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Polyhalogen compounds sheet
5-mark answer frame

Give the uses and harmful effects of (i) dichloromethane, (ii) trichloromethane, (iii) freons and (iv) DDT.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Define in one line. Polyhalogen compounds contain more than one halogen atom; many are useful but harmful.
  2. 2 · Dichloromethane. Uses: solvent, paint remover, aerosol propellant. Harm: affects the nervous system; burns skin and eyes.
  3. 3 · Trichloromethane. Uses: solvent; making freon R-22. Harm: depresses the nervous system, damages liver and kidneys; forms phosgene in light: 2CHCl₃ + O₂ → 2COCl₂ + 2HCl.
  4. 4 · Freons. Uses: refrigerants, aerosol propellants. Harm: destroy ozone in the stratosphere.
  5. 5 · DDT. Use: insecticide against malaria mosquitoes. Harm: insect resistance, toxic to fish, stored in fatty tissue.
  6. 6 · Link to real life. DDT was sprayed in India against malaria; its farm use is now banned.
  7. 7 · Close in one line. So most of these compounds are now restricted or replaced by safer chemicals.

Scoring tip: Give one use and one harm for each compound; a three-column table saves time.