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Why is it that adding $\ce{SiCl4}$ will cause the pH of a solution to become acidic? I thought that only salts (ionic bonds break in water) can change the pH once dissolved in water. $\ce{SiCl4}$ has covalent bonds, so why does the pH change?

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Silicon is very oxophilic and $\ce{SiCl4}$ irreversibly hydrolyzes in water as follows: $$\ce{SiCl4_{(l)} + 4 H2O_{(l)} -> 4 HCl_{(aq)} + Si(OH)4_{(s)}} $$

The release of $\ce{HCl}$ creates hydrochloric acid and lowers the pH.

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  • $\begingroup$ I wonder if the reaction in water would be: $$\ce{SiCl4 + 2 H2O -> 4 HCl + SiO_{2(s)}} $$ $\endgroup$ – MaxW Dec 14 '15 at 23:25
  • $\begingroup$ Depends on concentration I think $\endgroup$ – Mithoron Dec 14 '15 at 23:37
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    $\begingroup$ $\ce{Si(OH)4}$ will reversibly dehydrate and coagulate to some extend forming a sol over time. But when you use aqueous processes to form $\ce{SiO2}$ you get a substantial amount of $\ce{Si(OH)4}$ first. $\endgroup$ – A.K. Dec 14 '15 at 23:43
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$\ce{Si(OH)4}$ exists as $\ce{SiO2.2H2O}$ called Silica Gel. $\ce{Si(OH)4}$ as such does not stand for long, probably due to overcrowding of electrons around Si atom by $\ce{4 OH-}$ groups. Hence, $\ce{Si(OH)4}$ stabilises itself as $\ce{SiO2.2H2O}$

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