Quick Summary
- Soft water lathers easily with soap and is ideal for washing and industrial processes
- Hard water contains dissolved Ca²⁺ and Mg²⁺ ions that prevent lathering
- Hard water is further divided into temporary and permanent hardness
- Rainwater and distilled water are examples of soft water
- Borehole and well water in Nigeria often contain hardness
What is Soft Water?
Soft water is water that forms lather easily when soap is added. This happens because soft water does not contain dissolved salts of calcium or magnesium. When you wash your hands with soap under soft water, you get rich foam quickly without wasting soap.
Examples of soft water include rainwater, distilled water, and deionized water. In Nigeria, rainwater collected during the rainy season is typically soft. Many households in Lagos and other cities harvest rainwater for laundry because it saves soap and cleans better than tap water.
Characteristics of Soft Water
- Forms lather readily with soap
- Does not leave scale deposits in kettles and pipes
- Requires less soap for cleaning
- Suitable for industrial processes like dyeing and tanning
- Does not contain significant amounts of Ca²⁺ or Mg²⁺ ions
Uses of Soft Water
Domestic use: Soft water is excellent for washing clothes, bathing, and general cleaning. It makes soap work more effectively and leaves no residue on surfaces.
Industrial use: In textile factories, soft water is used for dyeing fabrics because dissolved salts can interfere with dye absorption. Leather tanning industries also prefer soft water because calcium and magnesium salts can damage the leather during processing.
Laboratory use: Scientists use distilled water (soft water) in experiments because impurities can affect chemical reactions and test results.
What is Hard Water?
Hard water is water that does not form lather easily with soap. Instead of producing foam, soap reacts with dissolved calcium and magnesium ions to form a sticky scum. You need to use more soap with hard water to get cleaning done.
Most borehole water, well water, and some tap water in Nigerian cities like Abuja, Kaduna, and Jos contain hardness. This is why many people notice white deposits inside their kettles after boiling water repeatedly. Those deposits are calcium carbonate formed from the dissolved salts in hard water.
Causes of Hard Water
Hard water contains dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺). These ions come from rocks and soil that water passes through underground. When rainwater seeps through limestone rocks (calcium carbonate) or dolomite rocks (calcium magnesium carbonate), it dissolves these minerals and becomes hard.
The chemical equation for limestone dissolving in water containing carbon dioxide is:
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
Problems Caused by Hard Water
- Wastes soap: Calcium and magnesium ions react with soap to form insoluble scum, wasting soap before lather forms
- Blocks pipes: Heating hard water causes scale formation that blocks water pipes and kettles
- Damages appliances: Scale reduces the efficiency of heaters, boilers, and industrial equipment
- Stains clothes: Hard water can leave mineral stains on washed fabrics
- Tastes unpleasant: Some people find hard water has a mineral taste
Types of Hard Water
1. Temporary Hard Water
Temporary hardness is caused by dissolved calcium hydrogen trioxocarbonate (Ca(HCO₃)₂) and magnesium hydrogen trioxocarbonate (Mg(HCO₃)₂). It is called “temporary” because it can be removed by simply boiling the water.
When you boil temporarily hard water, the hydrogen trioxocarbonates decompose to form insoluble carbonates that settle as scale:
Ca(HCO₃)₂(aq) → CaCO₃(s) + CO₂(g) + H₂O(l)
Mg(HCO₃)₂(aq) → MgCO₃(s) + CO₂(g) + H₂O(l)
The white coating inside kettles is calcium carbonate from boiling temporarily hard water.
2. Permanent Hard Water
Permanent hardness is caused by dissolved calcium tetraoxosulphate(VI) (CaSO₄) and magnesium tetraoxosulphate(VI) (MgSO₄), as well as calcium chloride (CaCl₂) and magnesium chloride (MgCl₂). These salts do not decompose when boiled, so the hardness remains even after boiling—hence the name “permanent.”
Permanent hardness requires special treatment methods like adding washing soda or ion exchange to remove it.
Comparison Table: Soft Water vs Hard Water
| Feature | Soft Water | Hard Water |
|---|---|---|
| Lather formation | Forms lather easily | Does not form lather easily |
| Dissolved ions | No Ca²⁺ or Mg²⁺ | Contains Ca²⁺ and Mg²⁺ |
| Soap usage | Requires less soap | Wastes soap (forms scum) |
| Scale formation | Does not form scale | Forms scale in kettles/pipes |
| Examples | Rainwater, distilled water | Borehole water, well water |
| Industrial use | Suitable for dyeing, tanning | Damages boilers, blocks pipes |
Common Exam Mistakes
WAEC Chief Examiner Reports show students often:
- Confuse temporary and permanent hardness causes—remember temporary is caused by HCO₃⁻ salts, permanent by SO₄²⁻ and Cl⁻ salts
- Write “soft water does not contain minerals” instead of “does not contain Ca²⁺ or Mg²⁺ ions”—soft water can contain other dissolved minerals
- Fail to balance chemical equations for hardness removal
- Cannot explain why temporary hardness forms scale when boiled
- Mix up the terms “lather” and “scum”—lather is foam, scum is the sticky residue
- Write vague answers like “hard water is bad” instead of explaining specific problems like pipe blockage or soap wastage
Practice Questions
Multiple Choice Questions
1. Which of the following is an example of soft water?
a) Borehole water
b) Sea water
c) Distilled water ✓
d) Well water
2. The ions responsible for hardness in water are:
a) Na⁺ and K⁺
b) Ca²⁺ and Mg²⁺ ✓
c) H⁺ and OH⁻
d) Fe²⁺ and Cu²⁺
3. When soap is added to hard water, it forms:
a) Lather immediately
b) A precipitate called scum ✓
c) Hydrogen gas
d) Carbon dioxide
4. Which substance causes temporary hardness in water?
a) Calcium tetraoxosulphate(VI)
b) Magnesium chloride
c) Calcium hydrogen trioxocarbonate ✓
d) Sodium chloride
Essay Questions
1. (a) Define soft water and hard water. (4 marks)
(b) State three problems caused by hard water in homes and industries. (3 marks)
(c) Explain why rainwater is soft but borehole water is often hard. (3 marks)
Tip: For part (a), give clear definitions mentioning lather formation. For part (b), be specific—don’t just say “causes problems,” explain exact problems like pipe blockage. For part (c), discuss what rainwater passes through vs what underground water dissolves.
2. (a) Distinguish between temporary and permanent hardness of water. (4 marks)
(b) Write a balanced equation to show what happens when temporarily hard water is boiled. (3 marks)
(c) Why is soft water preferred for industrial processes like dyeing and leather tanning? (3 marks)
Tip: In part (a), mention both the cause and method of removal for each type. For part (b), make sure your equation is balanced with correct state symbols. Part (c) requires explaining how Ca²⁺ and Mg²⁺ interfere with industrial processes.
3. A student collected water samples from four sources: rainwater (A), borehole water (B), distilled water (C), and tap water (D). When equal volumes of soap solution were added to each sample and shaken, samples A and C formed lather immediately while B and D formed scum.
(a) Which samples contain soft water? Explain. (3 marks)
(b) What causes the scum formation in samples B and D? (2 marks)
(c) Suggest a method to soften sample B for domestic use. (2 marks)
(d) State two advantages of using soft water for laundry. (3 marks)
Tip: This is a practical-based question. Use the observation (lather vs scum) to identify soft/hard water. Be specific about ions causing scum. For softening method, consider if the hardness is temporary or permanent.
Memory Aids
Remember types of water hardness:
TEMP = Trioxocarbonate (HCO₃) → Easily removed, Makes Precipitate when boiled
PERM = Persistent Even after boiling → needs Reagents or ion exchange Methods
Soft water uses: WILD
Washing (domestic)
Industrial processes (dyeing, tanning)
Laboratory work
Drinking (no mineral taste)
Hard water problems: SWEPT
Soap wasted
White scale forms
Equipment damaged
Pipes blocked
Taste unpleasant
Related Topics
- Methods of softening hard water (treatment with washing soda, ion exchange)
- Water purification and treatment
- Properties and uses of water
- Solubility and solution chemistry
- Soap and detergent chemistry