Hardness in water is caused principally by dissolved
Answer: A
These two ions form insoluble compounds with soap and with many dyes and finishing agents. Sodium and potassium salts raise the dissolved solids but do not cause hardness in the technical sense. Soap and many dyes react with them directly.
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Temporary hardness differs from permanent hardness in that it
Answer: B
Bicarbonates decompose on heating and deposit as carbonate scale, which is why kettles fur. Sulphates and chlorides of the same metals persist through boiling and make up the permanent part. Both kinds must be dealt with in a mill.
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Assertion (A): Temporary hardness is the more troublesome kind in a boiler.
Reason (R): Bicarbonates decompose on heating and deposit scale directly on the heated surfaces.
Answer: C
Both are true and the reason gives the mechanism: the scale forms exactly where the heat transfer occurs, so it insulates the surface and can lead to overheating of the metal. Feed water treatment is the standard defence.
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Match each water impurity with the problem it causes in a textile mill.
List I
A. Calcium and magnesium
B. Iron
C. Copper
D. High alkalinity
List II
1. catalytic degradation during peroxide bleaching
2. insoluble lime soaps and dye precipitates
3. shifts the pH of dyebaths away from specification
4. yellow or brown stains and catalytic fabric damage
Answer: D
Each impurity affects a different stage of processing, so a single specification cannot cover them all. Iron and copper matter far below the levels at which hardness becomes a problem. Each has its own place in the specification.
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Iron in process water is objectionable chiefly because it
Answer: A
A few parts per million produce visible yellow marks and local weakening of the fabric where the peroxide decomposes. The specification for iron is far tighter than that for hardness. Pipework and storage are common sources of it.
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Assertion (A): Traces of iron or copper are far more damaging in peroxide bleaching than an equivalent amount of calcium.
Reason (R): They catalyse the decomposition of peroxide, so it is spent uselessly and attacks the fibre locally.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Iron and copper are transition metals that catalyse the breakdown of hydrogen peroxide into radicals, so the bleach is consumed before it can do its work and the radicals attack the cellulose at the point where the metal sits - producing pinholes and catalytic damage. A few parts per million matter, where calcium is tolerated at far higher levels.
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Water contains 100 milligrams per litre of calcium carbonate equivalent hardness. Expressed in degrees of French hardness, where one degree is 10 milligrams per litre, what is that?
Answer: C
Correct answer: 10 degrees
\[ \frac{100}{10} = 10\ \mathrm{degrees\ French}. \] Hardness scales are simply different divisors of the same milligrams per litre of calcium carbonate equivalent: one French degree is 10 mg/L, one German degree about 17.8 and one English degree about 14.3. Quoting a hardness without naming the scale is therefore meaningless.
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Consider the following statements about expressing hardness.
1. It is usually quoted as calcium carbonate equivalent.
2. Several national degree scales exist and differ from one another.
3. A hardness figure is meaningful without stating its unit.
Answer: D
The first two are correct. The third is exactly the trap, since French, German and English degrees are all different and a bare number can be wrong by a factor of two. Every specification should name its scale explicitly.
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Ion exchange softening replaces calcium and magnesium with
Answer: A
Correct answer: sodium ions from the resin
The resin is charged with sodium, and as hard water passes through it the calcium and magnesium are held on the resin while sodium is released in their place. When the resin is exhausted it is regenerated with brine, which drives the exchange back the other way and flushes the hardness to drain.
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State whether the following is true or false.
Ion exchange softening reduces the total dissolved solids of the water.
Answer: B
Correct answer: False
Nothing is removed - ions are swapped. Each calcium ion leaving is replaced by two sodium ions, so the total dissolved solids stays much the same and may rise slightly by mass. What changes is which ions are present, and that is enough for textile purposes because sodium salts do not precipitate with soap or scale on the fibre. Reducing the total dissolved solids requires reverse osmosis or demineralisation.
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