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According to my chemistry textbook, an acid is a compound that gives hydronium ion on dissolving in water but amphoteric compounds like ZnO do not yeild hydronium ion on dissolving in water . I understand that they give salt and water on reacting with a base but still do not satisfy the definition then how can they said to be to also act like an acid?

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  • $\begingroup$ This confusion better should have been resolved before turning to amphoteric compounds. Look at H2SiO3: it's an acid, and so should give H+ on dissolving in water, but wait, it doesn't dissolve in water at all! Ditto for bases like Mg(OH)2 giving OH-. $\endgroup$ – Ivan Neretin Mar 19 '19 at 7:20
  • $\begingroup$ Technically, magnesium hydroxide does dissolve measurably in water. Milk of magnesia will turn red litmus indicator blue. The solubility of the hydroxide is sufficient to displace heavy metals in water and soil treatment. $\endgroup$ – Oscar Lanzi Mar 19 '19 at 14:38
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The trouble is that "amphoteric", when applied to oxides and sulfides, usually means: "what does the substance dissolve in?". (In the case of $\ce{ZnO}$, it would form either some $\ce{Zn^{2+}}$ complex when dissolved in an acid or $[\ce{Zn(OH)4]^{2-}}$ in a strongly basic solution.) This use must mentally be distinguished from "amphoteric" as applied to e.g. $\ce{NaH2PO4}$, which can accept and donate $\ce{H}^+$ ions (and their compounds with one, two or more water molecules).

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