NDA (I) 2018 General Ability Test · Question 68 of 150

Correct — A, It is used as a reducing agent in chemical industries. Bleaching powder is used industrially as an oxidising agent, and the option has reversed exactly that one word. The chemistry makes the reversal impossible to defend. Bleaching powder is calcium oxychloride, made by passing chlorine over dry slaked lime, and it works by releasing chlorine, which oxidises the coloured compounds in cloth and paper into colourless ones and oxidises the organic matter and microbes in drinking water. A substance that whitens and disinfects by giving up oxidising power cannot be a reducing agent.

• (b)It is used for bleaching wood pulp in paper factories — True, and it is one of the standard uses listed in the school chemistry chapter. Paper mills bleach brown wood pulp white before it is made into paper.
• (c)It is used for disinfecting drinking water — True. Bleaching powder has been used for generations to make water free of germs, and municipal chlorination rests on the same chemistry.
• (d)It is used for bleaching linen in textile industry — True, and the compound takes its very name from this use. Bleaching cotton and linen in textile mills, along with laundry work, is its oldest large-scale application.

Bleaching powder, calcium oxychloride, is manufactured by the action of chlorine on dry slaked lime, calcium hydroxide. The chlorine for the process comes from the electrolysis of brine, which ties the compound back into the chlor-alkali family. Bleaching powder acts by liberating chlorine, and the standard list of its uses is bleaching cotton and linen in textile mills and wood pulp in paper factories, disinfecting drinking water, and serving as an oxidising agent in the chemical industry.

The examiner has taken a four-item list straight from the textbook, kept three of them word for word, and inverted a single term in the fourth. That is a common and cheap way to build a NOT-true item, and the defence against it is to read each option for the one word that could have been flipped rather than for the general sense. Here 'reducing' is that word. It also helps to know why the compound behaves as it does. Oxidation in the old sense means addition of oxygen or removal of hydrogen, and chlorine achieves the same effect by taking electrons; bleaching, disinfecting and industrial oxidation are three faces of a single chemical action. One practical caution worth carrying is that bleaching powder loses strength on standing in air, because carbon dioxide slowly displaces the chlorine.

• Bleaching powder is calcium oxychloride, made by passing chlorine over dry slaked lime.
• It acts as an oxidising agent, not a reducing agent.
• Its standard uses are bleaching textiles and wood pulp, disinfecting drinking water and industrial oxidation.
• The chlorine used to make it comes from the electrolysis of brine.
• It loses its available chlorine on long exposure to air and so weakens with storage.

• Skimming a NOT-true option and missing a single reversed word.
• Confusing bleaching powder with washing soda or with plaster of Paris, all of which appear together in this chapter.
• Assuming that because bleaching removes colour it must be a reduction.

NDA sets it as a NOT-true list of uses, as a preparation question, or by pairing bleaching powder with another chlorine compound and asking what they share.

• practice — not a real PYQ

  Bleaching powder is prepared by the action of chlorine on

  • (a)quicklime
  • (b)dry slaked lime
  • (c)limestone
  • (d)gypsum

  Answer(b) dry slaked lime — chlorine is passed over dry calcium hydroxide to give calcium oxychloride.

• practice — not a real PYQ

  Bleaching powder removes colour from cloth because it

  • (a)reduces the coloured compounds
  • (b)oxidises the coloured compounds
  • (c)dissolves the coloured compounds
  • (d)coats the fibre with a white layer

  Answer(b) oxidises the coloured compounds — the chlorine it releases converts them into colourless products.