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Your proposed reaction is unlikely That is because the combination $\ce{CaF2 + H2O}$ (under tap water conditions) is much more stable than $\ce{CaO + 2 HF}$. In other words, the electronic energy of the former is lower and the system preferably stays in this state. That does not mean, however, that the reaction does not occur! It just means that the ...
Waste water, if not too polluted, is treated in wastewater treatment plants by adding aluminum salts like aluminum sulfate $\ce{Al2(SO4)3}$ in order to remove bacteria. But this solution is acidic, because aluminum sulfate produces aluminum ions that are partly hydrolyzed according to the equation $$\ce{Al^{3+} + H2O -> [Al(OH)]^{2+} + H+}$$ This acidity (...