Bleaching of a textile removes
Answer: A
Correct answer: the natural colour of the fibre
Bleaching destroys the coloured matter native to the fibre - the pigments and flavonoids that make scoured cotton creamy rather than white. Each of the other three belongs to a different preparation step: desizing removes the warp size, scouring the waxes and pectins, and singeing the protruding surface fibres.
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The bleaching agent most widely used for cotton today is
Answer: B
Correct answer: hydrogen peroxide
Peroxide is used for the great majority of cotton bleaching. It gives a good stable white, works in the same alkaline conditions as scouring so the two can be combined, and leaves nothing objectionable behind - which is the point the next item develops.
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Match each bleaching agent with its characteristic condition or feature.
List I
A. Hydrogen peroxide
B. Sodium hypochlorite
C. Sodium chlorite
D. Sodium dithionite
List II
1. acidic conditions, corrosive and needs special plant
2. alkaline conditions, effluent free of chlorine
3. a reducing bleach used for wool and synthetics
4. mildly alkaline, cheap but forms chlorinated compounds
Answer: C
The first three are oxidising and the fourth is reducing, and each works under quite different conditions. Choosing between them turns on the fibre, the plant available and the effluent constraints. Only one of the four is a reducing agent.
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Assertion (A): Hydrogen peroxide has displaced hypochlorite for most cotton bleaching.
Reason (R): Its decomposition products are only water and oxygen, so the effluent carries no chlorinated material.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the decisive advantage. Hypochlorite bleaches effectively but forms adsorbable organic halogens in the effluent and can leave chlorine on the fibre, causing yellowing and damage later. Peroxide decomposes to water and oxygen, so the effluent carries no chlorinated species at all - and that, more than any difference in whiteness, is what displaced hypochlorite.
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Hydrogen peroxide bleaching of cotton is carried out under
Answer: A
The perhydroxyl ion is the active species and its concentration rises with pH. Too high a pH causes rapid decomposition, so the alkalinity is controlled rather than simply maximised. Temperature and time complete the recipe.
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Consider the following statements about peroxide bleaching.
1. The perhydroxyl ion is the active bleaching species.
2. A stabiliser is needed to control the rate of decomposition.
3. Higher pH always gives a better result.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: the perhydroxyl ion is the bleaching species, and a stabiliser is needed because peroxide otherwise decomposes faster than it bleaches. Statement 3 is wrong in the way it is put - raising the pH increases the perhydroxyl concentration but also accelerates decomposition, so there is an optimum band around pH 10.5 to 11, above which the peroxide is spent uselessly and the fibre is attacked.
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Sodium silicate is added to a peroxide bleach bath as
Answer: C
It moderates the release of active oxygen and also sequesters some metal ions. Its drawback is silicate deposits on machinery and fabric, which is why organic stabilisers have been developed. Silicate scale is a maintenance burden.
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Assertion (A): A stabiliser is essential in a peroxide bleach bath.
Reason (R): Uncontrolled decomposition wastes the peroxide and attacks the fibre locally.
Answer: D
Both are true and the reason gives both consequences: the bleach is spent without doing its work and the oxygen released at a metal particle causes a pinhole in the fabric. Sequestering the metals is the other half of the answer.
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All of the following help control peroxide decomposition EXCEPT
Answer: A
Correct answer: raising the temperature as high as it will go
Temperature accelerates decomposition rather than controlling it, so pushing it as high as possible spends the peroxide before it can do its work and damages the cellulose. The three genuine controls are a stabiliser to moderate the rate, sequestrants to remove the catalytic metal ions, and pH held within its working band.
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State whether the following is true or false.
Traces of iron in the bleach liquor cause local fibre damage.
Answer: B
Correct answer: True
Iron and copper catalyse the breakdown of peroxide into radicals, and because the metal sits at one point on the fabric the radicals are generated there. The result is catalytic damage - pinholes and weak spots at the site of a rust particle or a trace of hardness - while the bleach as a whole is consumed for nothing. Sequestrants and clean water are the defence.
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