The chalk on a blackboard, the plaster on a wall, a tablet for acidity and the white drink before a stomach X-ray all come from Group 2. Section 17.8 of the book shows how two properties, basic nature and solubility, explain almost every use.
Chapter 17 of the PECTAA Chemistry 12 book is Group 2 Elements. After extraction and purification (section 17.7), section 17.8 covers the uses of Group 2 compounds in industry and everyday life, medicine and agriculture. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
The two key properties. The book links the uses of Group 2 compounds mainly to their basic nature and their varying solubility.
Construction (section 17.8.1 i).
- Calcium carbonate (CaCO₃) occurs as limestone, marble and chalk. It is used as a building material, in cement and concrete, and as a filler in paper and plastics.
- Calcium oxide (CaO, quicklime) and calcium hydroxide (Ca(OH)₂, slaked lime) are used in cement, mortar and plaster. Quicklime also removes impurities in steel making.
- Gypsum (CaSO₄·2H₂O) is used for plasterboard (drywall) and to make plaster of Paris (CaSO₄·½H₂O).
Environment (section 17.8.1 ii).
- Flue gas desulphurization: CaO and CaCO₃ are used in wet scrubbing to remove acidic SO₂ from power plant emissions, helping to prevent acid rain. Here the basic compound neutralises an acidic gas.
- Magnesium oxide (MgO) lines furnaces as a refractory material because of its high melting point.
Elements in use (the book's side box). Magnesium metal goes into lightweight alloys and into fireworks and flares for its bright burning. Strontium compounds give red colour in fireworks and flares. Beryllium strengthens alloys (springs, aerospace parts) and is used in X-ray tubes because X-rays pass through it.
Medicine (section 17.8.2).
- Magnesium hydroxide (Mg(OH)₂, milk of magnesia): neutralises excess stomach acid and acts as a mild laxative; its low solubility makes it safe.
- Calcium carbonate: an antacid in indigestion tablets.
- Calcium compounds (carbonate, citrate): for strong bones and teeth; dietary supplements to prevent osteoporosis.
- Magnesium compounds (sulphate, citrate): for muscle and nerve function; treat magnesium deficiency.
- Epsom salts (MgSO₄·7H₂O): a laxative and used in baths.
- Barium sulphate (BaSO₄): the "barium meal" for X-ray imaging of the digestive system. It is opaque to X-rays and, being highly insoluble, is not absorbed.
Agriculture (section 17.8.3).
- CaCO₃ and Ca(OH)₂ are added to acidic soils to raise the pH.
- Calcium and magnesium are essential plant nutrients: calcium for cell wall development, magnesium as the central atom of chlorophyll.
- Calcium nitrate and magnesium sulphate (Epsom salts) are used as fertilizers.
Linking uses to the trends in this chapter. Most uses follow from trends met earlier in Chapter 17:
| Trend from the chapter | Use it explains |
|---|---|
| Oxides and hydroxides are basic | antacids, soil treatment, SO₂ scrubbing |
| Mg(OH)₂ is only slightly soluble | a mild, safe antacid |
| Sulphate solubility falls down the group, BaSO₄ least | barium meal |
| Carbonates decompose on strong heating | quicklime for cement and mortar |
| Bright flames of Group 2 metals and salts | Mg in flares, Sr compounds for red colour |
Seeing the table this way turns a long list of uses into a few ideas you already know, which makes "explain why" questions much easier to answer.
Key points
- Basic nature → antacids, soil treatment, scrubbing SO₂.
- Low solubility → safe Mg(OH)₂ antacid and BaSO₄ barium meal.
- High melting point → MgO furnace linings.
- Mg sits at the centre of chlorophyll; Ca builds plant cell walls.
Worked examples
Example 1 (antacid equation). Write the neutralisation of stomach acid (HCl) by magnesium hydroxide.
- Base + acid → salt + water.
- Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
- Check: Mg 1, Cl 2, O 2, H 4 on each side.
Example 2 (book Quick Check 17.4 c). State one agricultural use of CaCO₃ and the chemistry behind it.
- Use: added to acidic soil.
- Principle: neutralisation. CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂, which removes acid and raises the pH.
Example 3 (scrubbing). Show how calcium carbonate removes sulphur dioxide.
- CaCO₃ + SO₂ → CaSO₃ + CO₂.
- The acidic gas is captured as a solid calcium salt instead of reaching the air.
Common mistakes in the exam
- Calling BaSO₄ safe because barium is harmless. It is safe because it is insoluble and not absorbed.
- Mixing quicklime and slaked lime. Quicklime is CaO; slaked lime is Ca(OH)₂.
- Writing plaster of Paris as CaSO₄·2H₂O. That is gypsum; plaster of Paris is CaSO₄·½H₂O.
- Giving magnesium oxide as the antacid. The book's antacids are Mg(OH)₂ and CaCO₃.
- Leaving out the reason when a question asks "why".
Exam technique
- Pair every use with its property: basic, insoluble or high melting point.
- Add a balanced equation where neutralisation is involved.
- Learn the formulas of gypsum, plaster of Paris and Epsom salts exactly.
- Check your board's notice for the sections in your paper.
Practice questions
- Give the formula of Epsom salts. Answer: MgSO₄·7H₂O.
- Why is MgO used to line furnaces? Answer: It has a high melting point.
- Which Group 2 compound is used in a barium meal? Answer: Barium sulphate, BaSO₄.
- Name the element at the centre of chlorophyll. Answer: Magnesium.
- Which strontium use does the book mention? Answer: Red colour in fireworks and flares.
Ready to practise this? Continue in 2nd Year Chemistry.