Silicon dioxide (silica): forms and uses
Silica is silicon dioxide, SiO₂. Each silicon atom is joined to four oxygen atoms and these join in a huge network, so silica is hard and melts only above 1700 °C. It occurs as quartz (clear crystals), sand (tiny quartz grains), flint and other forms. Pure silica melted and cooled gives quartz glass. Silica gel is a porous form that soaks up water vapour (packets in new shoes).
Uses: glass, cement and concrete, bricks and ceramics, optical fibres, abrasive sandpaper, and (after reduction) pure silicon for chips. SiO₂ is an acidic oxide: it reacts with hot concentrated bases but not with water. See also Silicon, silicates, glass and ceramics.
Glass: making, types and uses
Ordinary soda-lime glass is made by heating sand (SiO₂), soda ash (Na₂CO₃) and limestone (CaCO₃) to about 1500 °C. The melt is shaped (blown, rolled or floated) and cooled quickly. Because it cools fast the atoms do not get time to arrange: the network stays untidy, which makes glass transparent and brittle.
- Soda-lime glass: windows, bottles, jars.
- Borosilicate glass (with boron oxide): heat-resistant lab glassware and cookware.
- Lead crystal: shiny, heavy, for decoration.
- Coloured glass: metal oxides added, for example cobalt gives blue and iron or chromium gives green.
- Safety glass (toughened or laminated) and glass fibre for insulation and light cables.
Limestone, marble and chalk
All three are mostly calcium carbonate (CaCO₃): limestone (sedimentary, often with shell fossils), marble (limestone changed by heat and pressure, so it is hard, crystalline and takes a polish) and chalk (soft, fine, white limestone from tiny sea shells).
How to tell them from other rocks: put a drop of dilute hydrochloric acid. CaCO₃ fizzes, because CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Pass the gas into lime water: it turns milky. Granite or sandstone do not fizz.
Lime cycle: heating limestone (about 900 °C) gives quicklime: CaCO₃ → CaO + CO₂. Quicklime and water give slaked lime: CaO + H₂O → Ca(OH)₂. Slaked lime slowly takes CO₂ from the air and turns back to CaCO₃, which is how lime mortar hardens. Uses: cement, glass, steel-making (removes impurities), neutralising acidic soil and lakes, antacid tablets.
Hard water and removing temporary hardness
Hard water has dissolved calcium (and magnesium) ions and does not make lather easily with soap. Temporary hardness is caused by calcium hydrogencarbonate, Ca(HCO₃)₂. Boiling removes it: Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. The white solid is the scale in kettles and pipes. Permanent hardness comes from calcium sulfate or chloride and boiling does not remove it.
Other ways: add washing soda (Na₂CO₃), which makes CaCO₃ precipitate and removes both kinds; or use an ion-exchange softener that swaps Ca²⁺ for Na⁺.
Hydrates, anhydrous salts and the setting of plaster
A hydrate has water molecules built into its crystal (water of crystallisation), shown after a dot: CuSO₄·5H₂O is blue, and washing soda is Na₂CO₃·10H₂O. Heating drives the water off and leaves the anhydrous salt: CuSO₄·5H₂O → CuSO₄ (white) + 5H₂O. Add water and the blue colour returns, so white anhydrous copper(II) sulfate is a test for water.
Gypsum is CaSO₄·2H₂O. Heated gently (about 150 °C) it loses most of its water and becomes plaster of Paris, CaSO₄·½H₂O. Mixed with water, tiny gypsum needle crystals grow and lock together, so the paste sets hard in minutes and expands a little (good for casts and moulds): CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. Keep plaster of Paris dry or it sets in the bag.
Fertilisers: natural and artificial
Plants need nitrogen (N) for leaves, phosphorus (P) for roots and flowers, and potassium (K) for health. Soil runs low after many crops, so we add fertilisers.
- Natural: farmyard manure, compost, green manure, bone meal. Slow, improve soil structure, but weak.
- Artificial: urea CO(NH₂)₂ (46% N), ammonium nitrate NH₄NO₃ (35% N), ammonium sulfate (NH₄)₂SO₄, superphosphate (P), potassium chloride KCl (K), and NPK mixtures.
Percentage of an element = (mass of the element in the formula ÷ formula mass) × 100. For urea, 28 ÷ 60 × 100 ≈ 46.7%. Too much fertiliser can run into rivers and cause eutrophication: algae bloom and fish die. How ammonia for fertilisers is made: see the Haber process.
Try it: five quick tests
(1) Drop vinegar on chalk or a seashell: fizz means calcium carbonate. (2) Boil tap water in a clean pan and look for a white film. (3) Look at the 3D: heat the gypsum and see the blue water balls leave. (4) At home, mix plaster of Paris with water in a paper cup and time how fast it sets. (5) In free play, compare urea with superphosphate: which one gives only nitrogen?
Key formulas and definitions
- Glass: sand SiO₂ + soda ash Na₂CO₃ + limestone CaCO₃ → melt → cool fast
- CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; CaCO₃ → CaO + CO₂; CaO + H₂O → Ca(OH)₂
- Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂ (boiling)
- CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O (heat); reverse with water = setting
- % of element = (mass of element ÷ formula mass) × 100
Worked examples
1. Find the percentage of nitrogen in urea, CO(NH₂)₂ (C = 12, O = 16, N = 14, H = 1).
Formula mass = 12 + 16 + 2 × (14 + 2) = 60. N mass = 28. % N = 28 ÷ 60 × 100 = 46.7%.
2. Find the percentage of nitrogen in ammonium nitrate, NH₄NO₃.
Mass = 14 + 4 + 14 + 48 = 80. N = 28. 28 ÷ 80 × 100 = 35%.
3. What mass of quicklime (CaO) is obtained from 100 kg of pure limestone? (Ca 40, C 12, O 16)
CaCO₃ = 100, CaO = 56. So 100 kg gives 56 kg of CaO (and 44 kg of CO₂).
4. How would you show that a white rock is limestone and not granite?
Add a drop of dilute HCl. Limestone fizzes and the gas turns lime water milky. Granite does not fizz.
5. Write what happens when temporary hard water is boiled.
Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂. White CaCO₃ scale forms and the water becomes soft.
6. How many moles of water are lost when 1 mole of CuSO₄·5H₂O is heated fully? What mass of water is lost? (H₂O = 18)
5 moles of water are lost. Mass = 5 × 18 = 90 g.
Common mistakes
- Thinking glass is a crystal. It is an untidy (non-crystalline) network made by fast cooling.
- Believing boiling softens all hard water. Only temporary hardness goes; permanent hardness needs washing soda or ion exchange.
- Mixing up plaster of Paris (CaSO₄·½H₂O) with gypsum (CaSO₄·2H₂O). Plaster has less water.
- Saying marble and limestone are different chemicals. Both are CaCO₃, only the structure differs.