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Substances in Common Use — Class 9 Maharashtra SSC Science revision notes

A 3-page visual revision sheet for Substances in Common Use, written for Class 9 Maharashtra SSC Science. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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the chemistry behind cleaning, building and preserving the things around us

Soaps and detergents

  • soap is the sodium or potassium salt of a long chain fatty acid, made by heating a fat or oil with an alkali
  • that process is saponification, and it also yields glycerol as a by-product
  • a soap molecule has two ends: a long hydrocarbon tail that mixes with grease and an ionic head that mixes with water
  • in water the molecules gather into a micelle, tails inward around the grease and heads facing out, and the grease leaves with the rinse
  • hard water contains calcium and magnesium ions, which react with soap to form an insoluble scum instead of a lather
  • detergents work in hard water, because their calcium and magnesium salts stay soluble, which is exactly why they were developed

Cement and glass

  • cement is made by heating limestone with clay in a kiln and grinding the product with a little gypsum
  • the gypsum slows the setting, without which the cement would harden too fast to be worked
  • setting is a chemical reaction with water, called hydration, and not simply drying out
  • that is why concrete is kept wet for days after pouring, a process called curing, so the reaction can complete
  • concrete is cement with sand and stone, and reinforced concrete adds steel rods, which supply the tensile strength concrete lacks
  • glass is made by melting sand with soda and limestone, and it is not a true crystal but a very slowly flowing solid

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