Before we start, when sugar is dissolved in water, the sugar molecules diffuse in the gaps between the water molecules, which we understand as dissolving.

You might like to look at this animation. It illustrates what happens when a perfume bottle or room freshener is opened.

IG students look a little more deeply. To show that larger molecules diffuse more slowly than smaller ones, we use a long horizontal glass tube with stoppers fitted at both ends. Using tongs we dip one piece of cotton wool into concentrated hydrochloric acid and another piece into concentrated ammonia solution. This is nasty stuff, so we use a fume cupboard. Drain off excess liquid. Simultaneously, we put the soaked pieces of cotton wool inside the ends of the glass tube. Close the ends of the glass tube with the stoppers. Watch for a white ring forming where the ammonia gas and the hydrogen chloride gas meet after diffusing through the air towards each other.

Ammonia molecules are less massive therefore faster moving than heavier hydrogen chloride so the white ring of ammonium chloride should form nearer to the hydrogen chloride end of the glass tube.

The reaction is as follows:

hydrogen chloride    +    ammonia   <=>     ammonium chloride.
HCl(g) +     NH3(g) <=>           NH4Cl(s)

(The reaction is reversible, so the arrows go both ways)

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