ChemistryICSE Class 9 · Water

Hard and Soft Water: Temporary and Permanent Hardness, Removal and Purification (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Hard water does not lather easily with soap and forms scum, because calcium and magnesium salts are dissolved in it. Temporary hardness (bicarbonates) is removed by boiling or washing soda; permanent hardness (chlorides and sulphates) only by washing soda. Water is made potable by coagulation with alum, filtration, chlorination and, where needed, defluoridation.

Boiling removes only temporary hardness, but washing soda removes both temporary and permanent hardness.

Illustration drawn for QuestionBuddy.

Understand Hard and Soft Water: Causes, Removal of Hardness and Purifying Water

Soft and hard water. Soft water lathers easily with soap. Hard water does not; it forms a scum (curd) because calcium and magnesium ions react with soap:

$2 C_{17} H_{35} C O O N a + C a C l_{2} \to (C_{17} H_{35} C O O)_{2} C a + 2 N a C l$

Rainwater dissolves carbon dioxide and, on flowing over rocks, dissolves calcium and magnesium salts.

	Temporary hardness	Permanent hardness
Caused by	bicarbonates of Ca and Mg	chlorides and sulphates of Ca and Mg
Removed by boiling	yes	no
Removed by washing soda	yes	yes

Removal.

*Boiling (temporary only):* $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$ ; $M g (H C O_{3})_{2} \to M g C O_{3} + H_{2} O + C O_{2}$ . The insoluble carbonate settles and is filtered off.
*Washing soda (both):* $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ ; $M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$ ; $C a (H C O_{3})_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a H C O_{3}$ .

Mass calculations. Balance the equation, take relative masses with coefficients, and scale: for $C a (H C O_{3})_{2}$ ( $M_{r} = 162$ with Ca = 40, H = 1, C = 12, O = 16), 162 g gives 100 g of $C a C O_{3}$ , 18 g of water and 44 g of carbon dioxide.

Advantages and disadvantages.

*Soft water:* good lather, saves soap, no scale; but lacks minerals and can dissolve lead from old pipes.
*Hard water:* supplies calcium and magnesium, pleasant taste; but wastes soap, forms scale in kettles and boilers (wasting fuel and risking bursting) and is unsuitable for dyeing and many industries.

Making water potable.
1. *Coagulation* with alum: fine clay particles clump together and settle.
2. *Filtration* through sand and gravel beds.
3. *Chlorination*: $C l_{2} + H_{2} O \to H C l + H O C l$ ; hypochlorous acid kills germs.
4. *Defluoridation*: removing excess fluoride, which causes fluorosis (for example by the Nalgonda technique or activated alumina).

Diseases from polluted water: diarrhoea, typhoid, cholera, dysentery, hepatitis A and polio from germs; harm from heavy metals and pesticides.

Hard and Soft Water: Causes, Removal of Hardness and Purifying Water: formulas and key facts

Boiling temporary hardness: $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$; 162 g → 100 g CaCO₃.
Washing soda (permanent): $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$; Both kinds of hardness.
Soap and hard water: $2 C_{17} H_{35} C O O N a + C a C l_{2} \to (C_{17} H_{35} C O O)_{2} C a + 2 N a C l$; Scum formed.
Chlorination: $C l_{2} + H_{2} O \to H C l + H O C l$; HOCl kills germs.

How to solve Hard and Soft Water: Causes, Removal of Hardness and Purifying Water questions

Classify the hardness first, then pick the method.

1If the salt is a bicarbonate, boiling works; chloride or sulphate needs washing soda.
2Write and balance the equation; the precipitate is the carbonate.
3Take relative masses with coefficients and scale to the mass given.
4State the physical observation (turbidity, scale or scum).
5For potable water list coagulant, filtration, chlorine, fluoride removal in order.

Hard and Soft Water: Causes, Removal of Hardness and Purifying Water in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Scale in a kettle

Suppose the water in a house carries 0.162 g of calcium bicarbonate in each litre, and a kettle boils 2 L daily.

How much calcium carbonate scale forms in 100 days?

Try it first, then show the solutionSolution

Daily Ca(HCO₃)₂ $= 2 \times 0.162 = 0.324$ g.
Scale per day $= 0.324 \times \frac{100}{162} = 0.2$ g; in 100 days $= 20$ g.

Answer: About 20 g of scale.

Memory hook: 162 g gives 100 g.

Tricks to remember Hard and Soft Water: Causes, Removal of Hardness and Purifying Water

Short memory aids. Say each one aloud once, then use it on a question the same day.

Bicarbonate boils away
Temporary hardness is bicarbonate; boiling turns it into insoluble carbonate.
Use it when: Classifying hardness.
Washing soda for both
Na₂CO₃ precipitates Ca and Mg as carbonates.
Use it when: Removal methods.
Alum, sand, chlorine, fluoride
Coagulate, filter, kill germs, remove excess fluoride.
Use it when: Potable water steps.

Hard and Soft Water: Causes, Removal of Hardness and Purifying Water: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Removed by boiling · 1 mark · MCQBoard level

Hardness due to which salt can be removed by boiling the water?
(a) calcium sulphate
(b) magnesium chloride
(c) calcium bicarbonate
(d) magnesium sulphate

Try it first, then show the solutionSolution

Bicarbonates decompose on boiling; chlorides and sulphates do not.

Answer: (c) calcium bicarbonate

Example 2: Two equations · 2 marks · Short answerBoard level

Write balanced equations for removing (a) the temporary hardness caused by calcium bicarbonate by boiling and (b) the permanent hardness caused by calcium sulphate by washing soda.

Try it first, then show the solutionSolution

(a) $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$ . (1)
(b) $C a S O_{4} + N a_{2} C O_{3} \to C a C O_{3} + N a_{2} S O_{4}$ . (1)

Answer: See working

Example 3: Boiling hard water · 3 marks · Short answerHarder board level

A sample of water contains 8.1 g of calcium bicarbonate (Ca = 40, H = 1, C = 12, O = 16). It is boiled. Find the mass of calcium carbonate precipitated and of carbon dioxide released.

Try it first, then show the solutionSolution

$M_{r}$ of Ca(HCO₃)₂ $= 40 + 2 (1 + 12 + 48) = 162$ ; $M_{r}$ of CaCO₃ $= 100$ ; of CO₂ $= 44$ . (1)
CaCO₃ $= 8.1 \times \frac{100}{162} = 5.0$ g. (1)
CO₂ $= 8.1 \times \frac{44}{162} = 2.2$ g. (1)

Answer: 5.0 g CaCO₃; 2.2 g CO₂

Example 4: Washing soda and calcium chloride · 3 marks · Short answerHarder board level

11.1 g of calcium chloride (Ca = 40, Cl = 35.5) in hard water is treated with washing soda, Na₂CO₃ (Na = 23, C = 12, O = 16). Find the masses of Na₂CO₃ needed and of calcium carbonate precipitated.

Try it first, then show the solutionSolution

$C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ : 111 g, 106 g, 100 g, 117 g. (1)
Na₂CO₃ $= 11.1 \times \frac{106}{111} = 10.6$ g. (1)
CaCO₃ $= 11.1 \times \frac{100}{111} = 10.0$ g. (1)

Answer: 10.6 g Na₂CO₃; 10.0 g CaCO₃

Example 5: Kettle scale and purification · 4 marks · Case-basedHOTS / competency

A house gets water with 0.162 g of calcium bicarbonate per litre. A kettle boils 2 L daily.
(a) Find the calcium carbonate scale formed per day.
(b) Find the scale in 100 days.
(c) Name two harms of such scale.
(d) Write the equation of chlorination of drinking water and name the germicide.

Try it first, then show the solutionSolution

(a) Daily Ca(HCO₃)₂ $= 0.324$ g; scale $= 0.324 \times \frac{100}{162} = 0.2$ g. (1½)
(b) $0.2 \times 100 = 20$ g. (½)
(c) Wastes fuel (poor conductor of heat) and may cause overheating or bursting of boilers. (1)
(d) $C l_{2} + H_{2} O \to H C l + H O C l$ ; hypochlorous acid (HOCl). (1)

Answer: (a) 0.2 g (b) 20 g (c) fuel waste, bursting (d) HOCl

Where students lose marks in Hard and Soft Water: Causes, Removal of Hardness and Purifying Water

The slips examiners see most often, and what to write instead.

Mistake: Boiling prescribed for permanent hardness
Chlorides and sulphates do not decompose on boiling.
Wrong working
Boil to remove CaSO₄.
Right working
Use washing soda.
Do this: Match method to salt.
Usually costs: 2 marks.
Mistake: Equation left unbalanced
The bicarbonate equation has CO₂ and H₂O.
Wrong working
Ca(HCO₃)₂ → CaCO₃ + CO₂
Right working
Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂
Do this: Count H and O.
Usually costs: 1 mark.
Mistake: Chlorination described as bleaching
HOCl kills germs.
Wrong working
Chlorine bleaches the water.
Right working
Hypochlorous acid formed kills germs.
Do this: Write the equation.
Usually costs: 1 mark.

Exam tips

Always name the salts that cause each kind of hardness.
Write the scum equation with C₁₇H₃₅COONa when asked.
For mass questions list relative masses with coefficients.
Give advantages and disadvantages in pairs: two each.

Practice questions on Hard and Soft Water: Causes, Removal of Hardness and Purifying Water

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Water becomes hard because it contains dissolved salts of:
(a) sodium and potassium
(b) calcium and magnesium
(c) iron and copper
(d) ammonium and zinc
Hint
They form scum with soap.
Answer
(b) calcium and magnesium
Q2 · 1 mark · Very short answerBoard level
Name the substance used as a coagulant to settle fine clay particles in water treatment.
Hint
Potash...
Answer
Alum.
Q3 · 2 marks · Short answerHarder board level
Write the balanced equation for the action of washing soda on magnesium sulphate in hard water.
Hint
Magnesium carbonate precipitates.
Answer
$M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$ .
Q4 · 3 marks · Short answerHarder board level
Write the balanced equation for the reaction of sodium stearate soap with magnesium sulphate, name the scum and say what is observed.
Hint
Stearate is C₁₇H₃₅COO⁻.
Answer
$2 C_{17} H_{35} C O O N a + M g S O_{4} \to (C_{17} H_{35} C O O)_{2} M g + N a_{2} S O_{4}$ ; magnesium stearate forms as an insoluble scum and the water does not lather easily.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Hard water is unsuitable for use in boilers.
Reason (R): Scale of calcium carbonate forms on the boiler walls and wastes fuel.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Scale.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
500 L of water contains 0.081 g of calcium bicarbonate per litre. Find the mass of washing soda crystals (Na₂CO₃·10H₂O) needed to remove it by $C a (H C O_{3})_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a H C O_{3}$ (Ca = 40, H = 1, C = 12, O = 16, Na = 23).
Hint
First the Na₂CO₃, then scale to crystals.
Answer
Ca(HCO₃)₂ $= 0.081 \times 500 = 40.5$ g ( $M_{r} = 162$ ). Na₂CO₃ $= 40.5 \times \frac{106}{162} = 26.5$ g. Crystals ( $M_{r} = 286$ ) $= 26.5 \times \frac{286}{106} \approx 71.5$ g.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Cause of temporary hardness?Show answer
Bicarbonates of calcium and magnesium.
2.Cause of permanent hardness?Show answer
Chlorides and sulphates of calcium and magnesium.
3.What does alum do?Show answer
Coagulates fine particles so that they settle.
4.Which acid kills germs in chlorination?Show answer
Hypochlorous acid.
5.What does excess fluoride cause?Show answer
Fluorosis.

Hard and Soft Water: Causes, Removal of Hardness and Purifying Water: frequently asked questions

What is the difference between temporary and permanent hardness?

Temporary hardness is due to calcium and magnesium bicarbonates and is removed by boiling; permanent hardness is due to their chlorides and sulphates and needs washing soda.

How is hard water softened with washing soda?

Sodium carbonate precipitates calcium and magnesium ions as insoluble carbonates, for example CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl.

How is water made safe to drink?

By coagulation with alum, filtration, chlorination to kill germs and removal of excess fluoride if necessary.