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Elements, Compounds and Mixtures

Diffusion of Hydrogen Chloride and Ammonia.

Cotton wool soaked in concentrated ammonia solution, NH_3(aq)
and concentrated hydrogen chloride solution (hydrochloric acid) HCl_(aq)
are placed at each end of a sealed tube.
The cotton wool with ammonia solution gives off ammonia molecules (NH₃).
The cotton wool with hydrochloric acid gives off
hydrogen chloride molecules (HCl).

HCl and NH₃ molecules diffuse through the air towards each other.
When they meet, they react to form  a white powder
called ammonium chloride, NH₄Cl.

hydrogen chloride    +    ammonia        ammonium chloride.
HCl_(g)    +     NH_3(g)                 NH₄Cl_(s)

The sign shows that the reaction is reversible.
The ring of white powder is closer to the HCl than the NH₃.
This is because the NH₃ molecules are lighter (smaller)
and have diffused more quickly through the air in the tube
(you can work out which molecule is lighter by looking at the RFM).

Note that lighter (smaller) particles move more quickly
than heavier (larger) ones.

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Copyright © 2008 Dr. Colin France. All Rights Reserved.