STATES OF MATTER -
properties of gases and liquids (fluids) and solids
The gas particle model and diffusion
chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE chemistry, O level
and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
science students for national examinations in chemistry and also helpful for UK
advanced level chemistry students aged ~16-18 and US grades 11-12 K12 honors.
DIFFUSION in Gases:
- The natural rapid and random movement of the particles in
all directions means that gases readily ‘spread’ or
diffuse quite naturally without the need of mechanical mixing or adding
continues until the concentrations are uniform throughout the container of
gases, but ALL the particles keep moving with their ever present kinetic energy!
- The net movement of a particular gas will be in the direction
from a region of higher concentration to a region of lower concentration for a
particular molecule, down the so–called
A long glass tube (2–4 cm diameter) is filled at one
end with a plug of cotton wool soaked in conc. hydrochloric acid
sealed in with a rubber bung (for health and safety!) and the tube is
kept perfectly still, clamped in a horizontal position. A similar plug of
conc. ammonia solution
is placed at the other end. The soaked cotton wool plugs will give off
fumes of HCl
and NH3 respectively,
and if the tube is left
undisturbed and horizontal, despite the lack of tube movement, e.g. NO
shaking to mix and the absence of convection, a white cloud forms about
along from the conc. hydrochloric acid tube end.
Explanation: What happens is the colourless
gases, ammonia and hydrogen chloride, diffuse down the tube and
react to form fine white crystals of the salt ammonium chloride.
+ hydrogen chloride
- Diffusion is faster in gases than
liquids where there is more space for them to move
(experiment illustrated below) and
diffusion is negligible in solids due to the close packing of the particles.
- Diffusion is responsible for the
spread of odours even without any air disturbance e.g. use of perfume,
opening a jar of coffee or the smell of petrol around a garage.
- The rate of diffusion increases with increase in temperature as the particles
gain kinetic energy and move faster.
- Other evidence for random particle
movement including diffusion:
- When smoke particles are viewed under a
microscope they appear to 'dance around' when illuminated with a light
beam at 90o to the viewing direction. This is because the
smoke particles show up by reflected light and 'dance' due to the
millions of random hits from the fast moving air molecules. This is
called 'Brownian motion' (see
also in liquids). At any given instant of time,
the particle hits will not be evenly distributed over the surface, so the smoke particle get a greater bashing
in a random direction and then another, so they appear to dance and
zig-zag around at random.
An experiment to
illustrate diffusion in gases
- A two gaseous molecule
diffusion experiment is illustrated above and explained below!
Note the rule: The smaller the
molecular mass, the greater the average speed of the molecules
(but all gases have the same average kinetic energy at the same
NH3(g) + HCl(g)
- Therefore the smaller the
molecular mass, the faster the gas diffuses.
- e.g. Mr(NH3)
= 14 + 1x3 = 17, moves faster
than Mr(HCl) =
1 + 35.5 = 36.5
- AND that's why they meet nearer
the HCl end of the tube!
- So the experiment is not only evidence
for particle movement, it is also evidence that molecules
molecular masses move/diffuse at different speeds.
other page for a
treatment of Graham's Law of Diffusion
simple demonstration of diffusion with a coloured gas
gas, heavier than air (greater density), is put into the
bottom gas jar and a second gas jar of lower density colourless air is placed over it separated with a
glass cover. Diffusion experiments should be enclosed at constant temperature to
minimise disturbance by convection.
If the glass cover is removed
then (i) the colourless air gases diffuses down into the coloured brown
gas and (ii) bromine diffuses up into the air. The random particle movement
leading to mixing cannot be due to convection because the more dense gas starts at the
No 'shaking' or other means of mixing is required. The
random movement of both lots of particles is enough to ensure that both gases
eventually become completely mixed by diffusion (spread into each other).
This is clear evidence for diffusion
due to the random continuous movement of all the gas particles and,
initially, the net movement of one type of particle from a higher to a
lower concentration ('down a diffusion gradient'). When fully mixed, no further colour change
distribution is observed BUT the random particle movement continues! See
also other evidence in the liquid section after the particle
model for diffusion diagram below.
A particle model of diffusion in gases:
Imagine the diffusion gradient from left to
right for the green particles added to the blue particles on
So, for the green particles, net migration is from left to right
(from a higher to a lower concentration) and will continue, in
a sealed container, until all the particles are evenly
distributed in the gas container (as pictured). The particle
motion continues, but there is no change in concentration
throughout the mixture.
faster in gases compared to liquids/solutions because there
is more space between the particles for other particles to
move into at random.
See also advanced section 23.
Graham's Law of Diffusion
importance of diffusion and gas exchange in living organisms
For plant gas exchanges and photosynthesis
What is the
chemical process of photosynthesis?
Plant structure and photosynthesis - leaf adaptations
gas/nutrient exchanges see
Gas exchange in
the human lungs by diffusion, comments on breathing, COPD and ventilators
Gas exchange and the structure of fish gills
The function of villi in the exchange
surface of the small
structure adaptations in other animals
More on transport systems in plants and
A particle model and factors
affecting the rate of diffusion and Fick's Law of diffusion
The action of
partially permeable cell
membranes - selective diffusion and examples
Osmosis - examples and explanation
Some details of examples of osmotic action in individual animal or plant cell types
Osmosis experiments - demonstrations of osmotic action
with diffusion in gases
Be able to draw particle pictures to
illustrate and explain diffusion in gases.
Be able to describe and explain what diffusion is in gases using the
kinetic particle model.
Know that the net migration of gaseous particles due to their random
motion is from a region of higher concentration to a lower concentration.
Be able to interpret the tube experiment where ammonia and hydrogen
chloride gases are allowed to diffuse towards each other.
Be able to describe, interpret observations and why bromine vapour
diffuses into air, noting and explaining the even colour of the mixture in
Be able to describe the importance of diffusion of gases in and out
through the stomata of plant leaves i.e. the gas exchange of oxygen and
carbon dioxide in the process of photosynthesis.
Know the importance of
substance exchanges in the organs of plants and animals.
UK GCSE level (~US grade 8-10) school chemistry revision
All my UK advanced level (~US grades 11-12)
pre-university chemistry revision notes
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INDEX of all my notes on the states of matter
(GCSE level and advanced pre-university level
notes on the states of matter and their properties