# Why does ammonium chloride form white crystals? [closed]

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

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.

• 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?
– MaxW
Jan 29 '18 at 9:13
• @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. Jan 29 '18 at 13:19
• Almost there... but it does have a "color". It is white. Why is it not clear like a pane of glass?
– MaxW
Jan 29 '18 at 16:42
• @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. Jan 29 '18 at 16:44