The FDA Code requires that water supply systems include a backflow or back-siphonage prevention device
pipage composed only of PVC or other plastic materials
regularly spaced emergency shutoff valves
a method of aerating the water at designated checkpoints

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the answer is barium and copper bromide

a). in this case, dh> 0 and sd> 0. if t becomes larger, the term "-tds" will become more negative. at some point, it is possible that -dts will be more negative than the dh is positive, and the reaction will become spontaneous. this is probably the correct answer, but you should look at all of the other possibilities and make sure of that.