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I'm taking a first year university chemistry course, and I am reading a lot of contradicting things. Some people say that ionic compounds that are soluble in water are always electrically conductive, others say that it maybe conductive. Which one is it?

I have this problem to think about:

Suppose that an unknown chemical compound exhibits the following properties:

  • it is crystalline but shows no electric conductivity in the solid state
  • it melts at $\mathrm{300^\circ C}$ and decomposes at $\mathrm{400^\circ C}$
  • the compound is soluble in water, the solution shows no electrical conductivity.

What kind of chemical bond would you expect in the given compound? Try to describe this type of bond in detail.

Everything screams "ionic" (high melting point, solid at room temperature, crystalline structure, non-conducting in solid form) besides the fact that it doesn't conduct in water.

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    $\begingroup$ It's probably stable molecular crystal. $\endgroup$ – Mithoron Feb 20 '15 at 23:10
  • $\begingroup$ hmm, never even heard of those in the course! are all inonic compounds then conductors in water? $\endgroup$ – Arden Feb 20 '15 at 23:57
  • $\begingroup$ Covalent molecular solids can also exhibit high melting points if the molecules are large enough and have high symmetry (they pack well into a crystalline structure). See for example adamantane. Furthermore, simple ionic compounds usually don't decompose at such low temperatures. Lastly, I'd be quite interested to hear if there actually is such a thing as an ionic compound which dissolves in water but does not dissociate into electrolytes. $\endgroup$ – Nicolau Saker Neto Feb 21 '15 at 0:06
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    $\begingroup$ Melting points of 300 °C and above aren't unsual for organic compounds, think in terephthalic acid or trimesic acid. Both are soluble in water, but I'd expect some conductivity here. $\endgroup$ – Klaus-Dieter Warzecha Feb 21 '15 at 0:06
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DavePhD is right! Your material is an organic compound.

In order to be soluble in water, it should have some polar substituents. I'd however exclude carboxylic acids or phenolic $\ce{OH}$ (except maybe in the proximity of a carbonyl group) since these will partly dissociate and yield to minor conductivity.

If the melting point wouldn't be that high, inositol would be a candidate.

The nucleobases thymine and uracil do show melting points in the 300 °C range and are both soluble in water.

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All soluble ionic compound conduct according to Debye-Hückel-Onsager theory.

Think of something molecular.

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  • $\begingroup$ Oops I was only thinking of the melting point $\endgroup$ – DavePhD Feb 21 '15 at 0:15
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I wouldn't say "always". But yes, and by default, ionic compounds are usually going to conduct electricity. I would study electrolytes before trying to decide these things, if you know electrolytes, the answer would be obvious to you

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  • $\begingroup$ I don't know if you should have posted something like this as an answer. It would be really nice if you bring examples as to be the exceptions. $\endgroup$ – M.A.R. Feb 23 '15 at 14:41

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