Softening Water by Boiling

Introduction

The simplest way to soften some hard water needs no chemicals at all: just boil it. For centuries people have noticed that boiled water lathers better with soap than water straight from a well or stream. Boiling works because it turns dissolved calcium into a solid that can be separated from the water. But it only works on one kind of hardness, and it has real costs. This page looks at the chemistry and the practical limits of softening by boiling.

Core explanation

Softening means removing ions. Water is hard because it contains dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). To soften water, these ions must be taken out of solution. One way is to turn them into an insoluble compound — a precipitate — that can be filtered off or left behind.

What boiling does. Temporary hardness is caused by dissolved calcium hydrogencarbonate , Ca(HCO₃)₂ (and magnesium hydrogencarbonate). When the water is boiled, these compounds decompose. Calcium hydrogencarbonate breaks down into calcium carbonate , water and carbon dioxide. The calcium carbonate is insoluble, so it forms a white precipitate. The Ca²⁺ ions are now locked up in solid CaCO₃ and are no longer dissolved, so the water is softer.

Separating the solid. Some of the calcium carbonate sticks to the container as limescale, and the rest forms a fine suspension that settles out or can be removed by filtration . The clear water left behind has lost its temporary hardness.

What boiling cannot do. Permanent hardness, caused by dissolved calcium and magnesium sulfates (and chlorides), is not affected by boiling because these salts do not decompose at 100 °C. Boiled water from a region with permanent hardness is still hard.

Advantages and disadvantages of boiling:

Advantages Disadvantages --- --- Needs no chemicals Removes temporary hardness only Simple and familiar Uses a lot of energy for large volumes Also kills microbes Produces limescale in the container Leaves no added sodium Impractical for a whole household supply

Because of the energy cost, boiling is used for small volumes only — for example, water boiled in a kettle for drinks. Homes and industries use washing soda or ion exchange instead.

Formulae

Calcium hydrogencarbonate on boiling:

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

Magnesium hydrogencarbonate on boiling:

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

Step-by-step reasoning

Showing that boiling softens temporarily hard water:

1. Measure how much soap solution a sample needs to give a lasting lather. 2. Boil an equal volume of the same water for a few minutes, then cool it. 3. Filter off the white precipitate. 4. Test the filtrate with soap in the same way. 5. Less soap is needed, showing that hardness has been removed.

Visual explanation

Picture a clear beaker of water from a chalk spring on a hotplate. As it heats, tiny bubbles of carbon dioxide rise and the water turns slightly cloudy. After cooling, a thin white layer of calcium carbonate lies on the bottom and a faint crust rings the glass. Filtered, the water is clear again — but now it is softer.

Real-world analogy

Boiling is like a sieve that only catches one kind of object. It catches the "hydrogencarbonate" pieces by turning them into lumps of chalk, but the "sulfate" pieces are small enough to pass straight through, so they remain in the water.

Real-world example

In hard-water areas, tea made with freshly boiled water can develop a thin "skin" or film on top. Part of this film contains calcium carbonate formed during boiling, combined with compounds from the tea. It is a visible sign that boiling has pulled some calcium out of solution.

Why?

Why does boiling make calcium hydrogencarbonate decompose? Hydrogencarbonate ions are only stable while dissolved carbon dioxide is present to keep them in balance. Heating drives carbon dioxide out of the water, and the hydrogencarbonate turns into carbonate, which immediately combines with calcium ions to form insoluble calcium carbonate.

Common misconception

"Boiling removes all the minerals from water." Boiling removes only the calcium and magnesium that were present as hydrogencarbonates. Sulfates, chlorides, sodium and other dissolved ions all stay in solution, and some even become slightly more concentrated as water evaporates.

Worked example

Question: Sample A needs 10 cm³ of soap solution before and 2 cm³ after boiling. Sample B needs 10 cm³ before and 9 cm³ after. Which sample is mainly temporarily hard?

Reasoning: Boiling removes temporary hardness only. Sample A lost most of its hardness on boiling; sample B hardly changed, so its hardness is mostly permanent.

Answer: Sample A is mainly temporarily hard.

Quick check

1. Name the white precipitate formed when temporarily hard water is boiled. Answer: Calcium carbonate, CaCO₃.

Exam focus

State clearly that boiling removes only temporary hardness, and give the balanced equation with state symbols. For evaluation questions, mention both the benefit (no chemicals) and the drawbacks (energy cost, limescale, no effect on permanent hardness).

Advanced insight

Boiling does not remove quite all temporary hardness. Calcium carbonate is very slightly soluble, and some Ca²⁺ stays dissolved alongside sulfate or chloride ions. Magnesium carbonate is more soluble than calcium carbonate, and on prolonged boiling it tends to convert to magnesium hydroxide, which is less soluble. Real results therefore depend on how long the water is boiled.

Summary

Boiling softens water by decomposing dissolved calcium and magnesium hydrogencarbonates into insoluble carbonates, which precipitate and can be removed. It needs no chemicals and also kills microbes, but it removes only temporary hardness, uses a lot of energy and forms limescale. It is practical only for small volumes of water.

Practice questions

1. Explain, using an equation, how boiling removes temporary hardness. Answer: Calcium hydrogencarbonate decomposes to insoluble calcium carbonate: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g), removing Ca²⁺ ions from solution. 2. Why does boiling not remove permanent hardness? Answer: Permanent hardness is caused by calcium and magnesium sulfates, which do not decompose on heating, so the ions stay dissolved. 3. Give two reasons why boiling is not used to soften a town's water supply. Answer: It would use a huge amount of energy, and it would not remove permanent hardness. 4. How could you show that a sample of boiled, filtered water is softer than before? Answer: Compare the volume of soap solution needed to form a lasting lather before and after boiling; less soap is needed afterwards.