (1).
Example 2: Removing hardness
Explain, with equations, why boiling removes temporary but not permanent hardness.
Temporary hardness is due to Ca(HCO₃)₂, which decomposes on boiling: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂, precipitating the calcium. Permanent hardness is due to CaSO₄ or MgCl₂, which do not decompose on boiling, so the Ca²⁺ and Mg²⁺ remain dissolved.
Example 3: Interpreting BOD
Water upstream of a village has a BOD of 2 mg dm⁻³; downstream it is 25 mg dm⁻³. Suggest a reason and an effect.
The large increase suggests organic pollution, such as untreated sewage, entering the river from the village. Microorganisms decomposing this matter use up dissolved oxygen, which could cause fish and other aquatic organisms to die.
Example 4: Hardness titration
25.0 cm³ of water required 12.5 cm³ of 0.0100 mol dm⁻³ EDTA. EDTA reacts 1:1 with Ca²⁺. Calculate the concentration of Ca²⁺ in mol dm⁻³.
Moles EDTA = 0.0100 × 0.0125 = 1.25 × 10⁻⁴ mol = moles Ca²⁺. [Ca²⁺] = 1.25 × 10⁻⁴ ÷ 0.0250 = 5.00 × 10⁻³ mol dm⁻³.
Example 5: Expressing hardness in mg dm⁻³ of CaCO₃
Water hardness is often expressed as the equivalent mass of calcium carbonate. Using the result from Example 4 ([Ca²⁺] = 5.00 × 10⁻³ mol dm⁻³), express the hardness in mg of CaCO₃ per dm³. (Mᵣ CaCO₃ = 100)
Mass of CaCO₃ equivalent = 5.00 × 10⁻³ × 100 = 0.500 g dm⁻³ = 500 mg dm⁻³. This would be classed as very hard water — typical of some limestone aquifers.
Example 6: Why a chlorine residual matters
Explain why water companies leave a small residual concentration of chlorine in treated water, and why too much chlorine is undesirable.
A residual of chlorine (as HClO and ClO⁻) continues to kill microorganisms that may enter the water as it travels through pipes and storage tanks to homes. However, too much chlorine gives water an unpleasant taste and smell, and chlorine can react with organic matter in water to form chlorinated by-products, some of which may be harmful in the long term. Treatment works therefore remove as much organic matter as possible before chlorination and monitor chlorine levels carefully.
Example 7: Choosing a softening method
A hotel in Barbados has hard water that causes scale in its boilers. Suggest a suitable softening method and explain.
An ion-exchange water softener would be suitable. As water passes through the resin, Ca²⁺ and Mg²⁺ ions are exchanged for Na⁺ ions, removing both temporary and permanent hardness: 2RNa + Ca²⁺ → R₂Ca + 2Na⁺ The resin can be regenerated by flushing with concentrated brine. Boiling would only remove temporary hardness and would itself cause scale.
Example 8: Evaluating desalination for a small island
Evaluate reverse osmosis desalination as a water supply for a small Caribbean island.
Reverse osmosis provides a reliable supply of fresh water independent of rainfall, which is valuable on islands with few rivers or aquifers and growing tourist demand. However, it requires a lot of electricity — often generated from imported fossil fuels — making water expensive and adding to carbon emissions. Membranes need pre-treatment and replacement, and the concentrated brine discharged can damage seagrass and coral near the outfall. Powering plants with solar energy and carefully siting brine outfalls can reduce these drawbacks.
Water quality indicators summary
| Indicator | Good-quality water | What a poor value suggests |
|---|---|---|
| pH | about 6.5–8.5 | acidic (acid rain, mine drainage) or alkaline pollution |
| Dissolved oxygen | high (several mg dm⁻³) | organic pollution, warm water |
| BOD | low (a few mg dm⁻³) | sewage or food-industry waste |
| Turbidity | low | silt, erosion, poor treatment |
| Nitrate/phosphate | low | fertiliser run-off, sewage — risk of eutrophication |
| Coliform bacteria | none in drinking water | faecal contamination |
Common mistakes and how to avoid them
- Saying filtration removes dissolved salts — it removes suspended particles only.
- Saying chlorine itself is the main disinfectant — HClO is the active species.
- Mixing up temporary and permanent hardness.
- Forgetting the environmental impact of desalination brine.
- Describing treatment stages without linking each to the impurity removed.
How this links to your Internal Assessment
Water chemistry is one of the most practical and locally relevant SBA areas. You could compare the hardness of tap water, rainwater, well water and bottled water using EDTA titrations or soap tests; measure the pH, turbidity and chloride content of samples from different sources; investigate the effectiveness of different coagulants (alum versus iron salts, or natural coagulants such as moringa seeds) in clarifying muddy water; or test the chlorine content of tap water at different distances from a treatment works. Strong reports collect and label samples carefully, repeat analyses, compare results with drinking-water standards, and discuss the reliability and limitations of the methods.
Exam technique for Water treatment and analysis
- For each treatment stage, state what is added or done, which impurity is removed and how.
- Write equations for coagulation, chlorination and softening reactions.
- Distinguish temporary and permanent hardness clearly, with equations.
- When interpreting data, link changes in pH, DO, BOD or ion concentration to likely causes and effects.
Quick revision summary
- Caribbean water sources: surface water, groundwater, rainwater harvesting, desalination.
- Impurities: suspended solids, dissolved salts and nutrients, gases, microorganisms.
- Treatment: screening → aeration → coagulation/flocculation (alum) → sedimentation → filtration (sand, activated carbon) → disinfection (Cl₂ → HClO; or ozone, UV) → pH adjustment.
- Hard water: Ca²⁺ and Mg²⁺ from limestone; scum and scale; temporary (boiling) vs permanent (washing soda, ion exchange, distillation).
- Desalination: reverse osmosis or distillation; energy and brine issues.
- Sewage treatment: preliminary, primary, secondary (bacteria lower BOD), tertiary.
- Analysis: pH, DO, BOD, turbidity, ion tests, EDTA titration for hardness, coliform counts.