CDS (II) 2024 General Knowledge · Question 20 of 120

Correct — A, Chloride. Paper is made from wood pulp that is brown because of the lignin left in it, and bleaching is the chemistry of destroying or removing that lignin without damaging the cellulose fibres. The reagents that do it are chlorine chemicals — elemental chlorine historically, sodium hypochlorite before that, and chlorine dioxide today, which accounts for about ninety-five per cent of bleached kraft pulp worldwide in what the industry calls elemental-chlorine-free bleaching. Oxygen, hydrogen peroxide and ozone are the non-chlorine alternatives. Of the four options offered, only the first points to that family at all, which is why it is the intended answer. The wording repays a moment's care, and the card gives it rather than glossing over it: the chloride ion itself, the one in common salt, does no bleaching. What bleaches is chlorine in an oxidising form. The other three options are all definite chlorine compounds with definite uses, and none of those uses is bleaching.

• (b)Chlorobenzene — Chlorobenzene is an organic solvent and a starting material for other chemicals. It contains chlorine bound tightly into a benzene ring, where it cannot act as an oxidising bleach at all.
• (c)Ammonium chloride — Ammonium chloride is used as a nitrogen fertiliser, as the electrolyte paste in a dry cell and as a soldering flux. It is a simple salt with no oxidising power and no bleaching action.
• (d)Hydrochloric acid — Hydrochloric acid is a strong acid, and acidity is not bleaching. Its chlorine is already in the fully reduced chloride form, so it cannot oxidise lignin; it would attack the pulp rather than whiten it.

Wood is roughly half cellulose fibre and about a quarter lignin, the polymer that glues the fibres together and gives unbleached pulp its brown colour. Pulping removes most of the lignin; bleaching removes or decolourises what is left. The agents that do the work are oxidising: chlorine, sodium hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide and ozone. Chlorine dioxide dominates the industry today, and alkaline hydrogen peroxide is the usual choice for mechanical pulp.

Where an option is worded imprecisely, the reliable strategy is to eliminate what can be eliminated and then ask which surviving option the setter can only have meant. All four options here contain chlorine, so the word chlorine by itself decides nothing; what separates them is whether the chlorine is in a form that can oxidise. In chlorobenzene it is locked into an organic molecule, in ammonium chloride and hydrochloric acid it is already the fully reduced chloride ion. Only the bare first option leaves room for the oxidising chlorine chemistry that bleaches pulp. It is worth knowing why the industry has moved away from elemental chlorine: chlorine gas reacting with lignin produced chlorinated organic compounds, including dioxins, in the mill effluent, and switching to chlorine dioxide cut those sharply.

• Unbleached pulp is brown because of lignin; bleaching removes or decolourises the remaining lignin.
• Chlorine dioxide is the most important bleaching method worldwide, used for about 95 per cent of bleached kraft pulp in elemental-chlorine-free sequences.
• Sodium hypochlorite was the first bleach used on chemical pulps and was largely replaced by chlorine in the 1930s.
• Alkaline hydrogen peroxide is the commonest bleaching agent for mechanical pulp; oxygen and ozone are also used.
• Bleaching powder, the calcium chlorine compound made from chlorine and slaked lime, is the classroom example of a pulp and textile bleach.

• Assuming any chlorine compound can bleach; the chlorine must be in an oxidising form.
• Confusing acid action with bleaching — hydrochloric acid attacks material but does not whiten it.
• Reading a loosely printed option literally and discarding it; ask which option the setter can only have meant.

Usually through bleaching powder — its preparation, its uses in paper, textiles and water treatment, or a NOT-true statement about it.

• practice — not a real PYQ

  Bleaching powder is manufactured by the action of chlorine on

  • (a)dry slaked lime
  • (b)common salt
  • (c)washing soda
  • (d)quicklime dissolved in acid

  Answer(a) dry slaked lime — chlorine passed over dry calcium hydroxide gives bleaching powder, the classic bleach for pulp and textiles.

• practice — not a real PYQ

  Bleaching of wood pulp works chiefly by

  • (a)dissolving the cellulose fibres
  • (b)oxidising and removing the remaining lignin
  • (c)coating the fibres with a white pigment
  • (d)neutralising the acids in the pulp

  Answer(b) oxidising and removing the remaining lignin — lignin is what makes unbleached pulp brown, and the bleaching agents destroy it while sparing the cellulose.