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Chemistry 3% exam weight

Water and Hardness of Water

Part of the WAEC WASSCE study roadmap. Chemistry topic chem-16 of Chemistry.

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Water and Hardness of Water

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Hardness of water is the inability of water to lather easily with soap, caused by dissolved Ca²⁺ and Mg²⁺ ions. Temporary hardness comes from Ca(HCO₃)₂ and Mg(HCO₃)₂ and is removed by boiling. Permanent hardness comes from chlorides and sulphates of Ca²⁺ and Mg²⁺ and is removed by washing soda (Na₂CO₃) or the permutit (ion-exchange) process.

TypeCauseRemoval method
TemporaryCa(HCO₃)₂, Mg(HCO₃)₂Boiling, Ca(OH)₂
PermanentCaSO₄, MgSO₄, CaCl₂, MgCl₂Na₂CO₃, ion-exchange

Boiling equation: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g). Soap forms scum before lather in hard water, so more soap is wasted.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

What Hardness Means

When soap (sodium stearate, NaC₁₇H₃₅COO) is added to hard water, it reacts with Ca²⁺ and Mg²⁺ ions to form an insoluble scum: Ca²⁺(aq) + 2NaC₁₇H₃₅COO(aq) → Ca(C₁₇H₃₅COO)₂(s) + 2Na⁺(aq). The water only lathers after all Ca²⁺ and Mg²⁺ have been precipitated, which is why hard water wastes soap.

Temporary vs. Permanent Hardness

Temporary hardness is caused by calcium and magnesium hydrogencarbonates. Boiling decomposes them:

  • Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
  • Mg(HCO₃)₂(aq) → MgCO₃(s) + H₂O(l) + CO₂(g)

Adding slaked lime, Ca(OH)₂, also precipitates the salts as carbonates.

Permanent hardness is caused by soluble chlorides and sulphates of calcium and magnesium (CaSO₄, MgSO₄, CaCl₂, MgCl₂). Boiling cannot remove these; the named method uses washing soda:

  • Na₂CO₃(aq) + CaSO₄(aq) → CaCO₃(s) + Na₂SO₄(aq)

Exam tip: WAEC WASSCE essay questions (Paper 2) typically award marks for (i) a clear definition, (ii) distinguishing the two types, (iii) at least two removal methods with balanced equations, and (iv) naming Ca²⁺/Mg²⁺ salts correctly.

Removal Methods Compared

  • Boiling — only temporary hardness; cheap for homes.
  • Washing soda — removes both types; introduces Na⁺ ions.
  • Permutit / ion-exchange resin — swaps Ca²⁺ and Mg²⁺ for Na⁺ or H⁺; the resin is regenerated with brine.

Measuring Hardness

Quantitative hardness uses EDTA titration with Eriochrome Black T indicator and is reported in mg/L CaCO₃ equivalent or mmol/dm³.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Subtleties

Rainwater is naturally soft because it lacks dissolved Ca²⁺ and Mg²⁺, but it is not potable until treated. Groundwater crossing limestone (CaCO₃) or dolomite (CaCO₃·MgCO₃) beds picks up carbon dioxide and dissolves the minerals as hydrogencarbonates — this is why most borehole water in Nigeria is hard. Permanent hardness arises where water passes through gypsum (CaSO₄·2H₂O) or salt deposits.

The Permutit Process in Detail

Permutit is a synthetic zeolite (sodium aluminium silicate, Na₂Z). Hard water flowing over it exchanges ions: Na₂Z(s) + Ca²⁺(aq) → CaZ(s) + 2Na⁺(aq). When the resin is exhausted, it is flushed with concentrated NaCl brine to reverse the exchange and regenerate it. Modern domestic units use cation-exchange resins that release H⁺ instead of Na⁺.

Common Mistakes WAEC Candidates Make

  • Claiming boiling removes all hardness — it only removes temporary.
  • Saying CaCO₃ is the dissolved hardness ion — CaCO₃ is the insoluble precipitate formed during removal.
  • Forgetting to balance equations, especially forgetting CO₂ in boiling reactions.
  • Reporting hardness in ”°” without naming the CaCO₃ equivalent standard.
  • Confusing washing soda (Na₂CO₃) with baking soda (NaHCO₃); only Na₂CO₃ precipitates Ca²⁺/Mg²⁺.

Trap alert: A question may give a single salt such as MgSO₄ and ask which method works. Because MgSO₄ causes permanent hardness, boiling fails — students who pick boiling lose the mark.

Hardness connects to water treatment (chlorination, filtration), soap and detergents (why detergents work in hard water while soaps fail), and scale formation in boilers and kettles (CaCO₃ from decomposed hydrogencarbonates reduces heat transfer).

Practice Prompts

  1. A sample of water contains only Ca(HCO₃)₂. State and explain one household method of softening it, writing a balanced equation.
  2. Outline how the permutit process softens water containing CaSO₄, and state how the exhausted zeolite is regenerated.

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