Temporary Hardness

Introduction

Some hard water becomes soft simply by being boiled. If you boil hard water from a limestone area and let it cool, soap will lather in it much more easily — but the kettle gains a white coating in the process. This kind of hardness is called temporary hardness . Understanding it explains why limescale forms, why it is worst in hot-water systems, and why boiling is one of the oldest ways of softening water.

Core explanation

The cause. Temporary hardness is caused by dissolved calcium hydrogencarbonate , Ca(HCO₃)₂, and, to a lesser extent, magnesium hydrogencarbonate , Mg(HCO₃)₂. These form when rainwater containing carbon dioxide reacts with limestone, chalk or dolomite. In solution they exist as Ca²⁺ or Mg²⁺ ions together with hydrogencarbonate ions , HCO₃⁻.

What boiling does. When the water is heated, calcium hydrogencarbonate undergoes thermal decomposition :

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

Calcium carbonate is insoluble , so it forms a white precipitate . The calcium ions are removed from solution as solid calcium carbonate, and the carbon dioxide escapes as a gas. With far fewer Ca²⁺ ions left, the water no longer forms scum so easily — it has been softened.

This is the reverse of the reaction that made the water hard in the first place:

- In the ground: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (rock dissolves). - On heating: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂ (solid comes back out).

Heating drives carbon dioxide out of the water, and because the reaction is reversible, losing CO₂ pushes it towards solid calcium carbonate.

Limescale. The precipitated calcium carbonate sticks to hot surfaces as limescale — the crust inside kettles, on immersion heater elements and in hot-water pipes. So the removal of temporary hardness and the formation of limescale are two sides of the same reaction.

Distinguishing temporary from permanent hardness. A simple comparison shows which type is present. Soap solution is added drop by drop to a sample of water, shaking each time, until a lasting lather forms; the volume of soap needed indicates hardness. The test is repeated on a sample of the same water that has been boiled and cooled.

Sample Soap needed before boiling Soap needed after boiling Conclusion --- --- --- --- A a lot very little temporary hardness only B a lot still a lot permanent hardness only C a lot less, but still some both types

Step-by-step reasoning

To explain why boiling softens some hard water:

1. Identify the cause: dissolved Ca(HCO₃)₂. 2. Heating decomposes it to CaCO₃, H₂O and CO₂. 3. CaCO₃ is insoluble, so it precipitates. 4. Calcium ions leave the solution, so the water is softer.

Visual explanation

Picture a beaker of clear hard water, with Ca²⁺ and HCO₃⁻ ions drawn as coloured dots. As it heats, tiny bubbles of CO₂ rise and escape, and white specks of CaCO₃ appear and drift down to coat the bottom of the beaker. After boiling, only a few Ca²⁺ dots remain in the water.

Real-world analogy

Temporary hardness is like a sugar syrup held together by warmth-sensitive "glue". While conditions stay the same, the calcium stays dissolved. Change the conditions by heating and the glue (dissolved carbon dioxide) escapes, so the calcium drops out as a solid crust.

Real-world example

In hard-water areas, the heating element of an electric kettle becomes furred with limescale within weeks. The water poured out after boiling is softer than the water from the tap, which is why tea made with it may have less "scum" on its surface. The limescale itself, though, wastes energy, as it insulates the element from the water.

Why?

Why does boiling not remove all hardness? Only hydrogencarbonates decompose on heating. Calcium and magnesium sulfates or chlorides do not decompose at 100 °C and stay dissolved, so hardness caused by them — permanent hardness — remains after boiling.

Common misconception

"Boiling removes hardness because the calcium evaporates." Calcium ions do not evaporate with the steam. They stay in the kettle, but as solid calcium carbonate stuck to the walls and element rather than as dissolved ions in the water.

Worked example

Question: A 100 cm³ sample of tap water needs 12 cm³ of soap solution to produce a lasting lather. After boiling and cooling, a fresh 100 cm³ sample needs 3 cm³. Distilled water needs 1 cm³. What types of hardness are present?

Reasoning: Boiling reduced the soap needed from 12 to 3 cm³, so much of the hardness was temporary. The boiled sample still needs more soap than distilled water (3 compared with 1 cm³), so some permanent hardness remains.

Answer: Both temporary and permanent hardness are present; most of the hardness is temporary.

Quick check

1. Name the solid formed when temporarily hard water is boiled. Answer: Calcium carbonate (limescale).

Exam focus

Learn the equation Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂ and describe it as thermal decomposition. Explain softening in terms of ions: calcium ions are removed from solution as insoluble calcium carbonate. Be ready to interpret soap-titration results before and after boiling.

Advanced insight

The dissolving and precipitating of calcium carbonate are governed by one reversible equilibrium: CaCO₃ + H₂O + CO₂ ⇌ Ca²⁺ + 2HCO₃⁻. Carbon dioxide is less soluble in hot water, so heating removes it and shifts the equilibrium to the left, precipitating calcium carbonate. The same shift occurs when water drips into caves and loses CO₂ to the air, building stalactites without any heating at all.

Summary

Temporary hardness is caused by dissolved calcium and magnesium hydrogencarbonates. Boiling decomposes these compounds: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. Insoluble calcium carbonate precipitates as limescale, removing calcium ions and softening the water. Boiling does not remove permanent hardness, and comparing soap needed before and after boiling shows which type is present.

Practice questions

1. What compounds cause temporary hardness? Answer: Calcium hydrogencarbonate and magnesium hydrogencarbonate. 2. Write the balanced symbol equation for the effect of heat on calcium hydrogencarbonate. Answer: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g). 3. Explain why limescale forms in kettles in hard-water areas. Answer: Heating decomposes dissolved calcium hydrogencarbonate into insoluble calcium carbonate, which precipitates and sticks to the hot element and walls. 4. Water from two areas is tested. After boiling, sample X lathers easily but sample Y still does not. Which contains permanent hardness? Explain. Answer: Y, because its hardness was not removed by boiling, so it must be caused by calcium or magnesium sulfates or chlorides rather than hydrogencarbonates.