Pariksha SaathiLesson 2 of 8 · Science and its Pedagogy (Paper II)
Materials: Separation, Physical and Chemical Changes, Acids and Bases, Fibres, Metals, Fuels and Combustion
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Sorting and separating
Materials are grouped by properties such as lustre, hardness, solubility, floating or sinking, and transparency (glass is transparent, oiled paper translucent, wood opaque). Mixtures are separated by hand-picking, threshing, winnowing (wind carries away the lighter husk), sieving, sedimentation and decantation, filtration, evaporation and churning. Salt mixed with sand is dissolved in water, filtered and the filtrate evaporated.
Physical and chemical changes
A physical change alters shape, size, colour or state without forming a new substance and is often reversible (melting wax, dissolving sugar, crystallisation). A chemical change forms a new substance, often with heat, light, sound, a change of colour, a smell or a gas. Magnesium ribbon burns with a brilliant white light to form magnesium oxide, whose solution is basic; vinegar with baking soda gives carbon dioxide, which turns lime water milky; and an iron nail in blue copper sulphate solution gets a brown coat of copper as the solution turns green (iron sulphate). Rusting needs both oxygen and water and is prevented by paint, grease or galvanisation, a coating of zinc.
Acids, bases and salts
Acids taste sour and turn blue litmus red; bases taste bitter, feel soapy and turn red litmus blue. Litmus comes from lichens, turmeric turns red with bases, and china rose (gudhal) indicator turns acids dark pink and bases green. NCERT pairs: vinegar has acetic acid, an ant's sting formic acid, citrus fruits citric acid, curd lactic acid, spinach oxalic acid, amla ascorbic acid (vitamin C) and tamarind tartaric acid; lime water is calcium hydroxide and milk of magnesia magnesium hydroxide. An acid and a base neutralise each other to give a salt and water, with heat. So an ant bite is treated with baking soda or calamine (zinc carbonate), acidic soil gets quicklime or slaked lime, and basic soil gets organic matter.
Fibre to fabric
Cotton grows in black soil and a warm climate, and ginning separates the seeds from the picked bolls; jute fibre is taken from the stem of the jute plant by retting. Fibres are spun into yarn on a takli or charkha and woven or knitted into fabric. Wool comes from sheep, yak, Angora and Kashmiri goats and camel; it is sheared, scoured, sorted, dyed and rolled into yarn, and sorters risk anthrax, called sorter's disease. Silk comes from the cocoon of the silkworm, whose life cycle runs egg, larva (caterpillar), pupa and moth; rearing silkworms is sericulture, the mulberry silkworm feeds on mulberry leaves, and tussar, mooga and kosa are other Indian silks.
Synthetic fibres and plastics
Rayon, called artificial silk, is made by chemically treating wood pulp; nylon, the first fully synthetic fibre, polyester, and acrylic, which looks like wool, are made from petrochemicals. Thermoplastics such as polythene and PVC soften on heating; thermosetting plastics such as bakelite (switches, handles of utensils) and melamine (kitchenware) do not soften once moulded. Plastics are non-biodegradable and burning them releases poisonous fumes, hence the 4R principle: Reduce, Reuse, Recycle, Recover.
Metals and non-metals
Metals are lustrous, malleable, ductile, sonorous and good conductors of heat and electricity; non-metals are generally dull, brittle and poor conductors, graphite being a conducting exception. Mercury is a liquid metal and bromine a liquid non-metal; sodium, soft enough to cut with a knife, is stored in kerosene, and phosphorus is stored in water. Metal oxides are basic and non-metal oxides acidic. Many metals react with dilute acids to give hydrogen, which burns with a pop sound. A more reactive metal displaces a less reactive one from its salt solution: zinc displaces copper from copper sulphate, but copper cannot displace zinc.
Coal and petroleum
Coal, petroleum and natural gas are exhaustible fossil fuels formed from dead organisms over millions of years, while sunlight, air and wind are inexhaustible. Coal formed from forests buried about 300 million years ago, by carbonisation, and its processing gives coke (almost pure carbon, used to make steel), coal tar (for dyes, drugs, explosives, paints and naphthalene balls) and coal gas. Petroleum formed from organisms in the sea; refining separates LPG, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen. NCERT records that the world's first oil well was drilled in Pennsylvania, USA, in 1859. The PCRA advises driving at a steady moderate speed and switching off the engine at traffic lights.
Combustion and flame
Combustion is a chemical process in which a substance reacts with oxygen to give off heat. It needs a fuel, air and heat up to the ignition temperature, the lowest temperature at which a substance catches fire; petrol, LPG and alcohol are inflammable, with very low ignition temperatures. Water can be boiled in a paper cup because it absorbs the heat and keeps the paper below its ignition temperature. Combustion can be rapid (gas stove), spontaneous (phosphorus at room temperature) or an explosion (crackers). A candle flame has a dark inner zone of unburnt wax vapour, a luminous middle zone of partial combustion and a non-luminous outer zone of complete combustion, the hottest, which goldsmiths use. Water is not used on oil or electrical fires; carbon dioxide is best there.
Key facts
- Separation: winnowing - wind removes the lighter husk; sedimentation then decantation - pour off the clear water; filtration - insoluble solid from a liquid; salt from sand - dissolve, filter, evaporate.
- Physical change: no new substance, often reversible (melting, dissolving, crystallisation). Chemical change: a new substance, with heat, light, gas, colour change or smell (burning, rusting).
- Iron nail in copper sulphate solution: brown copper deposits and the blue solution turns green (iron sulphate) - a chemical change by displacement.
- Rusting needs both oxygen and water; galvanisation coats iron with zinc; a blacksmith heats an iron rim before fitting it because metals expand on heating.
- Natural acids: vinegar - acetic; ant sting - formic; citrus fruits - citric; curd - lactic; spinach - oxalic; amla - ascorbic (vitamin C); tamarind - tartaric.
- Indicators: litmus (from lichens) - blue turns red in acid; turmeric turns red in a base; china rose - dark pink in acid, green in base.
- Silkworm life cycle: egg, larva (caterpillar), pupa (cocoon), moth; sericulture = rearing silkworms; wool sorters may catch anthrax (sorter's disease).
- Thermoplastics (polythene, PVC) soften on heating; thermosetting plastics (bakelite, melamine) do not; 4R principle: Reduce, Reuse, Recycle, Recover.
- Coal products: coke (steel), coal tar (dyes, drugs, explosives, naphthalene balls), coal gas; the world's first oil well, as NCERT gives it: Pennsylvania, USA, 1859.
- Candle flame: inner zone dark (unburnt wax vapour), middle zone luminous (partial combustion), outer zone non-luminous and hottest (complete combustion).
Quick revision
Winnowing, sedimentation, decantation, filtration, evaporation; physical change forms no new substance, chemical change does; rusting needs oxygen and water, galvanisation is a zinc coat; blue litmus turns red in acid, turmeric red in base; ant sting has formic acid; acid + base gives salt + water; silkworm egg-larva-pupa-moth; thermoplastic vs thermosetting; metals malleable and ductile; coke, coal tar, coal gas; ignition temperature; outer flame zone hottest.