How to Use This Page
Welcome to the exercise solutions for Chapter 5 — Alkali and Alkaline Earth Metals (s-Block Elements). Here is how to get the most out of this page:
1. Try each question yourself first before checking the solution. 2. Compare your answer with the detailed solution and reasoning provided. 3. Memorise reaction equations — they are important for board exams. 4. Use the comparison tables for quick revision of Group 1 vs Group 2 trends. 5. Check the Exam Checklist at the end to verify complete preparation.
Key Concepts (Quick Recap)
Group 1 (Alkali Metals) vs Group 2 (Alkaline Earth Metals) Comparison
| Property | Group 1 (Alkali Metals) | Group 2 (Alkaline Earth Metals) |
|---|---|---|
| Elements | Li, Na, K, Rb, Cs, Fr | Be, Mg, Ca, Sr, Ba, Ra |
| Valence config | ns¹ | ns² |
| Oxidation state | +1 | +2 |
| Reactivity | More reactive | Less reactive than Group 1 |
| Ionisation energy | Lower | Higher (smaller atoms, +2 charge) |
| Flame colour | Yes (each has distinct colour) | Yes (each has distinct colour) |
| Reaction with water | Vigorous, produces H₂ | Less vigorous (Be does not react) |
| Oxides | M₂O (basic) | MO (basic, BeO is amphoteric) |
| Hydroxides | Strong bases (NaOH, KOH) | Less basic than Group 1 |
| Carbonates | Soluble (except Li₂CO₃) | Insoluble |
| Bicarbonates | Soluble | Soluble |
| Hydrides | Ionic (NaH) | Ionic (CaH₂) |
Flame Colours of s-Block Elements
| Element | Flame Colour |
|---|---|
| Lithium (Li) | Crimson red |
| Sodium (Na) | Bright yellow |
| Potassium (K) | Violet / Lilac |
| Rubidium (Rb) | Reddish violet |
| Caesium (Cs) | Blue |
| Calcium (Ca) | Brick red |
| Strontium (Sr) | Crimson red |
| Barium (Ba) | Apple green |
Diagonal Relationship
Elements of the second period show similarities with elements diagonally below them in the third period:
- Li resembles Mg: Both form nitrides (Li₃N, Mg₃N₂), both form covalent organometallic compounds, both carbonates decompose on heating.
- Be resembles Al: Both are amphoteric, both form covalent halides, both have high charge density.
Important Compounds of s-Block Elements
| Compound | Formula | Common Name | Use |
|---|---|---|---|
| Sodium hydroxide | NaOH | Caustic soda | Soap, paper, drain cleaner |
| Sodium carbonate | Na₂CO₃·10H₂O | Washing soda | Detergent, water softening |
| Sodium bicarbonate | NaHCO₃ | Baking soda | Baking, antacid |
| Sodium chloride | NaCl | Common salt | Food preservative |
| Calcium oxide | CaO | Quicklime | Cement, flux |
| Calcium hydroxide | Ca(OH)₂ | Slaked lime | White wash, water treatment |
| Calcium carbonate | CaCO₃ | Limestone | Cement, marble |
| Calcium sulphate | CaSO₄·2H₂O | Gypsum | Plaster of Paris |
| Magnesium sulphate | MgSO₄·7H₂O | Epsom salt | Laxative, medicine |
I. Choose the Best Answer (MCQs)
II. Fill in the Blanks
1. The general electronic configuration of alkali metals is ns¹.
2. The general electronic configuration of alkaline earth metals is ns².
3. Among alkali metals, Francium (Fr) is the most electropositive element.
4. Lithium shows a diagonal relationship with magnesium (Mg).
5. Sodium hydroxide (NaOH) is commercially known as caustic soda.
6. The chemical formula of washing soda is Na₂CO₃·10H₂O.
7. Quicklime is chemically CaO (calcium oxide).
8. When quicklime reacts with water, it forms slaked lime [Ca(OH)₂].
9. The flame colour of calcium is brick red.
10. BeO is amphoteric in nature (reacts with both acids and bases).
11. The bicarbonate of sodium used in baking is NaHCO₃ (sodium bicarbonate).
12. Plaster of Paris is obtained by heating gypsum (CaSO₄·2H₂O).
13. The solubility of alkali metal hydroxides increases down the group.
14. Among alkaline earth metals, Beryllium (Be) does not react with water even at red heat.
15. The oxide that is used in the manufacture of cement is CaO (quicklime).
III. True or False (with Corrections)
1. Statement: All alkali metals are soft and can be cut with a knife.
2. Statement: Beryllium oxide is a basic oxide.
3. Statement: Solubility of carbonates of alkaline earth metals increases down the group.
4. Statement: Alkali metals form ionic hydrides with hydrogen.
5. Statement: Sodium carbonate can be manufactured by the Solvay process.
IV. Match the Following
| Column A | Column B |
|---|---|
| 1. Caustic soda | (a) Na₂CO₃·10H₂O |
| 2. Washing soda | (b) CaO |
| 3. Quicklime | (c) NaOH |
| 4. Baking soda | (d) CaSO₄·½H₂O |
| 5. Slaked lime | (e) NaHCO₃ |
| 6. Plaster of Paris | (f) Ca(OH)₂ |
1 — (c): Caustic soda is NaOH (sodium hydroxide) 2 — (a): Washing soda is Na₂CO₃·10H₂O (sodium carbonate decahydrate) 3 — (b): Quicklime is CaO (calcium oxide) 4 — (e): Baking soda is NaHCO₃ (sodium bicarbonate) 5 — (f): Slaked lime is Ca(OH)₂ (calcium hydroxide) 6 — (d): Plaster of Paris is CaSO₄·½H₂O (calcium sulphate hemihydrate)
V. Answer Briefly (2-mark)
VI. Answer in Detail (5-mark)
Physical Properties
1. Atomic and ionic radii: Alkali metals have the largest atomic radii in their respective periods. Atomic and ionic radii increase down the group as new electron shells are added.
2. Ionisation energy: Alkali metals have the lowest ionisation energy in their periods (only one electron in ns¹). Ionisation energy decreases down the group because atomic radius increases and the valence electron is farther from the nucleus.
3. Melting and boiling points: Low and decrease down the group. This is because metallic bonding weakens as atomic size increases and only one valence electron contributes to the electron sea.
4. Density: Generally low. Lithium, sodium, and potassium are lighter than water (Li, Na, K float on water). Density increases down the group.
5. Softness: All alkali metals are soft and can be cut with a knife. Softness increases down the group.
Chemical Properties
Sodium Hydroxide (NaOH) — Caustic Soda
Preparation — Chlor-Alkali Process (Electrolysis of brine):
Brine (concentrated NaCl solution) is electrolysed using a mercury cathode cell, diaphragm cell, or membrane cell. Sodium hydroxide is obtained in the solution around the cathode.
Properties:
1. White, crystalline solid, highly soluble in water (deliquescent). 2. Strong base — completely dissociates in water:
3. Reacts with acids (neutralisation):
4. Reacts with metals (amphoteric metals like Al and Zn):
5. Reacts with CO₂ to form sodium carbonate:
6. Saponification — reacts with fats/oils to form soap:
Uses:
- Manufacture of soap and detergents
- Paper and pulp industry
- Textile industry (mercerisation of cotton)
- Drain cleaner
- Petroleum refining
- Manufacture of rayon
Sodium Carbonate (Na₂CO₃) — Washing Soda (Na₂CO₃·10H₂O)
Preparation — Solvay Process:
The key reactions:
- Ammonia is dissolved in brine.
- CO₂ is passed through, forming NaHCO₃ (less soluble, precipitates out).
- NaHCO₃ is heated to give Na₂CO₃.
- CaCO₃ (limestone) is heated to provide CO₂.
- NH₃ is recovered from NH₄Cl by treating with Ca(OH)₂.
Why the Solvay process cannot be used for K₂CO₃:
- KHCO₃ is more soluble than NaHCO₃ and does not precipitate easily.
- The process relies on the low solubility of NaHCO₃ in brine.
Properties:
1. White crystalline solid, soluble in water. 2. Alkaline solution in water (hydrolysis of CO₃²⁻):
3. Reacts with acids:
4. Reacts with calcium salts (water softening):
5. On heating, washing soda loses water of crystallisation:
Uses:
- Water softening (removes permanent hardness)
- Manufacture of glass (soda glass: SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂)
- Detergent and soap industry
- Paper industry
- Textile industry (dyeing)
- Household cleaning (washing soda)
Calcium Oxide (CaO) — Quicklime
Preparation: Quicklime is prepared by heating limestone (calcium carbonate) at about 900-1000 °C:
This is a decomposition reaction. The CO₂ is removed to prevent the reverse reaction.
Properties:
1. White, amorphous solid with a high melting point (2600 °C). 2. Reacts vigorously with water (slaking), producing heat:
3. Reacts with acids:
4. Reacts with CO₂ (re-forms limestone):
5. Reacts with silica at high temperature (flux in metallurgy):
Uses:
- Manufacture of cement
- As a flux in metallurgy (removes impurities as slag)
- Drying agent (desiccant)
- Soil treatment (reduces acidity)
- White wash
- Steel making (removes impurities)
Calcium Hydroxide [Ca(OH)₂] — Slaked Lime
Preparation: Slaked lime is prepared by adding water to quicklime (slaking):
The reaction is highly exothermic — the mixture boils and steam is produced.
Properties:
1. White powder, sparingly soluble in water. The solution is called lime water. 2. Lime water turns milky when CO₂ is passed through (test for CO₂):
3. On excess CO₂, the milky solution becomes clear (bicarbonate forms):
4. Reacts with acids (neutralisation):
Uses:
- White wash on walls
- Softening of hard water (Clark's method)
- Sewage treatment
- Sugar refining
- Mortar (mixture of slaked lime and sand)
Plaster of Paris (CaSO₄·½H₂O)
Preparation: Plaster of Paris is prepared by heating gypsum (calcium sulphate dihydrate) at about 150 °C:
If heated above 200 °C, anhydrous CaSO₄ (dead burnt plaster) is formed, which does not set with water.
Properties:
1. White powder that sets into a hard mass when mixed with water (setting reaction):
2. The setting is exothermic and involves volume expansion, allowing it to take the shape of the mould.
3. The setting time can be controlled:
- Retarders (e.g., borax, glue) slow setting.
- Accelerators (e.g., NaCl, alum) speed up setting.
Uses:
- Making casts and moulds (fracture casts in medicine)
- Sculpture and ornamental work
- Building material (plastering walls)
- Dentistry (dental casts)
- Fireproofing material
- Chalk (when mixed with colouring agents)
Common Mistakes to Avoid
1. Confusing quicklime, slaked lime, and limestone: CaO is quicklime, Ca(OH)₂ is slaked lime, and CaCO₃ is limestone. They are different compounds with different properties and uses. Do not mix them up.
2. Writing incorrect formulas for washing soda and baking soda: Washing soda is Na₂CO₃·10H₂O (not Na₂CO₃ alone). Baking soda is NaHCO₃ (not Na₂CO₃).
3. Confusing flame colours: Each s-block element has a distinct flame colour. Memorise them: Li = crimson red, Na = yellow, K = violet, Ca = brick red, Sr = crimson, Ba = green.
4. Forgetting that Be does not react with water: Among alkaline earth metals, Be does not react with water even at red heat. Mg reacts with hot water; Ca, Sr, Ba react with cold water.
5. Incorrect Plaster of Paris formula: Plaster of Paris is CaSO₄·½H₂O, NOT CaSO₄·H₂O. The half-water of crystallisation is important.
6. Assuming all s-block oxides are basic: BeO is amphoteric. Always remember the exception due to the diagonal relationship with Al.
7. Confusing Solvay process products: The Solvay process produces Na₂CO₃ from NaCl, NH₃, and CO₂. It cannot be used for K₂CO₃ because KHCO₃ is too soluble and does not precipitate.
8. Forgetting that alkali metals form peroxides and superoxides: Li forms oxide (Li₂O), Na forms peroxide (Na₂O₂), and K, Rb, Cs form superoxides (KO₂, RbO₂, CsO₂). Not all alkali metals form simple oxides.
Exam Checklist
- [ ] Can explain general trends in physical properties of Group 1 and Group 2 elements
- [ ] Can write balanced equations for reactions with water, oxygen, halogens, and hydrogen
- [ ] Can explain why alkali metals are stored in kerosene
- [ ] Can define and give examples of the diagonal relationship (Li-Mg, Be-Al)
- [ ] Can list flame colours of s-block elements
- [ ] Can describe the preparation, properties, and uses of NaOH and Na₂CO₃
- [ ] Can describe the preparation, properties, and uses of CaO, Ca(OH)₂, and Plaster of Paris
- [ ] Can compare Group 1 and Group 2 elements in terms of reactivity, ionisation energy, and compound solubility
- [ ] Can explain the Solvay process and why it cannot be used for K₂CO₃
- [ ] Can differentiate between quicklime, slaked lime, and limestone with correct formulas