Permanent Hardness

Introduction

If you boil some hard water and then test it with soap, you may find that it still refuses to lather properly. Boiling has removed part of the hardness but not all of it. The hardness that survives boiling is called permanent hardness . It is caused by dissolved salts that are perfectly happy to stay in solution however long the water is boiled. Understanding why it behaves differently from temporary hardness tells us which softening methods will actually work.

Core explanation

What causes permanent hardness? All hardness is caused by dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺) . What differs between the two kinds of hardness is the negative ion that came with them. In permanent hardness, the calcium and magnesium are present mainly as dissolved sulfates — calcium sulfate, CaSO₄, and magnesium sulfate, MgSO₄. Dissolved calcium chloride and magnesium chloride also count as permanent hardness.

Where do these salts come from? Rainwater trickling through the ground dissolves minerals. The rock gypsum is hydrated calcium sulfate (CaSO₄·2H₂O), and it dissolves slowly in water. Magnesium sulfate occurs in some rocks and mineral deposits and is quite soluble. Water that passes through these rocks picks up Ca²⁺, Mg²⁺ and SO₄²⁻ ions.

Why boiling does not help. Temporary hardness is caused by dissolved hydrogencarbonates, such as Ca(HCO₃)₂. These are unstable when heated: they break down into insoluble calcium carbonate, which falls out of solution. Sulfates and chlorides behave quite differently. They are thermally stable at 100 °C — heating the water does not change them into anything insoluble. The calcium and magnesium ions simply stay dissolved, so the water is just as hard after boiling as before.

Removing permanent hardness. Because heat will not do the job, a chemical or physical method is needed:

Method How it works --- --- Adding washing soda (Na₂CO₃) Carbonate ions precipitate Ca²⁺ and Mg²⁺ as insoluble carbonates Ion exchange A resin swaps Ca²⁺ and Mg²⁺ for Na⁺ (or H⁺) ions Distillation Water is evaporated and condensed, leaving all dissolved salts behind

These three methods remove both kinds of hardness, whereas boiling removes only the temporary kind.

Most natural water contains both. Water from chalk and limestone areas is rich in temporary hardness, while water that has passed through gypsum-bearing rock also carries permanent hardness. The total hardness of a sample is the sum of the two.

Formulae

Softening permanent hardness with washing soda:

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

Ionic equation: Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)

Step-by-step reasoning

To decide whether a water sample has permanent hardness:

1. Test a portion with soap solution and note how much is needed to form a lasting lather. 2. Boil a second portion, let it cool and filter off any solid. 3. Test the boiled portion with soap in the same way. 4. If the boiled water still needs more soap than distilled water, the remaining hardness is permanent.

Visual explanation

Imagine two beakers of hard water being heated. In the first, containing calcium hydrogencarbonate, a white cloud of calcium carbonate appears and settles as a crust. In the second, containing calcium sulfate, the water boils and bubbles but stays perfectly clear — the Ca²⁺ and SO₄²⁻ ions keep moving freely among the water molecules.

Real-world analogy

Temporary hardness is like chalk writing on a whiteboard that wipes away with a dry cloth. Permanent hardness is like permanent marker: the dry cloth (boiling) does nothing, and you need a special solvent (a chemical treatment) to remove it.

Real-world example

Some regions draw their water from aquifers in gypsum-bearing rock. Households there often find that kettles scale up less than expected, yet soap still lathers poorly and scum still forms. The cause is permanent hardness, and many homes install ion-exchange softeners to deal with it.

Why?

Why is it called "permanent" if it can be removed? The name only means that it cannot be removed by boiling, which was the simplest household treatment. Chemically, adding carbonate ions or using ion exchange removes it completely, so "permanent" describes its resistance to heat, not true permanence.

Common misconception

"Boiling always softens water." Boiling only removes temporary hardness caused by hydrogencarbonates. Dissolved calcium and magnesium sulfates stay in solution, so boiled water can still be very hard.

Worked example

Question: A water sample needs 12 cm³ of soap solution to form a lasting lather. After boiling, it needs 7 cm³. Distilled water needs 1 cm³. What kinds of hardness are present?

Reasoning: Boiling reduced the soap needed from 12 cm³ to 7 cm³, so some temporary hardness was removed. The boiled water still needs more soap than distilled water (7 cm³ compared with 1 cm³), so permanent hardness remains.

Answer: The sample contains both temporary and permanent hardness.

Quick check

1. Name the dissolved compound that most commonly causes permanent hardness. Answer: Calcium sulfate, CaSO₄ (magnesium sulfate also contributes).

Exam focus

Be ready to state that permanent hardness is caused by dissolved calcium and magnesium sulfates, that it is not removed by boiling, and that it is removed by washing soda or ion exchange. Write the equation for calcium sulfate reacting with sodium carbonate, with state symbols.

Advanced insight

Calcium sulfate is only slightly soluble — roughly 2 g dissolves per litre of water at room temperature — and, unusually, it becomes a little less soluble at higher temperatures. In industrial boilers operating well above 100 °C this can cause hard calcium sulfate scale, which is much harder to remove than carbonate scale because it does not fizz away with weak acids.

Summary

Permanent hardness is caused by dissolved calcium and magnesium sulfates and chlorides. These salts are thermally stable, so boiling leaves the Ca²⁺ and Mg²⁺ ions in solution. Permanent hardness is removed by precipitating the ions with washing soda, by ion exchange or by distillation, all of which also remove temporary hardness.

Practice questions

1. Explain why boiling removes temporary hardness but not permanent hardness. Answer: Boiling decomposes hydrogencarbonates into insoluble calcium carbonate, but sulfates are stable when heated, so calcium and magnesium ions stay dissolved. 2. Write a word equation for softening magnesium sulfate solution with sodium carbonate. Answer: Magnesium sulfate + sodium carbonate → magnesium carbonate + sodium sulfate. 3. Give two methods that remove permanent hardness. Answer: Adding washing soda (sodium carbonate) and passing the water through an ion-exchange resin; distillation also works. 4. Which rock dissolving in groundwater is a common source of permanent hardness? Answer: Gypsum, which is hydrated calcium sulfate.