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Elements, Compounds and Mixtures

The Diffusion of Hydrogen Chloride and Ammonia Gas
through Air to form Ammonium Chloride.

Cotton wool soaked in concentrated ammonia solution, NH3(aq)
and concentrated hydrogen chloride solution
(also called 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).

The Diffusion of Hydrogen Chloride and Ammonia through Air to make Ammonium Chloride

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)          +         NH3(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 at the same temperature.

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