ChemistryICSE Class 8 · Water

Hard Water and Soft Water: Temporary and Permanent Hardness and Softening (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

Hard water does not lather easily with soap and forms scum; it contains calcium and magnesium salts. Temporary hardness (hydrogencarbonates) is removed by boiling; permanent hardness (chlorides and sulphates) is removed by washing soda, which is also able to remove temporary hardness.

Understand Hard Water, Soft Water and Softening

Soft water lathers easily with soap. Hard water does not lather easily and forms a white scum. Hardness is caused by dissolved calcium and magnesium salts.

Temporary hardness: due to the hydrogencarbonates (bicarbonates) of calcium and magnesium. Removed by boiling.
Permanent hardness: due to the chlorides and sulphates of calcium and magnesium. Not removed by boiling; removed by washing soda.

Disadvantages.

Wastes soap: soap first reacts with the calcium and magnesium salts to form scum, and only after they are used up does it lather.
Scale (fur) in kettles, geysers and boilers: it conducts heat poorly so wastes fuel, and can block pipes or burst boilers.
Clothes look dull and feel rough; unsuitable for dyeing.
Pulses take longer to cook.

Softening by boiling (temporary hardness):
$C a (H C O_{3})_{2} boil C a C O_{3} + H_{2} O + C O_{2}$
The insoluble calcium carbonate settles and is filtered off.

Softening with washing soda (both kinds):
$C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$
$M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$
Calcium and magnesium are removed as insoluble carbonates; the sodium salts left do not cause hardness.

Soap test scheme. Shake the water with soap solution: lather = soft; scum = hard. Boil a fresh sample, cool, filter and test again: if lather now forms, the hardness was temporary; if scum still forms, the hardness is permanent (or both).

Hard Water, Soft Water and Softening: formulas and key facts

Boiling: $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$; Removes temporary hardness.
Washing soda: $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$; Removes permanent hardness.
Causes of hardness: Temporary: Ca and Mg hydrogencarbonates | Permanent: Ca and Mg chlorides and sulphates; Boiling removes only the first.

How to solve Hard Water, Soft Water and Softening questions

Identify the dissolved salt, then pick the softening method that removes it.

1Hydrogencarbonates: temporary hardness; boiling removes it.
2Chlorides and sulphates: permanent hardness; use washing soda.
3Write the equation showing the insoluble carbonate formed.
4For scheme questions, test with soap before and after boiling.

Hard Water, Soft Water and Softening 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 the kettle

Suppose a kettle in a hard-water area gets a white crust inside after months of use.

Explain the crust and its effect.

Try it first, then show the solutionSolution

Boiling hard water decomposes hydrogencarbonates and deposits calcium carbonate as scale.
Scale conducts heat poorly, so more fuel is needed to boil the water.

Answer: Scale of calcium carbonate wastes fuel.

Memory hook: Hard water leaves a crust.

Lather at the river

Imagine a washerman finds that soap forms a scum instead of a lather in a well, but lathers freely in rainwater.

Why?

Try it first, then show the solutionSolution

Well water contains dissolved calcium and magnesium salts.
Rainwater is soft: it has none of these salts.

Answer: The well water is hard; rainwater is soft.

Memory hook: Scum means hard.

Tricks to remember Hard Water, Soft Water and Softening

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

Temporary boils away
Temporary hardness goes on boiling; permanent stays.
Use it when: Methods.
Washing soda for both
Washing soda removes Ca and Mg as carbonates.
Use it when: Softening.
Scum, then lather
Hard water wastes soap: scum forms first.
Use it when: Soap.

Hard Water, Soft Water and Softening: solved examples

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

Example 1: Cause of hardness · 1 mark · MCQBoard level

Hardness of water is caused by dissolved salts of:
(a) sodium and potassium
(b) calcium and magnesium
(c) iron and copper
(d) chlorine and fluorine

Try it first, then show the solutionSolution

Calcium and magnesium salts cause hardness.

Answer: (b)

Example 2: Disadvantages · 2 marks · Short answerBoard level

State two disadvantages of using hard water.

Try it first, then show the solutionSolution

It wastes soap, forming scum before lather. (1)
It forms scale in kettles and boilers, wasting fuel and blocking pipes. (1)

Answer: Wastes soap; scale.

Example 3: Softening · 3 marks · Short answerHarder board level

Write balanced equations for removing (a) temporary hardness due to calcium hydrogencarbonate by boiling, (b) permanent hardness due to magnesium sulphate by washing soda. Name the solid in each.

Try it first, then show the solutionSolution

(a) $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$ ; solid: calcium carbonate. (1½)
(b) $M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$ ; solid: magnesium carbonate. (1½)

Answer: See the working.

Example 4: The soap test · 4 marks · Long answerHarder board level

Equal volumes of two samples A and B need 3 mL and 14 mL of soap solution respectively to give a lasting lather. After boiling, the volumes needed are 3 mL for A and 6 mL for B. (a) Which sample is soft? (b) Which sample is hard, and what kind of hardness does it have? (c) Explain the fall from 14 mL to 6 mL. (d) Why is 6 mL still greater than 3 mL?

Try it first, then show the solutionSolution

(a) A is soft: it lathers readily and boiling makes no difference. (1)
(b) B is hard, with both temporary and permanent hardness. (1)
(c) Boiling removed the temporary hardness. (1)
(d) The permanent hardness remains because boiling does not remove chlorides or sulphates. (1)

Answer: (a) A (b) B, both kinds (c) temporary removed (d) permanent remains

Example 5: The boiler · 4 marks · Case-basedHOTS / competency

A factory boiler uses hard water. (a) What forms inside it? (b) Why does it waste fuel? (c) How could the water be softened beforehand to remove both kinds of hardness? (d) Write one equation for it.

Try it first, then show the solutionSolution

(a) Scale (a hard crust of carbonates). (1)
(b) Scale conducts heat poorly, so more fuel is needed. (1)
(c) Treat with washing soda. (1)
(d) $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ . (1)

Answer: (a) scale (b) poor conductor (c) washing soda (d) as above

Where students lose marks in Hard Water, Soft Water and Softening

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

Mistake: Boiling removes all hardness
Only temporary hardness.
Wrong working
"Boiling softens any hard water."
Right working
"Boiling removes temporary hardness only."
Do this: Name the salts.
Mistake: Wrong salts
Temporary: hydrogencarbonates.
Wrong working
"Temporary hardness is due to chlorides."
Right working
"Temporary hardness is due to hydrogencarbonates."
Do this: Learn the pair.
Mistake: Unbalanced equation
Check CO₂ and H₂O.
Wrong working
"Ca(HCO₃)₂ → CaCO₃ + CO₂."
Right working
"Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂."
Do this: Count H and O.

Exam tips

Give salts with each kind of hardness.
Name the insoluble product in softening equations.
Use the soap test scheme for identification questions.
Mention scale and soap wastage.

Practice questions on Hard Water, Soft Water and Softening

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 · Very short answerBoard level
Which kind of hardness is removed by boiling?
Hint
Hydrogencarbonates.
Answer
Temporary hardness.
Q2 · 2 marks · Short answerHarder board level
Why does boiling not remove permanent hardness?
Hint
Chlorides and sulphates.
Answer
The chlorides and sulphates of calcium and magnesium are not decomposed by heat, so they stay dissolved.
Q3 · 3 marks · Short answerHarder board level
Why does hard water waste soap? Explain what happens when soap is added.
Hint
Scum.
Answer
Soap reacts first with the calcium and magnesium salts to form a white scum. Only after these salts are used up does the extra soap give a lather, so more soap is needed.
Q4 · 3 marks · Short answerHarder board level
A housewife adds washing soda to a bucket of water that gave scum with soap, then lets it stand and pours off the clear water. She says the soap now lathers well. Explain what the cloudy solid in the bucket was and write the equation if the hardness was due to calcium chloride.
Hint
Which solid settles?
Answer
The cloudy solid was calcium carbonate (or magnesium carbonate), formed when the washing soda removed the calcium and magnesium salts. $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ . The sodium salts left do not cause hardness, so the water lathers.
Q5 · 4 marks · Long answerHOTS / competency
A sample of water forms scum with soap. After boiling, cooling and filtering, it gives lather with soap. What kind of hardness did the sample have, and what is the solid in the filter paper? Write the equation if the dissolved salt was magnesium hydrogencarbonate.
Hint
Which hardness boils away?
Answer
Temporary hardness. The solid is a carbonate (magnesium carbonate). $M g (H C O_{3})_{2} \to M g C O_{3} + H_{2} O + C O_{2}$ .

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 hardness?Show answer
Calcium and magnesium salts.
2.Temporary hardness due to?Show answer
Hydrogencarbonates.
3.Permanent hardness due to?Show answer
Chlorides and sulphates.
4.Removes permanent hardness?Show answer
Washing soda.
5.Hard water with soap forms?Show answer
Scum.

Hard Water, Soft Water and Softening: frequently asked questions

What is hard water?

Hard water does not lather easily with soap and forms a scum; it contains dissolved calcium and magnesium salts.

What is the difference between temporary and permanent hardness?

Temporary hardness is due to hydrogencarbonates and is removed by boiling; permanent hardness is due to chlorides and sulphates and needs washing soda.

How does washing soda soften water?

It precipitates calcium and magnesium as insoluble carbonates, leaving sodium salts that do not cause hardness.