Preparation and Classification of Oxides
Senior 2 · Chemistry
Preparation and Classification of Oxides
Lesson 4 of 4
25 min
Walk through any Rwandan town and you are surrounded by oxides at work: cement and glass in buildings, white paint on walls, lime spread on fields, iron made from iron ore. One of the oldest chemical processes known, turning limestone into lime, is still carried out on a large scale today. The limestone deposits of Rusizi District supply Rwanda's cement industry, and lime is spread on many acidic hillside soils. This lesson looks at how slaked lime is produced and how useful oxides are.
Limestone is mainly calcium carbonate, CaCO3. In a lime kiln it is heated to about 900–1 000 °C, and it decomposes:
CaCO3(s) → CaO(s) + CO2(g) (heat)
The white solid, calcium oxide, is called quicklime. The reaction is reversible, so the carbon dioxide is carried away up the kiln to stop it from recombining with the lime.
Water is added to quicklime. The lumps hiss, swell, give off steam and crumble into a white powder, because the reaction releases a great deal of heat (it is strongly exothermic):
CaO(s) + H2O(l) → Ca(OH)2(s)
The product, calcium hydroxide, is called slaked lime (also hydrated lime or builders' lime). A little of it dissolves in water, about 1.7 g per litre at room temperature, to give a clear alkaline solution called limewater. A suspension of undissolved slaked lime in water is called milk of lime.
Slaked lime slowly absorbs carbon dioxide from the air and turns back into calcium carbonate:
Ca(OH)2(s) + CO2(g) → CaCO3(s) + H2O(l)
This is how lime mortar and whitewash harden over weeks, and it is the same reaction as the limewater test for carbon dioxide. The three steps form the lime cycle.
| Oxide | Uses |
|---|---|
| Silicon dioxide, SiO2 (sand, quartz) | making glass, concrete and cement |
| Iron(III) oxide, Fe2O3 (haematite) | main ore of iron; red-brown pigment in paints and bricks |
| Tin(IV) oxide, SnO2 (cassiterite) | ore of tin, mined in Rwanda; tin coats food cans and is used in solder |
| Aluminium oxide, Al2O3 | source of aluminium metal; very hard abrasive (corundum) for grinding; rubies and sapphires are coloured forms of it |
| Zinc oxide, ZnO | sunscreens and healing ointments; white pigment; rubber for tyres |
| Magnesium oxide, MgO | heat-resistant linings for furnaces; firebricks |
| Titanium dioxide, TiO2 | the bright white pigment in most white paints |
| Carbon dioxide, CO2 | fire extinguishers; fizzy drinks; raw material of photosynthesis |
| Sulfur dioxide, SO2 | making sulfuric acid; preserving dried fruit and juices; bleaching |
| Dinitrogen oxide, N2O | anaesthetic ("laughing gas") |
A kiln is loaded with 1 000 kg of pure limestone. What mass of quicklime is produced, and what mass of slaked lime can then be made? (Ca = 40, C = 12, O = 16, H = 1)
Warning
Quicklime and slaked lime are caustic
Tip
Heat it, wet it, air it
Limestone is heated to give quicklime (CaO + CO2); quicklime is wetted to give slaked lime (Ca(OH)2, strongly exothermic); slaked lime is aired, absorbing CO2 to return to CaCO3. Learn the three equations and the masses 100 → 56 → 74. Most lime calculations use them.
Write the equation for slaking lime and give two observations.
Solution: CaO + H2O → Ca(OH)2. The lumps get very hot, hiss and give off steam, then swell and crumble to a white powder.
Explain why farmers spread lime on acidic soil.
Solution: Slaked lime is a base. It neutralises the acids in the soil, raising the pH to a level at which most crops grow well.
What mass of quicklime can be made from 250 kg of pure limestone? (Ca = 40, C = 12, O = 16)
Solution: 100 kg of CaCO3 gives 56 kg of CaO, so 250 kg gives 250 × 56 ÷ 100 = 140 kg.
Why does whitewash made from slaked lime harden on a wall over several weeks?
Solution: The calcium hydroxide slowly reacts with carbon dioxide in the air to form hard calcium carbonate: Ca(OH)2 + CO2 → CaCO3 + H2O.
Match each oxide to one use: ZnO, SiO2, Fe2O3, CO2.
Solution: ZnO: sunscreen or ointment. SiO2: making glass. Fe2O3: iron ore or red pigment. CO2: fire extinguishers or fizzy drinks.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
