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Why does ammonium chloride ($\ce{NH4Cl}$) form white crystals on top of a test tube when heated?

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I suppose you are not asking why crystalline $\ce{NH4Cl}$ is white.

Ammonium chloride decomposes upon heat $$\ce{NH4Cl(s) ->[\Delta] NH3(g) + HCl(g)}$$

The top of the test tube is cold, and they recombine to generate the original salt. $$\ce{NH3(g) + HCl(g) -> NH4Cl(s)}$$

This is the white crystal you see.

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  • $\begingroup$ RE: "I suppose you are not asking why crystalline $\ce{NH4Cl}$ is white." // That is the part that you didn't explain... So why white? $\endgroup$
    – MaxW
    Jan 29 '18 at 9:13
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    $\begingroup$ @MaxW Because there are no chromophores (which account for the color in organic dyes) or $d-d$ $f-f$ transitions (which account for the color in metal aqua ions for transitional metals) or charge-transfer transitions (which account for the color in e.g. $\ce{MnO4^-}$). These things simply can't occur in such simple structure, and that's why simple ions of the main group elements and a molecule simple as $\ce{NH4^+}$ does not have color, even when hydrolyzed. $\endgroup$ Jan 29 '18 at 13:19
  • $\begingroup$ Almost there... but it does have a "color". It is white. Why is it not clear like a pane of glass? $\endgroup$
    – MaxW
    Jan 29 '18 at 16:42
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    $\begingroup$ @MaxW It can be if it is pure and compose of a single crystal. However when it is composed of huge amount of very small crystals they will show white color. It is the same for $\ce{NaCl}$ crystal carefully grown in its saturated solution vs. $\ce{NaCl}$ powder. $\endgroup$ Jan 29 '18 at 16:44

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