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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: ceraceanuboss on February 15, 2019, 03:12:37 AM

Title: Some melting points asap
Post by: ceraceanuboss on February 15, 2019, 03:12:37 AM

So basically tomorrow is the Chemistry Olympiad and I have one question that my teacher could not answer me:how can you put in order substances (ionic substances, acydes, etc) having the criteria the melting point (for eg:MgF2, NaCl, C(graphyte), CaF2, PbCl2, NiF2). Thanks!

Title: Re: Some melting points asap
Post by: AWK on February 15, 2019, 09:15:11 AM

Wrong question - no answer. Graphite does not melt - it sublimes. Graphite melts only at very high pressure and cannot be compared with other your compounds.

Textbooks inform (the same normal pressure is presumed) that covalent compounds show the lowest melting points, then ionic compounds, and the highest m.p. show eventually covalent networks (if exist at the temperature of melting - may decompose earlier, may sublime at normal pressure, and so on).

You can put in order, eg sodium halides (m.p.) NaF > NaCl > NaBr > NaI,
halides of the same group of the periodic table (but the group should contain only metals or non-metals) and you should compare each halogen separately, I mean - only chlorides, only bromides and so on).
Even then you may encounter some exceptions.