The following table gives the solubility of different salts at different temperatures.
| Temperature (in K) | Substance dissolved (in g) | |||
|---|---|---|---|---|
| KNO3 | NaCl | KCl | NH4Cl | |
| 283 | 21 | 36 | 35 | 24 |
| 293 | 32 | 36 | 35 | 37 |
| 313 | 62 | 36 | 40 | 41 |
| 333 | 106 | 37 | 46 | 55 |
| 353 | 167 | 37 | 54 | 66 |
Answer the following questions based on the table given above.
(a) What mass of KNO3 would be needed to produce a saturated solution of KNO3 in 50 grams of water at 313 K.
(b) If a saturated solution of KCl is made at 353 K and then cooled at room temperature, what would you observe? Explain.
(c) Find the solubility of each salt at 293 K.
(d) Which salt has the lowest solubility at 283 K?
(e) What is the effect of change of temperature on the solubility of a salt.
(a) From the table, the solubility of KNO3 at 313 K is 62 g per 100 g of water.
62 g of KNO3 per 100 g of water
Mass of solute per 50 g of water is
= x 50
= 31 g
∴ Mass of solute per 50 g of water is 31 g.
(b) The solubility of KCl at 353 K is 54 g per 100 g of water.
The solubility of KCl at 293 K (room temperature) is 35 G per 100 g of water.
When the solution cools, the solubility decreases.
The excess KCl will precipitate out of the solution as crystals.
(c) Considering given salts are dissolved in 100g of water
Solubility of KNO3 at 293 K
Mass of solute = 32 g
Mass of solvent = 100 g
Solubility = x 100
= x 100
= 32 g
∴ Solubility of KNO3 at 293 K is 32 g.
Solubility of NaCl at 293 K
Mass of solute = 36 g
Mass of solvent = 100 g
Solubility = x 100
= x 100
= 36 g
∴ Solubility of NaCl at 293 K is 36 g.
Solubility of KCl at 293 K
Mass of solute = 35 g
Mass of solvent = 100 g
Solubility = x 100
= x 100
= 35 g
∴ Solubility of KCl at 293 K is 35 g.
Solubility of NH4Cl at 293 K
Mass of solute = 37 g
Mass of solvent = 100 g
Solubility = x 100
= x 100
= 37 g
∴ Solubility of NH4Cl at 293 K is 37 g.
(d) From the table, when we calculate the solubility of KNO3 at 283 K,
Mass of solute = 21 g
Mass of solvent = 100 g
Solubility = x 100
= x 100
= 21 g
Whereas, the solubility of NaCl, KCl and NH4Cl will be 36 g, 35 g and 24 g respectively.
Hence, the least solubility will be of KNO3 at 283 K
(e) The solubility of most salts in water usually increases with rise in temperature. Example, Potassium nitrate.
There are some salts which show anomalous solubility. Their solubility first increases, and then decreases, with rise in temperature.
Example: Na2SO4.10H2O (Glauber's salt).
Solubility curve of Na2SO4.10H2O rises till it reaches 32.8°C, and then it falls slightly. This is because Na2SO4.10H2O is hydrous below 32.8°C and looses it water and become anhydrous above 32.8°C.
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.