Water Technology Questions and Answers

Answer: A

Correct answer: calcium and magnesium

Hardness is the property that prevents lather forming with soap, and it is caused by dissolved divalent cations, in practice almost entirely calcium and magnesium. Sodium and potassium salts are soluble and do not interfere; iron and manganese occur in far smaller amounts. The two hardness ions are also the ones that deposit scale on heating, which is why the same measure serves both problems.

Answer: B

Correct answer: bicarbonates of calcium and magnesium

Temporary hardness is the part removed by boiling, and it is due to the bicarbonates, which decompose on heating to give the insoluble carbonate, carbon dioxide and water. The sulphates and chlorides are unaffected by boiling and constitute the permanent hardness - which is why the two are treated by different chemicals in the lime-soda process.

Answer: C

These salts are unaffected by heat and remain in solution however long the water is boiled, so they must be removed chemically by lime-soda, zeolite or ion exchange. It is for that reason also called non-carbonate hardness.

Answer: D

Correct answer: calcium carbonate

A water contains several different hardness-producing salts, and expressing each as the mass of calcium carbonate that would cause the same hardness makes them additive, so a single figure describes the water. Calcium carbonate was chosen because its molar mass is a convenient 100 and it is the substance actually deposited as scale.

Answer: A

Correct answer: 100 divided by its own molar mass

The conversion scales the mass in the ratio of the two chemical equivalents, and since calcium carbonate has a molar mass of 100 with two equivalents per mole, that ratio comes out as 100 divided by the molar mass of the salt for any divalent hardness salt. So 111 mg/L of calcium chloride, whose molar mass is 111, is exactly 100 mg/L as calcium carbonate.

Answer: B

One part in a million by mass is one milligram in a kilogram, and a litre of water weighs essentially a kilogram at ordinary temperatures. The two units are therefore used interchangeably in practice, and both express the hardness as calcium carbonate.

Answer: C

Correct answer: 1 grain of CaCO\( _3 \) per gallon of water

The Clark degree is the old British unit, defined as one grain of calcium carbonate per imperial gallon, which works out at about 14.3 mg/L. It survives in older texts and in some water industry usage. The modern practice is to quote hardness directly in milligrams per litre as calcium carbonate, which avoids the conversion entirely.

Answer: D

It is defined as one part of calcium carbonate in a hundred thousand parts of water, which works out at ten milligrams per litre. Converting between parts per million, Clark and French degrees is a routine step in any hardness calculation.

Answer: A

Correct answer: Eriochrome Black T

Eriochrome Black T forms a wine-red complex with calcium and magnesium ions. As EDTA is added it takes the metal ions away, because the EDTA complex is the more stable, and at the end point the free indicator is released, turning the solution steel blue. Phenolphthalein and methyl orange are acid-base indicators and starch is for iodine.

Answer: B

Correct answer: the metal-EDTA complexes are unstable

EDTA is a weak acid, and at low pH its donor groups are protonated so it cannot chelate effectively - the metal complexes dissociate and the end point is lost. A pH 10 ammonia buffer keeps enough of the EDTA deprotonated for the complexes to be stable, while remaining low enough that magnesium hydroxide does not precipitate.