C33: Potable Water
Understanding potable water, how it is produced from fresh water and desalination, and how to analyse dissolved ions in water samples.
What is Potable Water?
Potable water is water that is safe to drink. It is not the same as pure water — potable water contains dissolved substances, but the levels of these substances must be low enough to be safe for human consumption.
Water is essential for life. In the UK, our drinking water comes from two main sources:
- Surface water — lakes, rivers and reservoirs
- Ground water — aquifers and water trapped in rocks
The source of water affects how much treatment it needs before it is potable.
Potable Water vs Pure Water
| Property | Potable Water | Pure Water |
|---|---|---|
| Contains dissolved substances | Yes — low levels | No |
| Safe to drink | Yes | Yes |
| pH | Between 6.5 and 8.5 | Exactly 7 |
| Dissolved oxygen | Present | Present |
| Minerals present | Yes (e.g. calcium, magnesium) | No |
| Microbes/bacteria | None or very low levels | None |
| Cost to produce | Relatively low | Very high (distillation) |
Water Treatment Process
In the UK, fresh water from rivers, lakes or aquifers is treated to make it potable. The process involves three main stages:
Stage 1: Filtration
Water is passed through filters made of sand and gravel. This removes solid particles such as leaves, dirt and other debris. Filtration does not remove dissolved substances or microbes.
Stage 2: Sedimentation
A coagulant (such as aluminium sulfate) is added to the water. This makes fine particles clump together and settle to the bottom as sediment. The clear water above the sediment is then removed for further treatment.
Stage 3: Chlorination
Chlorine gas or chlorine compounds are added to the water. This kills bacteria and other microorganisms, making the water safe to drink. The amount of chlorine is carefully controlled.
The order of water treatment is: Filtration → Sedimentation → Chlorination
Do not confuse filtration with distillation. Filtration removes solid particles; distillation removes dissolved substances by boiling and condensing.
Desalination
In some parts of the world, there is not enough fresh water. Sea water can be made potable through desalination — removing dissolved salts from sea water. Desalination requires a lot of energy, making it expensive.
Method 1: Distillation
Sea water is heated and boiled. The water evaporates, leaving the dissolved salts behind. The water vapour is then cooled and condensed back into liquid water, which is now pure. This process requires large amounts of thermal energy.
Method 2: Reverse Osmosis
Sea water is put under high pressure and forced through a semi-permeable membrane. The membrane allows water molecules to pass through but blocks dissolved salts and other impurities. This process requires large amounts of energy to create the high pressure needed.
| Feature | Distillation | Reverse Osmosis |
|---|---|---|
| Energy required | Very high (thermal) | High (pressure) |
| Equipment cost | High | High |
| Purity of product | Very high (pure water) | High (potable water) |
| Scale of use | Often used on large scale | Increasingly popular |
| By-products | Concentrated salt solution | Concentrated salt solution |
Desalination is only used where there is a shortage of fresh water because it is very expensive and energy-intensive compared to treating fresh water.
The Fluoride Debate
Some water supplies have fluoride added to help reduce tooth decay. This is a controversial issue with arguments on both sides.
Fluoride is added to water supplies in some areas to help prevent dental decay, particularly in children.
Arguments for fluoridation:
- Reduces tooth decay, especially in children
- Benefits people who cannot afford dental care
- Fluoride occurs naturally in some water supplies
Arguments against fluoridation:
- People have no choice — it is added to their water supply without consent
- Excessive fluoride can cause dental fluorosis (mottling of teeth)
- Long-term health effects are debated
- Topical fluoride (toothpaste) may be sufficient
In the exam, you may be asked to discuss both sides of the fluoride debate. Always present balanced arguments and avoid giving a one-sided opinion.
Analysing Dissolved Ions in Water
Water samples can be tested to identify dissolved ions present. Different tests give different results depending on the ions present.
Testing for Sulfate Ions (SO₄²⁻)
Add dilute hydrochloric acid followed by barium chloride solution. If sulfate ions are present, a white precipitate of barium sulfate forms.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Testing for Halide Ions
Add dilute nitric acid followed by silver nitrate solution:
- Chloride (Cl⁻) — white precipitate of silver chloride
- Bromide (Br⁻) — cream precipitate of silver bromide
- Iodide (I⁻) — yellow precipitate of silver iodide
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Ag⁺(aq) + Br⁻(aq) → AgBr(s)
Ag⁺(aq) + I⁻(aq) → AgI(s)
Nitric acid is added before silver nitrate to remove any carbonate ions that would also form a precipitate with silver ions, giving a false positive result.
Testing for Calcium and Magnesium Ions (Hardness)
Add sodium hydroxide solution. Calcium ions form a white precipitate of calcium hydroxide. Magnesium ions form a white precipitate of magnesium hydroxide. These dissolved ions cause water hardness.
Remember that pure water is produced by distillation, but potable water only needs to meet safety standards — it does not need to be pure.
Practice Questions
1. Describe the three stages used to treat fresh water to make it potable.
Filtration removes solid particles as water passes through sand and gravel. Sedimentation uses a coagulant to make fine particles settle out. Chlorination adds chlorine to kill bacteria and microbes.
2. Explain why desalination is not widely used in the UK.
The UK has plenty of fresh water from rivers, lakes and aquifers, so desalination is unnecessary. Desalination is very expensive and energy-intensive compared to treating fresh water, making it impractical where fresh water is available.
3. State the difference between potable water and pure water.
Potable water contains low levels of dissolved substances and is safe to drink. Pure water contains no dissolved substances at all and has a pH of exactly 7. Pure water is much more expensive to produce.
4. Describe the test for sulfate ions in water.
Add dilute hydrochloric acid followed by barium chloride solution. A white precipitate of barium sulfate indicates sulfate ions are present.
5. Why is nitric acid added before silver nitrate when testing for halide ions?
Nitric acid removes carbonate ions that would react with silver nitrate to form a white precipitate of silver carbonate, giving a false positive result for chloride ions.
Required Practical
Water Purification by Distillation
Aim: To purify a water sample by simple distillation and test the purity of the distillate.
Method: Set up a simple distillation apparatus — round-bottom flask with the impure water sample, connected to a condenser with cold water running through it. Heat the flask gently using a Bunsen burner. The water evaporates and the vapour travels into the condenser where it cools and condenses as pure water. Collect the distillate in a clean beaker. Test the distillate for dissolved ions using silver nitrate (for chloride) and barium chloride (for sulfate). Also measure the boiling point of the distillate.
Expected results: The distillate should contain no dissolved ions (no precipitate with AgNO₃ or BaCl₂) and boil at exactly 100 degrees C, confirming it is pure water.
Safety: The apparatus will be very hot — handle with care. Use boiling chips to prevent bumping. Ensure the condenser has cold water flowing through it before heating.
Control variables: Volume of water sample, rate of heating, temperature of condenser water.
Maths Skills
Calculating Yield from Distillation
When purifying water by distillation, not all the water will be collected as distillate because some is lost as vapour or remains in the flask.
Percentage yield = (mass of distillate collected / mass of water originally in sample) x 100
Example: A student started with 150 g of impure water and collected 120 g of pure water distillate.
Percentage yield = (120 / 150) x 100 = 80%
Calculating mass of dissolved solids removed: If 200 g of impure water produces 195 g of pure water distillate, the mass of dissolved solids removed = 200 - 195 = 5 g.
Concentration of dissolved solids: Concentration (g/dm3) = mass of dissolved solids (g) / volume of water (dm3). If 5 g of solids were in 200 cm3 of water: volume = 0.2 dm3, so concentration = 5 / 0.2 = 25 g/dm3.
Common Misconceptions
Misconceptions About Potable Water
Potable water is pure water — it contains nothing else.
Potable water is not pure water. Potable water contains dissolved substances (minerals, chlorine, fluoride) but at safe levels for human consumption. Pure water contains only H₂O molecules with nothing dissolved — this is different from potable water. Drinking pure distilled water long-term would actually be harmful as it lacks essential minerals.
Water from a natural spring is always safe to drink without treatment.
Even spring water can contain harmful microorganisms and dissolved substances that make it unsafe. All water from natural sources must be tested and treated (filtration, chlorination) before it is considered potable, regardless of how clear it looks.
Desalination is the main way we get drinking water in the UK.
In the UK, most potable water comes from surface water (lakes and rivers) and groundwater (aquifers), which only requires filtration and chlorination. Desalination is very expensive and energy-intensive, so it is only used in countries with limited fresh water (e.g. Middle Eastern countries) or during severe droughts.
6-Mark Question
Explain the difference between potable water and pure water. Describe how potable water is produced.
Pure water contains only H₂O molecules and has a sharp boiling point of exactly 100 degrees C at standard pressure. Potable water is water that is safe to drink — it contains dissolved substances (such as minerals and added chlorine) but at levels that are safe for human consumption. To produce potable water from fresh water sources in the UK, water is taken from lakes, rivers or aquifers. It passes through mesh screens to remove large debris, then through filter beds containing sand and gravel to remove suspended solids. Chlorine or chlorine compounds are added to kill bacteria and other microorganisms. The pH may be adjusted and fluoride may be added for dental health. The water is then stored in reservoirs and piped to homes. In areas without fresh water, desalination is used — either by distillation (evaporating water and condensing the vapour) or by reverse osmosis (forcing water through a semi-permeable membrane under high pressure). Desalination produces pure water but is very expensive and energy-intensive compared to treating fresh water.
AO3: Analysis and Evaluation
Comparing Water Treatment Methods
A country needs to increase its supply of potable water. The options are:
- Option 1: Build a new reservoir and treatment works for surface water
- Option 2: Build a desalination plant using reverse osmosis
- Option 3: Recycle treated waste water to drinking standard
Evaluate each option, considering cost, energy use, environmental impact and reliability of supply.
Answer: Option 1 is the cheapest and most energy-efficient, and the treated water quality is well understood. However, it depends on sufficient rainfall, floods valleys to create reservoirs (habitat loss), and during droughts the supply may run low. Option 2 is very expensive and uses large amounts of energy (high carbon footprint), but it provides a reliable supply regardless of rainfall and uses seawater which is abundant. Option 3 recycles water that would otherwise be wasted, reducing demand on fresh water sources. However, public acceptance of recycled sewage is low ("yuck factor"), the treatment required to reach drinking standard is complex and expensive, and there are concerns about trace pharmaceuticals or hormones remaining. For a country with reliable rainfall, Option 1 is best. For arid coastal countries, Option 2 is necessary despite the cost.
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