Question

Which of the following items are not bleached by bleaching powder? (This question has multiple correct options).
A.Straw
B.Silk
C.Ivory
D.None of the above

Answer 1

Bleaching can be defined as a treatment in which an oxidative chemical is used that alters the light absorbing or light reflecting nature of a material structure , thereby increasing its perception of whiteness.
Bleaching powder is produced by the action of chlorine on a dry slaked lime according to the following reaction
$Ca{\left( {OH} \right)_2}\, + \,C{l_2}\, \to \,CaOC{l_2}\, + \,{H_2}O$
So bleaching powder smells strongly of chlorine.

Complete answer:
For the bleaching treatment we use bleaching agents. A bleaching agent is a chemical substance that can whiten or decolourise other substances. Bleaching agents are of two types: oxidative bleaching agents and reductive bleaching agents.
Oxidative bleaching agent: The oxidative bleaching agents are used more than that of reductive bleaching agents. It is a chemical reagent which decomposes in alkali solution and produces active oxygen. This active oxygen is used for bleaching.
Examples are hydrogen peroxide, bleaching powder, sodium chlorite
Reductive bleaching agents: In reductive bleaching many colouring matter can be reduced to colourless compounds. Double bond is converted to single bond in the case of reductive bleaching.
Examples are sulphur dioxide , sodium hydrosulphite etc.
Bleaching powder is calcium oxo-chloride $CaOC{l_2}$ or $Ca\left( {OCl} \right)Cl$. It is a mixed salt and is represented as $Ca{\left( {OCl} \right)_2}.\,Ca{\left( {OH} \right)_2}.CaC{l_2}.2{H_2}O$.
It is prepared by the action of chlorine on dry slaked lime . chlorine used here is obtained as a by product in the chlor-alkali process of manufacture of sodium hydroxide .
Natural colouring matters are present in silk, straw and ivory and that are eliminated during the degumming process. So the fabric made after degumming is not white but has cream colour , it is because the natural pigments are absorbed by fibroin. So chlorine based bleaching agents are such bleaching powder not used for bleaching of silk , straw and ivory since these agents tend to chlorinate the fibroin, which is used for absorbing the natural pigments.

Hence the correct options are A , B and C.

Note:
We will learn about the main uses of bleaching powder
-It is used as a disinfectant for cleaning process
-It is used for the sterilisation of drinking water
-It is used to bleach cotton fabric and wood pulp
-It is used as an oxidising agent