Diffusion of gases - ammonia and hydrogen chloride
Concentrated ammonia solution is placed on a bud in one end of a tube and concentratedhydrochloric
acid on a bud at the other. After about a minute the gases diffuse far enough to meet and a ring of
solid ammonium chloride is formed.
Apparatus and chemicals
For one demonstration:

Eye protection (goggles)
 Protective gloves
 A length of glass tube about half a metre long with an inside diameter of about 2 cm
 Cotton wool buds, 2
 Bungs with small hole to hold cotton buds
 Concentrated hydrochloric acid (Corrosive), a few mL
 880 ammonia solution (Corrosive, Dangerous for the environment), a few mL
Abstract
Procedure
The demonstrator should wear goggles and protective gloves.
1. Put the end of one of the cotton buds (held in the bung) into the ammonia solution. Push the
bung into one end of the tube. Replace the lid on the bottle of ammonia.
2. Repeat this procedure quickly with the second bung/cotton bud and the hydrochloric acid. Put
this into the other end of the tube. Replace the lid on the bottle of hydrochloric acid.
3. Watch the tube and observe a ring of white powder forming near the middle of the tube. This
is ammonium chloride.
Teaching notes
The reaction which is taking place is:
ammonia + hydrogen chloride → ammonium chloride
NH3 (g) + HCl (g) → NH4Cl (s)
The exact time taken for the ring to form will depend on the dimensions of the tube, the amount of the
solutions which are put on the cotton wool buds and the temperature of the room. The ring usually
forms nearer to the hydrochloric acid end of the tube because hydrogen chloride diffuses more slowly
than ammonia. This is because hydrogen chloride has almost twice the molecular mass of ammonia,
and the rate of diffusion is inversely proportional to the square root of the molecular mass of the gas. It
is worth noting that the rate of diffusion is not the same as the speed at which the gas molecules travel
(which is hundreds of meters per second). The gas molecules follow a zigzag path through the tube as
they collide with the molecules of the gases in the air that are present in the tube. The purpose of the
glass tube is to eliminate air currents and to see if the gas molecules will move on their own.
Graham's Law of Diffusion Calculator
The equation at the top of this page is Graham's Law of Diffusion, named after the Scottish chemist
Thomas Graham (1805 - 1869). It states that the diffusion rate of a gas is inversely proportional to the
square
root
of
its
molar
mass
(molecular
weight).
Stated in simpler terms, lighter gases (in terms of density or molecular weight) will diffuse faster than
those gases with higher densities or molecular weight.
Diffusion is the process in which one gas distributes itself within another, such as the way perfume is
disseminated throughout a room. Effusion is the manner in which a gas escapes from a container. All
containers have extremely small "holes" in them through which a gas may escape such as the way
helium
"escapes"
from
a
balloon
in
just
a
few
days.
Graham's Law of Diffusion applies to both the diffusion and effusion rates of gases.