Ammonia Diffusion
This Investigation should follow a demonstration of the diffusion of ammonia and
hydrogen chloride along a tube. The position and shape of the “smoke ring” so
produced indicates…
 Different gases diffuse at different speeds. This links in to the rate of
diffusion being related to molar mass.
 The shape of the ring shows ammonia to be less dense than hydrogen
chloride, since it slants in such a way as to indicate that ammonia floats
over the top of the hydrogen chloride. Leaving the ring to develop
illustrates this clearly.
 Also, observing the ring closely shows small convection currents which can
be stimulated by holding two fingers below the ring and waiting for heat to
diffuse through the glass.
This gives pupils imagination to tackle the investigation…
“Does Ammonia gas diffuse equally fast in all directions?”

The question prompts a discussion on the density of gases – brighter
pupils can calculate that ammonia is less dense than air (approximates to
nitrogen or oxygen/nitrogen mix?) Others can look up data in a book, or
simply be told.
 Diffusion works well in boiling tubes, typically taking 5 to 15 minutes to
cover the distance from the neck to base, depending on conditions.
 Progress of the gas is measured by using thin, pink litmus sensors. (Long
litmus strips give a diffuse boundary which makes taking measurements
harder.)
 Diffusion (boiling) tubes can be set up along Cartesian axes, and brighter
pupils may want to further divide the angles. This should not be
encouraged too much, especially initially.
 There is scope for a reasonable, simple theory and prediction.
Complicated or simple plans to suit pupils’ abilities are soon ready allowing
lots of results and graphs to be produced. As differences are random and
not directional, there is plenty of scope for evaluations, explanations and
suggested improvements.
 Some considerations for key factors include: amounts of ammonia solution
used, water inside the tube reabsorbing ammonia gas, holding the tubes
and making one warmer than another, ammonia already in the air…
In spite of the difficulties, this investigation provides a short, simple assessment
that even the most challenged can attempt and the brightest can score maximum
marks in 2 – 3 weeks. It also teaches that experiments do not always work the
way you expect, but for GCSE that does not matter providing it is realized that the
experiment has not worked as expected – ideally with an explanation.
Pupils will assume the ammonia advances down the tube with a flat diffusion front
perpendicular to the direction of diffusion. This is clearly not so. When difficulties
arise, a demo of dropping a few cm3 of milk into a 250 cm3 beaker of still water
illustrates the misconception and answers some of the more confusing anomalous
results.
The following represents a distillation from a few bright (A* grade) pupils - for your
ideas.
Does Ammonia diffuse equally in all directions?
Theory
Gases diffuse because their particles are moving very fast and are at very large
distances apart compared to their actual size. As a result particles of one gas can
move between and mingle with particles of another gas.
The particles spread out evenly throughout a container over time.
It is known that gases of low formula mass diffuse faster than gases of large
molecular mass, as demonstrated by allowing ammonia
Ammonia, NH3 is a gas at room temperature with a molecular mass of 17 gmole-1.
Air is a mixture of gasses at room temperature. For the purpose of this
investigation it is assumed that the composition of air is 80% N2 and 20% O2.
Therefore average molar mass of air
=
0.8 x MN2 + 0.2 x MO2
(where MN2 and MO2 represent formula masses of nitrogen and oxygen
respectively)
=
0.8 x 28 + 0.2 x 32
=
28.8 gmole-1.
Therefore the molar mass of ammonia is much less than the average molar mass
of air.
The molar volume of any gas
and pressure)
=
24.0 dm3 (at room temperature
Therefore molar density of ammonia
=
=
17.0 gmole-1 / 24.0 dm3mole-1.
0.71 g dm-3.
Therefore molar density of air
=
=
28.8 gmole-1 / 24.0 dm3mole-1.
1.2 g dm-3.
Because ammonia gas is less dense than the surrounding air, I predict that
flotation will assist diffusion in the vertically upward direction, oppose diffusion in
the vertically downward direction and have no noticeable effect on the horizontal
directions.
That is I predict diffusion upward will be faster than diffusion downward and that
horizontal diffusion will be an intermediate speed.
Method – Plan
Ammonia can be detected using damp, pink litmus paper that turns blue on
contact with ammonia.
Ammonia can be controlled conveniently by using 0.1M ammonia solution on
cotton wool. The gas escapes from the solution and diffuses through air to the
litmus paper turning it blue. The time taken for the litmus to turn blue is recorded.
By placing at least five papers at measured intervals in a boiling tube, the speed
(or rate) of diffusion can be measured along the tube.
The ammonia is made to diffuse upwards, downwards and horizontally in
separate experiments, each experiment being repeated to check measurements
and to obtain an average value.
Cotton wool with ammonia
solution
Strips of damp litmus paper stuck to
the inside of the tube by surface
tension.
cm scale (graph
paper) taped to the
glass
Upward
Diffusion
Downward
Diffusion
Horizontal
Diffusion
Key Factors to Control
The experiment was tested several times to determine the optimum conditions.
Based on preliminary tests, the following key factors to be controlled were noted.
Temperature should be the same for each test, since diffusion is due to the kinetic
motion of particles and the speed of the particles is related to temperature.
The amount of ammonia solution used (4 drops from a dropper), as more solution
means more ammonia evaporating per second and hence a higher rate of
diffusion. (Diffusion from a point is concentration or partial pressure dependant,
hence the need to control the amount of ammonia.)
Parallax, hence the need to stick a scale as close to the ammonia as possible.
Need to use dry boiling tubes, as ammonia dissolves readily in water. If the
ammonia dissolves in water drops inside the tube, the measured rate of diffusion
will be affected. (This may be discounted providing the tubes are ALL equally
damp at the start.
Use thin strips of litmus, as litmus paper changes through several shades of blue
if diffusion is slow. Thinner strips make the change sharper.
Handling of the tube needs to be minimal, as heat from the hand can cause
localised warming and hence change the rate of diffusion.
Method – Procedure
A boiling tube was cleaned with a paper towel to make sure it was dry.
Thin pieces of damp, pink litmus were placed at approximately 1.5 to 2.0 cm
intervals inside the boiling tube using a glass rod.
The tube was clamped in position and four drops of ammonia solution were
placed on a piece of cotton wool that had been glued to a rubber stopper. The
stopper was placed firmly into the boiling tube.
As soon as the first litmus turned blue, the stopwatch was started and the times
taken for other papers to turn blue were recorded.
This method was repeated for diffusion in vertically up, vertically down and
horizontal directions. Measurements for each direction were taken three times
each and an average was calculated.
A graph of distance versus average time was plotted.
Distance
/cm
Prediction - Upwards
Prediction - Horizontal
Prediction - Downwards
Average time taken /sec
Results
NOTE: The diffusion of any gas is not so simple. Some pupils will achieve results
that agree with their predictions by chance, most will not. A simple way of
illustrating the point for pupils is to show a large tube of still water in a measuring
cylinder. The cylinder represents the air in the boiling tube. By adding a few cm 3
of milk, to represent the ammonia, the milk will be seen to swirl as it travels down
the tube. Sometimes it will move down one side of the cylinder before returning
up the opposite side. Pupils often get analogous results to this with the ammonia
apparently diffusing the “wrong way”. This illustration of turbulent flow is enough
for pupils to explain any anomalous results. The turbulence is random and can be
influenced by many factors, such as holding the tube in one place, setting up
convection currents, moisture inside the tube (including the moist litmus paper)
absorbing ammonia from the air. This gives good opportunities for the student to
suggest improvements and explain anomalous results. It also has the potential to
develop into a discussion about the differences between a model of a situation
and the reality, with the need to refine models to obtain more accurate predictions.