Class 10 Chemistry Chapter 6 Water illustration with water cycle, molecules, and treatment processes.

Chapter 6: Water (Environmental Chemistry II)

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Water is an essential compound for life and a vital natural resource for ecosystems, industry, and human survival. This chapter explores the unique properties of water, its role as a universal solvent, sources and types of impurities, and the processes involved in its natural cycle. It also addresses water pollution, waterborne diseases, hardness of water, and treatment methods such as filtration, chlorination, boiling, distillation, and softening. Students will also learn about the causes and prevention of temporary and permanent hardness, as well as the importance of water conservation.

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.

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