|MadSci Network: Chemistry|
There is such an activity series, at least in the following sense: It is possible to arrange hydrated salts in order of how strongly they hold on to their water of hydration. Some salts, like calcium sulfate dihydrate, hold on to their water of crystallization very strongly, and are quite difficult to dehydrate; others, like potash alum, tend to lose their water of crystallization to the air on dry days. For any hydrated salt, at a fixed temperature, there will be a vapour pressure of water vapour that is in equilibrium with it. If the humidity of the air drops, so that the vapour pressure falls below this value, the hydrated salt will tend to lose its water, or "effloresce". the higher this equilibrium vapour pressure is, the less strongly is the hydrated salt holding its water. In theory, if a hydrated salt A holds its water more strongly than a hydrated salt B, then if hydrated B and anhydrous A are mixed in a closed vessel, anhydrous B and hydrated A should be produced. There are many factors that make things more complicated than that. Here are a few of them: 1. Many anhydrous salts and oxides take in water by adsorption - sticking it onto the surface - as well as or instead of absorption - a change in crystal structure to form a hydrated salt. e.g. aluminium oxide, calcium chloride. 2. Many salts and oxides do not stop at the hydrated salt level, but continue to take in water from the air or from another salt until they dissolve themselves (deliquescent materials). e.g. lithium chloride, copper nitrate, sodium hydroxide. 3. Many salts form several different hydrates, with quite different affinities for water, e.g. sodium carbonate, sodium sulfate. Your other questions are not really very directly related to this sort of area, because in aqueous solution there is plenty of water around, and so there is no question of competition between salts for a limited amount of water. A solution of a salt is identical whether it was made up from the hydrated or anhydrous solid, or prepared in some other way. Nevertheless, here are some answers: barium chloride dihydrate and sodium sulfate decahydrate (or heptahydrate) can each be dissolved up in water, and the two solutions mixed to produce an anhydrous precipitate of barium sulfate. calcium formate I think does not form a hydrate; nor does ammonium sulfate. Aqueous solutions of these two salts can be mixed to produce a precipitate of calcium sulfate dihydrate. I think these two examples work to show that the answer to both of your later questions is "yes". If they do not quite fit there will be others. But I really doubt that such examples have much significance, because there is plenty of water around, not competition for a limited amount of water! Hope I haven't missed the point. John.
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