Changes Around Us, Fibres, Plastics, Coal, Petroleum, Combustion and Air

What to remember

• A physical change alters shape, size, state or appearance but makes no new substance; a chemical change makes new substances and is usually irreversible, with energy change, gas, colour change or a precipitate as signs.
• Fibres are natural (cotton, jute, wool, silk) or synthetic (rayon, nylon, polyester, acrylic); plastics are polymers that are thermoplastic (soften on heating) or thermosetting (do not).
• Coal and petroleum are fossil fuels; combustion needs a fuel, air (oxygen) and heat to reach ignition temperature; air is mainly nitrogen (78 percent) and oxygen (21 percent).

Physical and chemical changes

• Signs of chemical change: change of colour, release of a gas, formation of a precipitate, change in temperature, light or sound, and change in smell.
• Rusting: iron + oxygen + water gives hydrated iron(III) oxide (reddish brown).
• Crystallisation is a physical change; galvanisation is a protective coating process; burning of magnesium ribbon gives white magnesium oxide (bright white light) and is chemical.
• Law of conservation of mass: in a chemical change total mass of reactants equals total mass of products (Lavoisier).
• Reversible and irreversible: melting wax is reversible; burning a candle's wick is not. Some chemical changes are reversible under suitable conditions.
• Periodic and non-periodic changes: day and night, tides and seasons are periodic; earthquakes are not.

Fibres

• Natural plant fibres: cotton (from the cotton boll; plant), jute (stem). Animal fibres: wool (sheep, goat, yak, camel; rearing, shearing, sorting, scouring, combing, spinning), silk (silkworm cocoon; sericulture).
• Synthetic fibres (man-made): made from petrochemicals.
• Rayon: artificial silk, made from wood pulp (cellulose), so semi-synthetic.
• Nylon: first fully synthetic fibre (1930s); strong, elastic, light; used in ropes, parachutes, toothbrush bristles, socks.
• Polyester: wrinkle resistant, easy to wash; terylene and PET (bottles, fabrics).
• Acrylic: soft and warm, resembles wool (artificial wool); sweaters and blankets.
• Properties: synthetic fibres absorb little water, dry fast, are strong and cheap. Disadvantages: they melt and stick to skin on catching fire; they are not biodegradable. Cotton clothes are better in summer; synthetic ones are not comfortable in heat.
• Polymer: a large molecule from many small units called monomers. Cellulose is a natural polymer; polyester is a synthetic polymer.

Plastics

• Polythene is made from ethene. PVC is polyvinyl chloride (pipes, raincoats). Teflon (polytetrafluoroethylene) is non-stick coating. Bakelite is a poor conductor of heat and electricity (electric switches, handles). Melamine is fire and heat resistant (kitchenware, floor tiles, fire-proof uniforms).
• Plastics are non-biodegradable. They persist for ages, choke drains and harm animals, and burning them releases poisonous gases. The 4R principle: reduce, reuse, recycle, recover. Use jute or cloth bags. Bioplastics from starch or corn are biodegradable.
• Plastics are light, strong, durable, corrosion resistant and cheap. Plastic cookware is not suitable for direct flame.

Coal and petroleum

• Fossil fuels: formed from buried plants and animals over millions of years. Exhaustible (non-renewable).
• Coal formation (carbonisation): plant matter buried and compressed becomes peat, then lignite, then bituminous, then anthracite. Carbon content increases in this order. Anthracite is the hardest and purest, with the highest calorific value.
• Products from coal by destructive distillation (heating in absence of air): coke (almost pure carbon, used in steel and extraction of metals), coal tar (thick black liquid, source of benzene, naphthalene, phenol, dyes, drugs, explosives and synthetic materials; bitumen-like uses for roads), coal gas (used as fuel in industry).
• Naphthalene balls are made from coal tar derivatives and repel moths.
• Petroleum ("rock oil"): a dark, oily mixture of hydrocarbons, found under rock layers, pumped out as crude oil. Refined by fractional distillation in a refinery. Fractions are separated by boiling points.

• Petroleum is also called liquid gold or black gold. A product stored as CNG (compressed natural gas) is a cleaner fuel for vehicles; natural gas is also used as raw material for fertilisers. LPG is mainly butane with propane; a smell (ethyl mercaptan) is added to detect leaks.
• Conservation: use public transport, switch off engines at signals, regular maintenance, use solar energy and biogas. India's oil comes mostly from imports; petroleum products need careful use.

Combustion

• Combustion is the chemical process in which a substance reacts with oxygen to give heat and light. A substance that burns is combustible; the burning substance is the fuel.
• Conditions for burning: fuel + oxygen + heat to reach ignition temperature, the lowest temperature at which a substance catches fire. Remove any one and the fire stops.
• Fire control: water cools and removes heat but is not suitable for oil or electrical fires. For oil, petrol and electric fires use CO2 extinguishers, foam or dry chemicals. CO2 is heavier than air and blankets the fire. A blanket on a person smothers the flame by cutting off oxygen.
• Types of combustion:
• Rapid: quick burning with heat and light (gas burner).
• Spontaneous: burning without any external flame (phosphorus in air; in coal mines).
• Explosive: sudden reaction with heat, light, sound and gas, such as fire crackers.
• Slow: respiration is slow oxidation that gives energy.
• Flame structure: the outer zone is blue, non-luminous and the hottest (complete combustion); the middle zone is yellow and moderately hot; the innermost dark zone near the wick is the coolest and has unburnt wax vapour. Goldsmiths use the outermost zone for melting gold.
• Complete and incomplete combustion: complete in plenty of air gives CO2 and water; incomplete (limited air) gives carbon monoxide and soot, which is poisonous.
• Calorific value: heat produced on complete burning of 1 kg of fuel, in kJ/kg. Hydrogen has the highest value, then LPG, natural gas, petrol and diesel; wood and cow dung have low values. Worked example: if a fuel has calorific value of 45,000 kJ/kg, burning 2 kg gives 90,000 kJ.
• A good fuel is cheap, easily available, easy to store, burns without smoke, has a high calorific value and leaves little ash.
• Environmental effects: carbon dioxide causes global warming; carbon monoxide poisons; oxides of sulphur and nitrogen cause acid rain; unburnt particles cause smog and respiratory disease.

Air

• Composition by volume (dry air): nitrogen about 78 percent, oxygen about 21 percent, argon about 0.9 percent, carbon dioxide about 0.04 percent, and traces of neon, helium, methane, hydrogen and water vapour.
• Worked example: in 500 L of air the oxygen volume is about 105 L (21 percent of 500).
• Oxygen: supports burning and breathing. Nitrogen: does not support burning, dilutes oxygen, used in fertilisers and as a liquid coolant. Carbon dioxide: used in photosynthesis, fire extinguishers and soft drinks. Argon: filling light bulbs.
• Nitrogen cycle and oxygen cycle: green plants release oxygen during photosynthesis; animals use oxygen and release CO2.
• Air pollution: by smoke, dust, carbon monoxide, oxides of sulphur and nitrogen, hydrocarbons and CFCs. Ozone layer in the stratosphere absorbs harmful UV; CFCs deplete it.
• Greenhouse gases: CO2, methane, nitrous oxide, water vapour and CFCs trap heat and cause global warming.
• Tests: oxygen relights a glowing splinter; carbon dioxide turns limewater milky.

Exam traps

• Burning of a candle has both a physical change (melting wax) and a chemical change (burning).
• Dissolving salt in water is physical; reacting zinc with acid is chemical.
• Rayon is made from wood pulp, but it is called artificial silk, not natural silk.
• Nylon is a thermoplastic fibre; Bakelite is thermosetting.
• Coal is not refined; it is processed by destructive distillation. Petroleum is refined by fractional distillation.
• Petrol and diesel are fractions; bitumen is the residue left at the bottom.
• Calorific value is measured in kJ/kg, not kJ/mol.
• Water is not used on oil or electrical fires.
• The innermost zone of a flame is cool; the outermost zone is hottest.
• Nitrogen is the main gas in air, but oxygen supports burning.

One-liners

• 1. Chemical changes make new substances and are mostly irreversible.
• 2. Nylon was the first fully synthetic fibre.
• 3. Rayon comes from cellulose (wood pulp).
• 4. Teflon coats non-stick pans.
• 5. Bakelite and melamine are thermosetting plastics.
• 6. Anthracite is the purest form of coal.
• 7. Coke, coal tar and coal gas come from coal.
• 8. Petroleum is refined by fractional distillation.
• 9. LPG leaks are detected by adding ethyl mercaptan.
• 10. Ignition temperature is the lowest temperature at which a substance starts burning.
• 11. Hydrogen has the highest calorific value among common fuels.
• 12. Air has about 78 percent nitrogen and 21 percent oxygen.

Practice questions

1. The first fully synthetic fibre was

   1. rayon
   2. acrylic
   3. nylon
   4. cotton
   Answer

   C. nylon

   Nylon was made entirely from petrochemicals.

2. Rayon is obtained from

   1. petroleum only
   2. silkworm cocoons
   3. sheep wool
   4. wood pulp (cellulose)
   Answer

   D. wood pulp (cellulose)

   Rayon is a semi-synthetic fibre called artificial silk.

3. Which of these is a thermosetting plastic?

   1. PVC
   2. Bakelite
   3. Nylon
   4. Polythene
   Answer

   B. Bakelite

   Bakelite sets permanently on moulding.

4. The non-stick coating on cookware is made of

   1. Teflon
   2. Bakelite
   3. PVC
   4. polythene
   Answer

   A. Teflon

   Teflon is polytetrafluoroethylene.

5. The purest and hardest variety of coal is

   1. bituminous coal
   2. peat
   3. anthracite
   4. lignite
   Answer

   C. anthracite

   Carbon content rises from peat to anthracite.

6. Which product is obtained from coal by destructive distillation?

   1. coke
   2. kerosene
   3. LPG
   4. petrol
   Answer

   A. coke

   Coke, coal tar and coal gas are the products.

7. The residue of petroleum refining used for road surfacing is

   1. kerosene
   2. bitumen
   3. diesel
   4. naphtha
   Answer

   B. bitumen

   Bitumen is the heavy residue.

8. Petroleum is refined by

   1. simple distillation of coal
   2. electrolysis
   3. filtration
   4. fractional distillation
   Answer

   D. fractional distillation

   Components separate by their different boiling points.

9. The substance added to LPG so that leakage can be detected by smell is

   1. ethanol
   2. ethyl mercaptan
   3. methane
   4. benzene
   Answer

   B. ethyl mercaptan

   It has a strong, unpleasant smell.

10. The most abundant gas in air is

    1. oxygen
    2. carbon dioxide
    3. argon
    4. nitrogen
    Answer

    D. nitrogen

    Dry air is about 78 percent nitrogen by volume.

11. Among common fuels the highest calorific value is that of

    1. hydrogen
    2. cow dung
    3. wood
    4. coal
    Answer

    A. hydrogen

    Hydrogen releases the most heat per kg.

12. The hottest zone of a candle flame is the

    1. innermost dark zone
    2. middle luminous zone
    3. outer (non-luminous) zone
    4. wick
    Answer

    C. outer (non-luminous) zone

    Complete combustion in plenty of air happens in the outer zone.

13. The unit of calorific value is

    1. kg/kJ
    2. kJ/kg
    3. J/m
    4. kJ/s
    Answer

    B. kJ/kg

    It is heat produced per kg of fuel.

14. The best extinguisher for an electrical fire is

    1. water
    2. sand only
    3. carbon dioxide
    4. wet cloth
    Answer

    C. carbon dioxide

    Water conducts electricity; CO2 does not.

15. The lowest temperature at which a substance catches fire is called its

    1. boiling point
    2. freezing point
    3. melting point
    4. ignition temperature
    Answer

    D. ignition temperature

    Fuel must be heated to this temperature to burn.

16. Which of these is a physical change?

    1. Dissolving sugar in water
    2. Souring of milk
    3. Burning paper
    4. Rusting of iron
    Answer

    A. Dissolving sugar in water

    No new substance forms; sugar can be recovered.

17. Which of these is a chemical change?

    1. Cutting a paper
    2. Rusting of an iron nail
    3. Melting of ice
    4. Boiling of water
    Answer

    B. Rusting of an iron nail

    Rust is a new substance.

18. Polythene is made from the monomer

    1. ethene
    2. ethanol
    3. propene
    4. methane
    Answer

    A. ethene

    Ethene units join to form polyethene.

19. Naphthalene balls used against moths are made from a product of

    1. petroleum gas
    2. coke
    3. natural gas
    4. coal tar
    Answer

    D. coal tar

    Naphthalene is obtained from coal tar.

20. CNG consists mainly of

    1. hydrogen
    2. ethanol
    3. methane
    4. butane
    Answer

    C. methane

    CNG is compressed natural gas.

21. The gas that turns limewater milky is

    1. carbon dioxide
    2. oxygen
    3. hydrogen
    4. nitrogen
    Answer

    A. carbon dioxide

    CO2 forms insoluble calcium carbonate.

22. A greenhouse gas among the following is

    1. argon
    2. helium
    3. neon
    4. carbon dioxide
    Answer

    D. carbon dioxide

    CO2 traps heat in the atmosphere.

23. Synthetic clothes are dangerous near fire because they

    1. give off oxygen
    2. do not burn at all
    3. melt and stick to the skin
    4. turn into water
    Answer

    C. melt and stick to the skin

    Synthetic fibres are thermoplastic polymers.

24. Incomplete combustion of a fuel gives the poisonous gas

    1. carbon dioxide
    2. carbon monoxide
    3. nitrogen
    4. oxygen
    Answer

    B. carbon monoxide

    Insufficient oxygen produces CO and soot.

25. Water should not be used on an oil fire because

    1. water is a fuel
    2. water reacts to give oxygen
    3. oil floats on water and spreads the fire
    4. oil dissolves in water
    Answer

    C. oil floats on water and spreads the fire

    Oil is lighter than water, so the burning layer floats.

26. Plastics cause environmental harm mainly because they

    1. are metals
    2. are non-biodegradable
    3. are soluble in water
    4. are heavy
    Answer

    B. are non-biodegradable

    They persist in the soil and choke drains.

27. A thermoplastic is one that

    1. softens on heating and can be re-moulded
    2. is a metal alloy
    3. hardens permanently once
    4. is a natural fibre
    Answer

    A. softens on heating and can be re-moulded

    Polythene and PVC are examples.

28. Anthracite is the best fuel among coal types because it

    1. has most moisture
    2. is a liquid
    3. is the softest
    4. has the highest carbon content
    Answer

    D. has the highest carbon content

    More carbon gives higher calorific value.

29. The ozone layer protects life by absorbing

    1. harmful ultraviolet rays
    2. infrared rays only
    3. sound waves
    4. visible light
    Answer

    A. harmful ultraviolet rays

    Ozone in the stratosphere absorbs UV.

30. A fuel has calorific value 45,000 kJ/kg. Heat produced by burning 2 kg is

    1. 45,000 kJ
    2. 90,000 kJ
    3. 22,500 kJ
    4. 180,000 kJ
    Answer

    B. 90,000 kJ

    45,000 × 2 = 90,000 kJ.

31. The approximate volume of oxygen in 500 L of dry air is

    1. 390 L
    2. 250 L
    3. 105 L
    4. 21 L
    Answer

    C. 105 L

    21 percent of 500 L = 105 L.

32. The approximate volume of nitrogen in 200 L of dry air is

    1. 78 L
    2. 42 L
    3. 200 L
    4. 156 L
    Answer

    D. 156 L

    78 percent of 200 L = 156 L.

33. A fuel of calorific value 40,000 kJ/kg is burnt in a quantity of 500 g. The heat produced is

    1. 2,000 kJ
    2. 40,000 kJ
    3. 20,000 kJ
    4. 80,000 kJ
    Answer

    C. 20,000 kJ

    0.5 kg × 40,000 = 20,000 kJ.

34. The mass of carbon dioxide formed on completely burning 12 g of carbon (C = 12, O = 16) is

    1. 44 g
    2. 12 g
    3. 28 g
    4. 32 g
    Answer

    A. 44 g

    C + O2 gives CO2; molar mass of CO2 is 44.

35. When 24 g of magnesium burns completely with 16 g of oxygen, the mass of magnesium oxide formed is

    1. 8 g
    2. 40 g
    3. 24 g
    4. 16 g
    Answer

    B. 40 g

    Mass is conserved: 24 + 16 = 40 g.

36. Which of the statements is/are correct? 1. Burning of a candle involves both physical and chemical changes. 2. Rusting of iron is a physical change.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Rusting forms a new substance, so it is chemical.

37. Which of the statements is/are correct? 1. Nylon is a thermosetting plastic. 2. Bakelite is a thermoplastic.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Nylon and polythene soften on heating; Bakelite is thermosetting.

38. Which of the statements is/are correct? 1. Coal is refined by fractional distillation. 2. Petroleum is refined by fractional distillation.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Coal is processed by destructive distillation; petroleum by fractional distillation.

39. Which of the statements is/are correct? 1. Fuel, oxygen and ignition temperature are all needed for burning. 2. Removing any one of them can put out the fire.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

40. Which of the statements is/are correct? 1. Rayon is semi-synthetic and made from cellulose. 2. Acrylic fibres resemble wool.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

41. Which of the statements is/are correct? 1. The innermost zone of a flame is the hottest. 2. The outer zone of a flame is the hottest.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    The innermost zone has unburnt vapour and is the coolest.

42. Which of the statements is/are correct? 1. Plastics are good conductors of electricity. 2. Plastics are non-biodegradable.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Plastics are poor conductors.

43. Which of the statements is/are correct? 1. Synthetic fibres absorb little water. 2. Cotton is a natural fibre obtained from plants.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

44. Match the petroleum fraction with its use: (a) bitumen (b) paraffin wax (c) kerosene (d) LPG; (i) candles (ii) road surfacing (iii) domestic gas (iv) stoves and jet fuel

    1. a-ii, b-i, c-iii, d-iv
    2. a-i, b-ii, c-iv, d-iii
    3. a-iv, b-i, c-ii, d-iii
    4. a-ii, b-i, c-iv, d-iii
    Answer

    D. a-ii, b-i, c-iv, d-iii

    Bitumen for roads, wax for candles, kerosene for stoves, LPG for cooking.

45. Match the fibre with its source: (a) wool (b) silk (c) jute (d) rayon; (i) wood pulp (ii) plant stem (iii) cocoon (iv) sheep

    1. a-i, b-iii, c-ii, d-iv
    2. a-iii, b-iv, c-ii, d-i
    3. a-iv, b-iii, c-i, d-ii
    4. a-iv, b-iii, c-ii, d-i
    Answer

    D. a-iv, b-iii, c-ii, d-i

    Wool from sheep, silk from cocoons, jute from stems, rayon from wood pulp.