Introduction
Water is one of the most important substances on Earth because it is essential for life, agriculture, industry, and the environment. In Class 10 Chemistry Chapter 6, students learn about the physical and chemical properties of water, its importance as a universal solvent, natural sources, impurities, hardness, purification methods, and water pollution. The chapter also explains the water cycle, different methods of water treatment, and the importance of conserving fresh water for future generations.
Understanding the properties of water helps students explain many natural processes and industrial applications. This chapter introduces important concepts such as temporary and permanent hardness, filtration, boiling, chlorination, distillation, and ion-exchange methods. These topics are commonly tested in school examinations and board exams, making this chapter an important part of the Class 10 Chemistry syllabus.
On this page, you will find carefully selected Class 10 Chemistry Chapter 6 MCQs with answers and explanations, along with learning outcomes, quick revision notes, important definitions, important chemical equations, exam tips, and common mistakes to help you prepare effectively for your examinations.
Learning Outcomes
After studying this chapter, students will be able to:
- Describe the physical and chemical properties of water.
- Explain why water is known as the universal solvent.
- Identify different types of water impurities.
- Differentiate between temporary and permanent hardness.
- Explain various methods of water purification.
- Understand the causes and prevention of water pollution.
- Describe the importance of water conservation.
Quick Notes – Chapter Summary
- Water is essential for living organisms and many industrial processes.
- Water is called the universal solvent because it dissolves many ionic and polar substances.
- Natural water contains suspended, dissolved, and biological impurities.
- Temporary hardness is caused by calcium and magnesium bicarbonates.
- Permanent hardness is caused by calcium and magnesium sulfates and chlorides.
- Boiling removes temporary hardness but not permanent hardness.
- Ion-exchange methods remove both temporary and permanent hardness.
- Chlorination destroys harmful microorganisms in drinking water.
- Distillation produces highly purified water by evaporation and condensation.
- Water pollution affects human health, aquatic life, and ecosystems.
- Water conservation helps ensure a sustainable supply of fresh water.
- The water cycle continuously recycles water through evaporation, condensation, and precipitation.
Important Definitions
Universal Solvent: A substance capable of dissolving a wide variety of ionic and polar compounds.
Hard Water: Water containing dissolved calcium and magnesium salts.
Temporary Hardness: Hardness caused by bicarbonates of calcium and magnesium.
Permanent Hardness: Hardness caused by sulfates and chlorides of calcium and magnesium.
Distillation: A purification process involving boiling and condensation.
Chlorination: The addition of chlorine to destroy harmful microorganisms in water.
Water Pollution: Contamination of water by harmful substances that affect its quality and usability.
Water Conservation: The careful and sustainable use of water resources.
Important Chemical Equations
Removal of Temporary Hardness by Boiling
Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ ↑ + H₂O
Formation of Hypochlorous Acid during Chlorination
Cl₂ + H₂O → HOCl + HCl
Water Cycle (Evaporation and Condensation)
H₂O (l) ⇌ H₂O (g)
Students should understand these reactions and their practical applications rather than memorizing them without understanding.
Class 10 Chemistry Chapter 6 – Water MCQs
1. Which property of water makes it known as the “universal solvent”?
- A. Low boiling point
- B. Ability to dissolve a wide range of substances ✅
- C. High density
- D. Odourless nature
Explanation: Water’s polarity allows it to dissolve many ionic and polar covalent substances.
2. What is the percentage of fresh water available on Earth for direct use by humans?
- A. About 10%
- B. About 5%
- C. Less than 1% ✅
- D. About 15%
Explanation: Most of Earth’s water is saline; less than 1% is fresh and accessible for human use.
3. Which type of impurity in water includes clay and silt particles?
- A. Suspended impurities ✅
- B. Dissolved salts
- C. Microorganisms
- D. Organic waste
Explanation: Suspended impurities are insoluble particles like clay and silt that can be removed by filtration or sedimentation.
4. Which gas is most responsible for the corrosion of water pipes?
- A. Oxygen
- B. Carbon dioxide ✅
- C. Nitrogen
- D. Methane
Explanation: Dissolved CO2 forms carbonic acid, which corrodes metal pipes.
5. Temporary hardness of water is caused by:
- A. Calcium sulfate
- B. Calcium bicarbonate and magnesium bicarbonate ✅
- C. Sodium chloride
- D. Iron(III) oxide
Explanation: Temporary hardness is due to dissolved bicarbonates of calcium and magnesium.
6. Which process removes temporary hardness?
- A. Adding slaked lime
- B. Boiling ✅
- C. Ion-exchange resin
- D. Filtration
Explanation: Boiling decomposes bicarbonates into insoluble carbonates, which can be filtered out.
7. Chemical equation for removal of temporary hardness by boiling is:
- A. Ca(HCO3)2 → CaCO3 ↓ + CO2 ↑ + H2O ✅
- B. CaCO3 + H2O → Ca(HCO3)2
- C. MgSO4 + H2O → Mg(OH)2
- D. CO2 + H2O → H2CO3
Explanation: Heating decomposes bicarbonates into insoluble carbonates, releasing CO2.
8. Permanent hardness of water is caused by:
- A. Bicarbonates
- B. Sulfates and chlorides of calcium and magnesium ✅
- C. Organic matter
- D. Suspended solids
Explanation: Permanent hardness is due to dissolved calcium/magnesium sulfates and chlorides, which cannot be removed by boiling.
9. Which method removes both temporary and permanent hardness?
- A. Boiling
- B. Sedimentation
- C. Ion-exchange method ✅
- D. Aeration
Explanation: Ion-exchange resins replace calcium and magnesium ions with sodium or hydrogen ions, softening water completely.
10. Which disease is most commonly spread by contaminated water?
- A. Influenza
- B. Cholera ✅
- C. Tuberculosis
- D. Malaria
Explanation: Cholera is a waterborne bacterial disease caused by Vibrio cholerae.
11. Chlorination of water mainly kills:
- A. Suspended solids
- B. Dissolved salts
- C. Pathogenic microorganisms ✅
- D. Gases
Explanation: Chlorine is a disinfectant that destroys harmful bacteria and viruses in water.
12. Which reaction represents disinfection of water with chlorine?
- A. Cl2 + H2O → HOCl + HCl ✅
- B. Cl2 + NaOH → NaCl + H2O
- C. Cl2 → 2Cl
- D. Cl + O2 → ClO2
Explanation: Chlorine in water forms hypochlorous acid (HOCl), an effective germicide.
13. Which process is best for producing distilled water?
- A. Filtration
- B. Sedimentation
- C. Boiling followed by condensation ✅
- D. Aeration
Explanation: Distillation involves boiling water and condensing the steam to remove all dissolved solids and impurities.
14. Which compound is used in the lime-soda process for water softening?
- A. Sodium chloride
- B. Calcium hydroxide and sodium carbonate ✅
- C. Magnesium sulfate
- D. Sodium bicarbonate
Explanation: Lime (Ca(OH)2) and soda ash (Na2CO3) precipitate calcium and magnesium salts.
15. Water conservation is important because:
- A. Water resources are unlimited
- B. Fresh water is limited and essential for life ✅
- C. Sea water can be easily used for all purposes
- D. It prevents water from evaporating
Explanation: Only a small fraction of Earth’s water is fresh and directly usable; conservation ensures sustainable supply.
Exam Tips
– Learn the difference between temporary and permanent hardness.
– Memorize the important water purification methods and their uses.
– Revise the important chemical equations given in this chapter.
– Understand the causes and effects of water pollution.
– Practice MCQs regularly to improve conceptual understanding.
Common Mistakes
❌ Confusing temporary hardness with permanent hardness.
❌ Assuming boiling removes all types of hardness.
❌ Forgetting that chlorination removes microorganisms, not dissolved salts.
❌ Mixing up filtration and distillation.
❌ Believing that all fresh water on Earth is available for human use.
Conclusion
Water is one of the most valuable natural resources and plays an essential role in sustaining life and supporting industrial development. Understanding the properties of water, methods of purification, hardness, pollution, and conservation helps students appreciate its importance and prepares them for school and board examinations. Regular practice of these MCQs and revision of the important definitions and chemical equations will strengthen conceptual understanding and improve examination performance.