Explain why:
(a) Boiled or distilled water tastes flat.
(b) Ice at zero degree centigrade has greater cooling effect than water at 0°C.
(c) Burns caused by steam are more severe than burns caused by boiling water.
(d) Rivers and lakes do not freeze easily?
(e) Air dissolved in water contains a higher proportion of oxygen.
(f) If distilled water is kept in a sealed bottle for a long time, it leaves etchings on the surface of the glass.
(g) Rain water does not leave behind concentric rings when boiled.
(a) Pure water is tasteless. The taste in water is due to the gases and solids dissolved in it i.e., impurities present in it. As boiled and distilled water are pure containing no impurities hence they taste flat i.e., are tasteless.
(b) The latent heat of fusion of ice is 336 J g-1 so it absorbs 336 J of heat and changes to water at 0°C and then water at 0°C absorbs heat and temperature is raised. Therefore, ice at 0°C absorbs extra heat in comparison to water at 0°C (due to latent heat of fusion). Hence, ice at 0°C has greater cooling effect than water at 0°C.
(c) Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g-1. Hence, steam at 100°C carries more heat than water. Therefore, burns caused by steam are more severe than burns caused by boiling water.
(d) Rivers and lakes do not freeze easily because the specific latent heat of fusion of ice is sufficiently high (= 336 J g-1). The water in lakes and rivers will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water surface, being a poor conductor of heat, will also prevent the loss of heat from the water of lake, hence the water does not freeze all at once.
(e) Air dissolved in water contains a higher proportion of oxygen because oxygen is more soluble in water compared to nitrogen. The composition of air dissolved in water is 33% oxygen compared to 21% in ordinary air.
(f) Substances that are apparently insoluble in water actually dissolve in it in traces. Even, when we put water in a glass vessel, an extremely small amount of glass dissolves in it. It is for this reason that when distilled water is kept in a sealed bottle for a long time, it leaves etchings on the inside surface of the glass.
(g) As rainwater does not contain dissolved solids hence, it does not leave behind concentric rings when boiled.
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.