Chapter 13 of 23

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Tamil Nadu (TNBSE)Class 8 Science★ 4-7 marks in the TN Class 8 annual exam through book-back MCQs, fill-ups and short answers

Water

Samacheer Kalvi (TN State Board) Class 8 Science — Chemistry Chapter 13, Water. Water (H₂O) and its three states, water as the universal solvent, solubility and how it changes with temperature and pressure, electrolysis of water, hard and soft water (temporary and permanent hardness) and how to remove hardness, and water pollution. Notes with the actual book-back questions, common mistakes and quick revision.

⏱ 16 min readEasy difficulty✓ Free · NCERT-aligned

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By the end of this chapter you'll be able to…

1State the formula and three states of water
2Explain solubility and its change with pressure
3Describe the electrolysis of water (hydrogen at cathode)
4Differentiate temporary and permanent hardness and remove them
5Identify water pollutants

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Why this chapter matters

Water explains the universal solvent, its solubility behaviour, electrolysis and hardness — including how to soften water. Permanent hardness (sulphates/chlorides) and ion-exchange are directly tested book-back content in the TN Class 8 exam.

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Class 7 — Water

Water cycle and uses.

Water — Class 8 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 8 Science, Chemistry — Chapter 13. The universal solvent — its states, solubility and hardness.

1. About this chapter

This chapter covers water (H₂O) and its states, solubility, electrolysis, hard and soft water, and water pollution.

2. Water, its states and solubility

Water has the formula H₂O and exists in all three states — ice (solid, at 0 °C), water (liquid) and steam (gas).
Water is the universal solvent — it dissolves solids (salt, sugar), liquids (milk) and gases (oxygen, CO₂).
Solubility of gases increases at high pressure (and low temperature) — the solubility of carbon dioxide is high when the pressure is high. The boiling point of water increases with pressure.

3. Electrolysis of water

Passing electric current through water (electrolysis) splits it into hydrogen at the cathode and oxygen at the anode, in the ratio 2 : 1 by volume.

4. Hard and soft water

Soft water lathers easily with soap; hard water does not — it forms scum with soap, wasting it.
Hardness is due to dissolved calcium and magnesium salts.
- Temporary hardness (bicarbonates) is removed by boiling.
- Permanent hardness is due to sulphates and chlorides of calcium and magnesium; it is removed by passing the water through ion-exchange resins (Ca²⁺/Mg²⁺ replaced by Na⁺).
Water pollution: substances like lead make water unsafe.

5. Worked examples

Example 1. At what temperature does water change to ice? 0 °C.

Example 2. Which gas is collected at the cathode on electrolysis of water? Hydrogen.

Example 3. What causes permanent hardness of water? Dissolved sulphates and chlorides of calcium and magnesium.

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

Water changes to ice at — (a) 0 °C / (b) 100 °C. Ans: (a) 0 °C.
The solubility of carbon dioxide in water is high when the pressure is — (a) low / (b) high. Ans: (b) high.
The gas collected at the cathode on electrolysis of water is — (a) oxygen / (b) hydrogen. Ans: (b) hydrogen.
A water pollutant among the following is — (a) oxygen / (b) lead. Ans: (b) lead.
Permanent hardness of water is due to the presence of — (a) bicarbonates / (b) sulphates and chlorides. Ans: (b) sulphates and chlorides.

II. Fill in the blanks 6. The chemical formula of water is H₂O. 7. The boiling point of water increases with an increase in pressure. 8. Hardness of water is due to dissolved salts of calcium and magnesium.

III. Answer briefly 9. Why is water called the universal solvent? 10. How is permanent hardness removed?

7. Common mistakes

Mistake: Saying the electrolysis ratio is 1 : 2. Fix: Hydrogen : oxygen = 2 : 1 (hydrogen at the cathode).
Mistake: Thinking boiling removes permanent hardness. Fix: Boiling removes only temporary hardness; permanent hardness needs ion-exchange.
Mistake: Confusing temporary and permanent hardness. Fix: Temporary = bicarbonates (boiling); permanent = sulphates/chlorides (ion-exchange).

8. Quick revision

Chemistry Ch 13 · water, solubility, electrolysis, hardness.
Water (H₂O) in three states; ice at 0 °C; universal solvent.
Gas solubility ↑ at high pressure; boiling point ↑ with pressure.
Electrolysis → hydrogen (cathode) : oxygen (anode) = 2 : 1.
Hardness from Ca/Mg salts; temporary (boiling), permanent = sulphates/chlorides (ion-exchange).

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Water

H₂O; ice at 0 °C; universal solvent

Exists in three states.

Solubility / boiling point

gas solubility ↑ at high pressure; b.p. ↑ with pressure

CO₂ dissolves more at high pressure.

Electrolysis

hydrogen (cathode) : oxygen (anode) = 2 : 1

By volume.

Hardness

temporary (bicarbonates) vs permanent (sulphates/chlorides)

Boiling vs ion-exchange.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying the electrolysis ratio is 1 : 2

✓ Hydrogen : oxygen = 2 : 1 (hydrogen at the cathode).

WATCH OUT

✗ Thinking boiling removes permanent hardness

✓ Boiling removes only temporary hardness; permanent hardness needs ion-exchange.

WATCH OUT

✗ Confusing temporary and permanent hardness

✓ Temporary = bicarbonates (boiling); permanent = sulphates/chlorides (ion-exchange).

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on states, solubility, electrolysis, pollutants and hardness.

Questions

Fill in the blanks

Formula of water, boiling point and hardness salts.

Questions

Answer briefly

Universal solvent, removing hardness, water pollution.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

Water changes to ice at ____.

▶ Show solution

0 °C.

Q2EASY· MCQ

The gas collected at the cathode on electrolysis of water is ____.

▶ Show solution

hydrogen.

Q3EASY· MCQ

Permanent hardness of water is due to the presence of ____.

▶ Show solution

sulphates and chlorides (of calcium and magnesium).

Q4EASY· Fill in the blanks

The boiling point of water ____ with an increase in pressure.

▶ Show solution

increases.

Q5EASY· Answer briefly

Why is water called the universal solvent?

▶ Show solution

Because it dissolves more substances than any other liquid — solids, liquids and gases alike.

Q6MEDIUM· Answer briefly

How is permanent hardness of water removed?

▶ Show solution

By passing the water through ion-exchange resins, where calcium and magnesium ions are replaced by sodium ions.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Chemistry Chapter 13 of Samacheer Kalvi Class 8 Science.
•Water (H₂O) in three states; ice at 0 °C; universal solvent.
•Gas solubility ↑ at high pressure; boiling point ↑ with pressure.
•Electrolysis → hydrogen (cathode) : oxygen (anode) = 2 : 1.
•Hardness from Ca/Mg salts; temporary = bicarbonates (boiling).
•Permanent hardness = sulphates/chlorides, removed by ion-exchange.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 4-7 marks across book-back MCQ, fill-ups and short answers

Question type	Marks each	Typical count	What it tests
MCQ / Fill	1	3-5	States, solubility, electrolysis, hardness
Short Answer	2-3	1-2	Universal solvent, removing hardness
Application	2	1	Water pollution

Prep strategy

Learn the formula and states of water
Note solubility and boiling point change with pressure
Remember hydrogen is at the cathode (2 : 1)
Separate temporary and permanent hardness

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Safe water

Removing hardness and pollutants gives clean, usable water.

Industry

Soft water is needed for boilers and washing.

Beverages

Carbon dioxide is dissolved under high pressure in fizzy drinks.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Quote 0 °C and the 2 : 1 electrolysis ratio
State hydrogen is at the cathode
Separate temporary and permanent hardness
Name ion-exchange for permanent hardness

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Explain why fizzy drinks lose gas when opened (pressure and solubility).
Describe a method to test whether water is hard or soft.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 8 Annual ExamHigh

Foundation / NMMS ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

The dissolved calcium and magnesium salts react with soap to form an insoluble scum, so the soap is wasted and does not lather.

They replace the calcium and magnesium ions that cause hardness with sodium ions, converting hard water into soft water.

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