Water Technology Questions and Answers

Answer: C

Correct answer: stable complex

EDTA is a hexadentate chelating agent: its four carboxylate groups and two nitrogen atoms wrap around a metal ion and bind it at six points, giving a very stable one-to-one complex. Nothing precipitates and nothing is evolved, so the titration is followed by the indicator colour change rather than by any visible reaction.

Answer: D

Correct answer: pH 12, where magnesium is precipitated as hydroxide

Raising the pH to 12 with sodium hydroxide precipitates the magnesium as its hydroxide, so it is no longer in solution to be titrated and the EDTA reacts only with calcium. Subtracting the calcium hardness from the total hardness measured at pH 10 gives the magnesium hardness, and a separate indicator such as murexide is used at the higher pH.

Answer: A

The phenolphthalein end point at about pH 8.3 accounts for hydroxide and half the carbonate, and the methyl orange end point near pH 4 completes the carbonate and the bicarbonate. Comparing the two readings identifies which species are present.

Answer: B

Carbonate is neutralised half way at the phenolphthalein end point and completely at the methyl orange end point, giving exactly that ratio. Hydroxide alone would make the two readings equal and bicarbonate alone would make the first zero.

Answer: C

The hydroxide takes a proton from the bicarbonate to give carbonate and water, so the two are consumed the moment they meet and an analysis reporting both is in error. Hydroxide with carbonate, or carbonate with bicarbonate, are both possible.

Answer: D

Correct answer: hard and adherent to the heating surface

Scale is a hard crystalline deposit bonded to the metal, formed where the salt crystallises directly on the hot surface. Sludge is a loose slurry that forms in the bulk of the water and can be removed by blowdown. The distinction matters entirely because of adherence: sludge is a nuisance, scale is a hazard.

Answer: A

Correct answer: insulates the tube and causes overheating

Scale has a thermal conductivity a small fraction of that of steel, so heat from the furnace cannot pass into the water. The tube metal beneath it rises in temperature until it loses strength and bulges or bursts under the internal pressure. Even a thin layer costs fuel; a thick one is a failure waiting to happen.

Answer: B

Correct answer: decreases with rising temperature

Calcium sulphate has retrograde solubility, so it becomes less soluble as the water is heated. It therefore reaches saturation first at the hottest surface - the tube wall - and crystallises directly on to it as a hard adherent scale. A salt whose solubility rose with temperature would precipitate in the bulk instead and form a removable sludge.

Answer: C

Violent boiling, a high water level or a sudden change in load throws water into the steam space, so the steam leaves wet and carries dissolved solids into the plant. Foaming is the related trouble caused by oil and alkali.

Answer: D

Correct answer: oil and suspended organic matter in the water

Foaming is the formation of a persistent layer of bubbles at the water surface, and it is caused by substances that lower the surface tension - oil, grease and suspended organic matter. The foam carries water over with the steam. The remedy is to remove the contaminant, by coagulation or by adding an antifoam, rather than to change the boiler conditions.