The lime-soda process is used for large-scale water-softening operations. CaO is
ID: 991890 • Letter: T
Question
The lime-soda process is used for large-scale water-softening operations. CaO is added cause precipitation of iron and magnesium as Fe2Mg04 reduce the pH to 3-4 oxidize Fe^* to insoluble Fe2C>3 remove most Al^* as solid Al(OH)3 cause precipitation of magnesium as Mg(OH)2 6) Of the reactions involved in the photodecomposition of ozone (shown below), which are photochemical? 02(g) + hv - O(g) + O(g) O (g) + 02(g) + M (g) - 03(g) + M* (g) 03(g) + hv - 02(g) + O (g) O (g) + O(g) + M (g) - 02(g) + M* (g) 1 only 1, 2, and 4 1 and 3 2 and 4 all of themExplanation / Answer
Answer (E) Cause precipitation of magnesium as Mg(OH)2.
The raw water is brought to a rapid mixer where calcium hydroxide Ca(OH)2 is added to it. In the majority of cases soda ash is also added to it, for what is termed a "lime-soda ash" softening process. (On a side stream, dry lime (CaO) is "slaked" by adding water to it to create the Ca(OH)2.) This slurry is added to the water being treated to increase the pH to 10 for calcium removal or to a pH of 11 for magnesium removal. The addition of the lime and soda ash to the hard water creates a precipitate consisting of calcium carbonate and magnesium hydroxide.
Answer (C) 1 and 3 (photochemical reaction)
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