Question 29
A sample of water is given in a trough. State how would you prove experimentally whether the given sample is hard water or soft water.
A piece of ordinary soap is rubbed inside the sample of water. If the sample of water lathers, it is soft else it is hard.
Key Concepts Covered
This question tests your understanding of the following concepts from the chapter Water: Question, Sample, Water, Trough, Experimentally, Whether. These are fundamental topics in Chemistry that students are expected to master as part of the ICSE Class 9 curriculum.
A thorough understanding of these concepts will help you answer similar questions confidently in your ICSE examinations. These topics are frequently tested in both objective and subjective sections of Chemistry papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This Question
Read the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to Remember
- Balance chemical equations before solving numerical problems.
- Learn the periodic table trends and exceptions.
- Understand reaction mechanisms, not just outcomes.
- Use correct IUPAC nomenclature in your answers.
Practice more questions from Water — Chemistry, Class 9 ICSE
Chapter Overview: Water
Water (H2O) is essential for life and is called the "universal solvent" due to its ability to dissolve a wide range of substances. This chapter covers the composition of water (hydrogen and oxygen in 2:1 ratio by volume), its physical and chemical properties, and water treatment processes. The synthesis of water by burning hydrogen in oxygen and the analysis of water by electrolysis demonstrate its composition. Water exists in three states - ice (solid), water (liquid), and steam (gas). It has unique properties like anomalous expansion (maximum density at 4°C), high specific heat capacity, and high latent heat. Chemical properties include its reactions with metals (Na, Ca produce hydroxides and H2), non-metals (C reacts with steam), and oxides (metal oxides form bases, non-metal oxides form acids). Hard water contains dissolved calcium and magnesium salts and does not lather easily with soap. Hardness can be temporary (removed by boiling) or permanent (removed by chemical treatment). Water purification methods include filtration, chlorination, distillation, and reverse osmosis.
Key Concepts & Reactions
| Concept | Details |
|---|---|
| Synthesis of Water | 2H2 + O2 → 2H2O (H2 and O2 combine in 2:1 volume ratio) |
| Analysis of Water | Electrolysis: 2H2O → 2H2 + O2 (confirms composition) |
| With Na | 2Na + 2H2O → 2NaOH + H2↑ (vigorous, exothermic) |
| Anomalous Expansion | Water expands when cooled below 4°C; ice is less dense than water |
| Temporary Hardness | Due to Ca(HCO3)2 or Mg(HCO3)2; removed by boiling |
| Permanent Hardness | Due to CaSO4, CaCl2, MgSO4, MgCl2; removed by washing soda |
| Soft Water | Water that lathers easily with soap; free from Ca2+ and Mg2+ salts |
Must-Know Concepts
- Water has maximum density at 4°C - this is why ice floats and aquatic life survives in winter
- Metal oxide + Water → Base (e.g., Na2O + H2O → 2NaOH)
- Non-metal oxide + Water → Acid (e.g., CO2 + H2O → H2CO3)
- Boiling removes temporary hardness: Ca(HCO3)2 → CaCO3↓ + H2O + CO2↑
- Washing soda removes permanent hardness: CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4
- Water of crystallisation: CuSO4·5H2O (blue) loses water to form CuSO4 (white) on heating
Temporary vs Permanent Hardness
| Feature | Temporary Hardness | Permanent Hardness |
|---|---|---|
| Cause | Bicarbonates of Ca, Mg | Sulphates, chlorides of Ca, Mg |
| Removal | By boiling or adding Ca(OH)2 | By adding washing soda (Na2CO3) |
| Principle | Bicarbonates decompose on boiling | Insoluble carbonates precipitate out |
Important Diagrams to Practice
- Electrolysis of water (Hofmann voltameter) with gas collection
- Graph showing anomalous expansion of water (volume vs temperature)
- Water purification flowchart (sedimentation → filtration → chlorination)
Common Mistakes
- Saying water has maximum density at 0°C (it is at 4°C)
- Confusing temporary and permanent hardness removal methods
- Writing that boiling can remove all types of hardness (it only removes temporary)
- Forgetting that water is a covalent compound but can ionise slightly (autoprotolysis)
Scoring Tips
- For hardness questions, always specify the type (temporary/permanent) and the chemical method with equation
- Explain anomalous expansion with the biological significance (aquatic life survival)
- Know the ratio: electrolysis produces H2 and O2 in 2:1 volume ratio (cathode:anode)
- Remember water's role as both acid and base (amphoteric nature)
Frequently Asked Questions
Why does ice float on water?
Water expands when cooled below 4°C due to hydrogen bonding forming an open crystalline structure. Ice at 0°C is less dense (0.92 g/cm3) than water at 4°C (1.00 g/cm3), so ice floats.
Why is water called the universal solvent?
Water can dissolve a very large number of substances (both ionic and many covalent compounds) due to its polar nature and ability to form hydrogen bonds. However, it cannot dissolve non-polar substances like oils and fats.
How does chlorination purify water?
Chlorine (Cl2) or bleaching powder is added to water to kill bacteria and other pathogens. Chlorine reacts with water to form hypochlorous acid (HOCl), which is a strong disinfectant. This is the final step in municipal water treatment.