School Science Lessons
10. Molecular motion, diffusion, osmosis, molecular spacing
2012-05-04bb SPP
Please send comments to: J.Elfick@uq.edu.au
Table of contents
10.0.0 Molecular motion
10.1.0 Diffusion, particles of matter
9.14.0 Osmosis
10.3.0 Molecular spacing
12.4.0 Siphons
10.1.0 Diffusion, particles of matter
3.55 Brownian movement
8.1.1.0 Candle, paraffin wax (diffusion
flame)
3.58 Clay soil suspension
9.12.0 Colloids, diffusion, semipermeable
membrane, dialysis
3.7 Congo red, microscopy stain
10.0.1 Diffusion experiments
10.1.3 Diffusion in liquids, potassium manganate
(VII) or potassium dichromate
10.1.2 Diffusion of ammonia and hydrogen chloride
gases
10.1.1 Diffusion of carbon dioxide
3.55.1 Diffusion of heavier than air
gas, carbon dioxide
3.55.3 Diffusion of liquids
10.0.2 Graham's law of diffusion
3.55 Matter as particles
3.3.3 Measure the size of the stearic
acid molecule
3.56 Particles of matter and dilution
3.57 Size of a molecule
3.3.0 Sizes of particles of matter
19.4.2.1 Stain removal
10.3.0 Molecular spacing
24.0.0 Change of state
10.3.4 Container holds more
10.3.3 Container not leaking
12.3.5.2 Inverted drinking glass
3.4.1.1 Stretched rubber bands
3.55 Matter
as particles
10.3.5 Shrinking balloons
10.3.2 Shrinking mixture of liquids
10.3.1 Shrinking solution
3.57 Size of a molecule
12.4.0 Siphons
12.4.0 Siphons
12.4.1.0 Simple siphon
2.12 Siphon and water spray (Primary)
12.4.1.1 Siphon fountain
12.4.1.4 Siphon in a bell jar
12.4.1.2 Siphon replaces water for fish tank
12.4.1.5 Siphon mechanism apparatus
12.4.1.9 Mariotte flask and siphon
12.4.1.7 Mechanical siphon
12.4.1.6 Pressure in a siphon
12.4.1.8 Self-starting siphon
See pdf
Self-siphoning gel, Poly-Ox, polyethylene oxide
12.4.1.10 Turnover siphon
12.4.1.3 U-tube siphon
10.0.1 Diffusion experiments
Diffusion is the mutual penetration of molecules of substances in contact.
In diffusion, the
molecules of a substance, being in permanent motion, penetrate
into the spaces between
the molecules of another substance, which is in
contact with the first substance, and are
distributed among them. Molecular
motion causes the concentration of a substance in
an in homogeneous material
to become homogeneous. Diffusion always occurs from high
concentration
to low and diffusion occurs at a faster rate when the difference between
concentrations is larger. The smaller the molecules, the faster the rate
of diffusion.
The rate of diffusion increases with temperature. Diffusion
occurs in all the states of
aggregation but to different extents. You can
observe diffusion in gases easily. Diffusion
in liquids occurs at a much
slower rate than in gases. Diffusion in solids occurs at a still
slower rate
than in liquids.
Open bottles of perfumes or other aromatic substance at the end of the
room with all
windows and doors shut. Note when you can first smell the
substances and estimate
the rate of diffusion in air per metre at that temperature.
10.0.2 Graham's law of diffusion
The rates of diffusion of particles of gas are inversely proportional
to the square root of
the molar mass of the particles.
Rate of diffusion A / Rate of diffusion B = √ molar mass B / √ molar
mass A
10.1.1 Diffusion of carbon dioxide
See diagram 3.34.1: Limewater test for carbon
dioxide
1. Fill a jar with carbon dioxide and invert it over a similar jar full
of air. After a few moments
separate the jars, pour a little lime water
in the lower one and shake it. The limewater will
turn milky indicating that
the carbon dioxide has fallen into the lower jar because it is the heavier
gas. Now repeat the experiment but this time put the carbon dioxide in the
lower jar and invert
a jar of air over it. If the jars are left for about
five minutes, some carbon dioxide will be carried
into the upper jar by diffusion,
in the same way some air will be carried into the lower jar. The
limewater
test shows the presence of carbon dioxide in the upper jar.
2. The molecules of gases are always in a random motion. They may not
only move downwards
under the gravity to form distribution according to concentrations,
but also diffuse in every
direction. Use a wide and thin test-tubes so that
the thin test-tube just fits into the wide test-tube.
3. Fill the thin test-tube
with carbon dioxide gas and fill the wide test-tube full of air. Insert the
open end of the thin test-tube into the wide test-tube. Let them stand vertical.
After some minutes,
separate them quickly. Add some limewater into the wide
test-tube and shake it gently. Note the
change in colour. The solution changes
from clear into muddy and finally becomes milk white.
It shows some carbon
dioxide gas has entered the thick test-tube because denser carbon dioxide
molecules go down from the thin test-tube into the wide test-tube under
the effect of gravitation.
3. Fill the wide test-tube with carbon dioxide
gas and inflate the thin test-tube full of air. Insert the
open end of the
thin test-tube into the wide test-tube and let them stand vertical. After
some minutes,
separate them then quickly add some limewater into the thin
test-tube and shake it gently. Observe
the change in colour at the thick
one. The solution changes from clear into muddy and becomes milk
white liquid
finally. It shows some carbon dioxide gas has entered the thin test-tube
because carbon
dioxide molecules in random motion diffuse in various directions
including upward and downward.
So some carbon dioxide molecules go up from
the thick test-tube into the thin test-tube.
10.1.2 Diffusion rates of
ammonia and hydrogen chloride gases
See diagram 10.1.2: White ring of ammonium
chloride
Be careful! Only teachers should do this experiment
because hydrochloric acid solution and
ammonia water are strongly corrosive
solutions.
1. Use a long horizontal glass tube with stoppers fitted at both ends.
Use tongs to dip one piece
of cotton wool into concentrated hydrochloric
acid and another piece into concentrated
ammonia solution. Drain off excess
liquid. Simultaneously, 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 is less
dense than hydrogen chloride so the white
ring of ammonium chloride should
form nearer to the hydrogen chloride end of the glass tube.
2. The long glass tube should be horizontal. Corks should fit at both
ends. Using a pair of tongs
or tweezers, dip a piece of cotton wool into
concentrated hydrochloric acid and dip another
piece into concentrated ammonia
solution, NH3 (aq) ("ammonium hydroxide") solution. Drain off
excess liquid. Simultaneously, put the ammonia in cotton wool in one end
of the tube and the acid
in cotton wool in the other end. Close the ends
of the tube with corks. Later, look for a white ring
that will form where
the ammonia gas and the hydrogen chloride gas meet after diffusing through
the air towards each other. Ammonia is the less dense gas and the white ring
of ammonium chloride
should form nearer to the hydrogen chloride end than
from the ammonia end of the tube.
3. Different gases diffuse at different rates at the same temperature.
Use a glass tube 1 metre long and
2 cm diameter, open at each end, two stoppers,
two identical balls of cotton wool, hydrochloric acid,
ammonia, water. Place
the glass tube flat on a table. Immerse one cotton wool ball in hydrochloric
acid solution. Immerse the other cotton wool ball in ammonia solution. Take
them out of the solutions
and press them until no liquid drops. Insert the
cotton wool balls into each end of the glass tube
simultaneously and instantly
insert the stoppers so that air cannot enter the tube. Observe the position
of the white circle band of ammonium chloride formed by diffusion of two
gases. Ammonia molecules
diffuse at a faster rate than hydrochloric acid
molecules so the two kinds of molecules do not meet at
the middle of the
glass tube.
10.1.3 Diffusion in liquids
See diagram 10.1.3: Diffusion in liquids
1. Place a crystal of potassium dichromate, potassium dichromate (VI),
or ammonium dichromate
at the bottom of a beaker of water. To do this, put
a glass tube into the beaker of water so that it
touches the bottom, then
to drop the crystal down the tube. Close the top of the tube with your
finger and remove the tube gently, leaving the crystal in the beaker. The
colour of the dissolving
crystal will spread throughout the water in quite
a short time.
2. Fill a very small open bottle with a strong solution of potassium permanganate,
potassium
manganate (VII). Place this in a larger jar. Fill the larger
jar very carefully by pouring water down
the side until the water level is
above the top of the small bottle. Leave this for a few days. The
potassium
permanganate solution diffuses evenly through the water.
3. Use two beakers containing water at 90oC and at room temperature.
Put the same quantity of
potassium dichromate crystals into each beaker.
Note in which beaker the rate of diffusion of
potassium dichromate in water
is faster. Use a glass tube opened at both ends. Close one end
with a stopper.
Then place the tube in water, the open end up. Put potassium dichromate
crystals
in the glass tube then gently tap it so that all the potassium dichromate
falls to the bottom. Hold the
glass tube in a beaker of water with your left
hand to keep it upright and not touch the bottom.
With the right hand, hold
a glass rod, longer than the glass tube, in a vertical position so you can
push down on the cork and remove it from the glass tube. Note how the colour
produced by the
dissolving crystals of potassium dichromate diffuse through
the water. Fill a small bottle with
potassium permanganate solution then
put it in an empty beaker. Fill the container with water so
that the water surface just reaches the top of the bottle. After some days the potassium
permanganate solution diffuses completely through the water.
4. Use forceps to place a crystal of lead nitrate and a crystal of potassium
iodide in a Petri dish of
deionized water. Observe the crystals dissolving
then forming a yellow lead iodide solid between
them. The solid forms
closer to the lead nitrate crystal because the iodide ion moves the fastest.
PbNO3 + KI --> KNO3 + PbI
10.3.1 Shrinking solution
See diagram 10.3.1: Shrinking solution
1. Put about 30 mL sodium chloride crystals into a measuring cylinder.
Add water until the
measuring cylinder is exactly full. After a few minutes,
note that as the sodium chloride dissolves
the liquid level drops.
2. Repeat the experiment using sucrose crystals instead of sodium chloride
crystals. The liquid
level does not drop. The sodium chloride crystals
dissolve to form ions that can fit between the
water molecules. The sucrose
crystals dissolve to form sucrose molecules that are much bigger
than the
sodium ions or chloride ions.
10.3.2 Shrinking mixture of liquids
1. Half fill a test-tube with water. While holding this test-tube at an
angle, pour ethanol slowly
from a beaker until the test-tube is full. Hold
the test-tube by placing your thumb on the mouth
of the tube so that no
air bubble is trapped. The test-tube appears to be full. Invert the test-tube
several times while keeping thumb on the opening. Do not release the pressure.
The liquid level
becomes lower. The alcohol or water did not evaporate
and no liquid spilt because of inverting
the test-tube. By inverting the
test-tube, mixing of water and alcohol occurs and the alcohol
molecules slip
in between the water molecules in the spaces between the molecules, thus
making
the total volume of the mixture become less. The spaces between the
molecules cannot be seen
by the naked eye.
2. Repeat the experiment with methanol or propan-2-ol (rubbing alcohol,
isopropyl alcohol).
10.3.3 Container not leaking
Fill a measuring cylinder by 1 / 3 with water softener pellets (Calgon,
sodium hexametaphosphate)
or with table salt (sodium chloride). Add water
until full and mark the liquid level with a grease
pencil or rubber band.
Leave it to stand for 5 minutes and note the liquid level. The drop in water
level is not because water is absorbed by the salt. Pour out some water
before all the salt dissolves
and refill it with fresh water. The water
level drops again because the crystals of the dissolving salt
breaks down
into ions that can slip in between the water molecules and make the total
volume
decrease. Repeat the experiment with other salts that dissolve in
water and sucrose. The sugar
molecule is large and does not ionize when dissolved
in water, so that the water level will not drop.
10.3.4 Container holds more
Fill a large beaker with marbles and note the top level of the marbles.
Slowly add sand to the
beaker while tapping to make the sand settle between
the marbles. Tip out the sand water and
measure its volume. Replace the
sand in the beaker of marbles. Add water to the marbles and
sand. Tip out
the water and measure its volume.
10.3.5 Shrinking balloons
Inflate 3 balloons fully. Let about half the air out of one balloon. Let
out about one quarter of the
air out from another balloon. Measure the
diameter of the three balloons. Leave the balloons
and the next day measure
the diameters again. Compare the diameters of the three balloons.
The full
balloon shrinks faster than the half-filled balloon than the quarter filled
balloon. The rate
of shrinkage depends on the pressure in the balloon.
12.4.0 Siphons
See diagram 12.4.1.0: Difference in heights of water columns
See diagram 12.4.05: Two siphon systems and as syringe
Siphons are tubes used to move liquids from a higher container to a
lower container, e.g. you
can siphon gasoline out of a gasoline tank in
a motor car. Pumps are machines used to move
liquids or gases.
1. You can define the siphon, as a pipe system consisting of two legs
as an inverted J, used to carry
a liquid from one vessel to another vessel
at a lower level, over an intermediate level higher than both
levels. When
both legs of the siphon are full, the hydrostatic force due to gravity is
larger on the longer
leg, thus causing the liquid to move up the shorter
leg, over the bend, and down the longer leg. Start
the siphoning process
by:
1.1 filling a siphon with liquid before placing it into its operating
location, or
1.2 by applying suction at the lower end after the tube is in
position. Once started, the flow will continue
until the liquid level in
both vessels is equal, or until the level in the higher vessel falls below
the inlet of
the tube when air is sucked in and the siphoning action stops.
2. There is more than one explanation of how the siphon works
Explanation
1: The forces of cohesion, surface tension between water molecules, allows
the water in
the short side of the siphon to be pulled up the tube by the
greater weight of the water in the long side.
The siphon acts in much the
same way as the longer end of a chain hanging out of a bucket can pull the
rest of the chain from the bucket.
Explanation 2: Both surface tension and
atmospheric pressure are required to make the siphon work.
Gravity pulls down
on the water in both sides of the siphon. However, because there is more water
in the long side, the weight is greater in the long side than in the short
side. If the columns of water
are not allowed to separate, the water in the
long side will pull the liquid up the short side.
Atmospheric pressure keeps
the water from separating. The water flowing down the long side
reduces the
pressure in the siphon and thus atmospheric pressure pushes water up into
the short
side of the siphon if it is greater than the pressure of the column
of water in the short side of the
siphon. If atmospheric pressure did not
exist then there would be no reason for water to enter
the short side of
the tube. The following experiment, the siphon fountain, shows that the principle
of operation for a siphon does not rely on surface tension alone.
3. After "Understanding the siphon" Kevin C. de Berg and Cedric E. Grieve,
Australian Science Teachers Journal, 45 (4)
The explanatory principle based on comparing the weights of fluid in
both legs of a siphon occurs in
most textbooks analysis and in the work of
the French scientist Pascal, 1663. The advantage of this
principle is that
it is relatively simple and applies to siphons as commonly employed. Its
disadvantage
is are that it applies only to siphons and not other fluid flow
devices, e.g. syringes, and applies only
where the siphon fluid is more dense
than the external fluid. The following explanatory principle
overcomes this
disadvantages: The siphon in diagram 12.4.05 1. has a fluid disc of negligible
thickness at X.
You can consider the contending pressures either side, S
and S1 of this disc at the moment the
stopper is removed from X. The pressure
on side S1 is atmospheric pressure, AP. To locate the
pressure on side S
of the disc, follow the pressure changes on the other side of the siphon
starting
at the surface of the fluid in the container where the pressure is
atmospheric pressure, AP. At A
the pressure is (Ap + 2). At B the pressure
is (AP + 2 - 18). At X, the pressure is (AP + 2- 18 + 6),
i.e. (AP -
10). The pressure on side S1 exceeds the pressure on side S so the liquid
will not siphon
from the tube but move back into the longer leg and into the
container.
In diagram 12.4.05 2. for this siphon, at X, the pressure on side S1
is again AP. On side S, the
pressure (AP + 2 - 12 + 14), i.e. (AP + 4). The
pressure on side S exceeds the pressure on side S1
and so fluid siphons from
the beaker out of the longer leg of the siphon. In the syringe in diagram
12.4.05.
The pressures on either side of a fluid disc of negligible thickness
at the opening of the needle after
the plunger has been moved back but before
fluid has begun to flow. On side S the pressure is less
than atmospheric pressure
because a small sample of air at atmospheric pressure was expanded into
larger
volume. On side S1 the pressure is (AP + h). So the pressure on side S1 exceeds
the pressure
on side S and fluid moves into the syringe from high pressure
to low pressure.
12.4.1.0 Simple siphon
See diagram 12.4.0: Siphons
Use two tall glass bottles and fill each about half full of water. Connect
two 30 cm lengths of glass
tube with a 30 cm rubber or plastic tubing. Fill
the tube with coloured water and pinch it. Put a
glass tube in each bottle
of water. Siphon the water back and forth by varying the height of the bottles.
12.4.1.1 Siphon fountain
See diagram 12.4.1: Siphon fountain
1. Fit a flask with a two-holes stopper. Through one hole put a jet
tube that extends to about half
way to the top of the flask and about 2
cm outside the stopper. Through the other hole push a
short length of a
glass tube so that it is just flush with the bottom of the stopper. Connect
a 20 cm
length of a rubber tube to the jet tube. Connect a 1 m length to
the other glass tube. Put some water
in the flask and insert the stopper.
Put the short rubber tube in a container of coloured water on a
table, let
the longer rubber tube go into a pail on the floor and then invert the
siphon. Coloured water
spurts from the jet. You can make a double siphon
fountain by making another flask unit similar to the
first one and connecting
them. While the water is maintained in the container, the fountain cannot
be
stopped.
2. Insert 2 pieces of glass tubing through a two-holes stopper such
that one extends farther than the other.
Connect 30 cm of rubber tubing
to each glass tube. Fill a large bottle with cold water. Pour 100 mL of
water in a large jar and insert the two-holes stopper with attached tubing.
Invert the jar. The ends of the
tubing in the bottle should be under water
at all times. Observe what happened after inverting the jar.
Observe the
volume of the air pocket above the water in jar as the water poured in the
empty bottle.
Note that water was drawn up into the jar. The bottle filled
with water had to stand higher than the other
bottle. See how to make the
fountain flow more or less. The water in the jar was needed to "prime" the
siphoning action. After inverting the jar, the water ran down and into the
empty bottle causing an
increase in volume of the air pocket above the water
in the jar and decreasing the pressure. The lower
pressure caused the sucking
up of the water from the bottle filled with water. The atmospheric pressure
pushes water up into the larger jar. The greater the difference in height
of the water levels in the two jars,
the greater the flow of water. As soon
as the water levels are the same height in both bottles the water
flow stops
as in a siphon.
3. Fit a glass container, or a flask made from a used electric bulb
with a two-holes rubber stopper.
Through one hole place a jet tube which
will extend to about half way to the top of the flask and about
2 cm outside
the stopper. Through the other hole push a short length of glass tube so
that it is just flush
with the bottom of the stopper. Connect a 20 cm length
of rubber tube to the jet tube. Connect a 1 m
length to the other glass tube.
Place some water in the flask and insert the stopper. Put the short rubber
tube in a container of water on a table, let the longer rubber tube go into
a bucket on the floor and then
invert the siphon. Add ink to the water to
see the fountain better. Make a double siphon fountain by
making another flask
unit similar to the first one and connect them.
12.4.1.2 Siphon replaces water for fish tank
Use a one metre length of 10 millimetre diameter rubber hose. Fill the
hose with water then block both
ends with fingers. Put one end into water
in a tank for goldfish then move away the finger the end under
water. Put
another end into a bucket or basin on the floor below the end of the tube
in the tank, then
move away the finger from the end. Water streams out of
the tank towards the bucket or basin.
12.4.1.3 U-tube siphon
Use two beakers half full of water with a connecting U-tube full of water.
Lift one beaker then the other.
12.4.1.4 Siphon in a bell jar
Transfer water through an U-tube from a sealed flask to an open beaker
when the assembly is placed
in a bell jar and evacuated.
12.4.1.5 Siphon mechanism apparatus
This apparatus that shows atmospheric pressure, not cohesion, to be the
basis for the siphon action.
12.4.1.6 Pressure in a siphon
Hook a manometer to the upper portion of a siphon.
12.4.1.7 Mechanical siphon
A mechanical model of a siphon consists of chain hung over a pulley
to a lower level.
12.4.1.8 Self-starting siphon
Seal an inverted U-tube in the side of a beaker to make a self-starting
siphon.
12.4.1.9 Mariotte flask and siphon
Use a gas siphon to transfer carbon dioxide from one beaker to another,
intermittent siphon,
Tantalus cup
Use a Mariotte flask to make a siphon with a constant flow because the
height of an open
tube inserted through the stopper of a jug with an outlet
at the bottom regulates flow.
12.4.1.10 Turnover siphon
See diagram 12.4.10: Turnover siphon
1. Insert two pieces of glass tubing through a two-holes stopper with
a longer piece drawn out to
form a jet. Connect the shorter glass tube with
rubber tubing to a lower jar, A. Connect the longer
glass tube with rubber
tubing to a higher jar, B. Add water to an elevated jar, C. Insert the
two-holes
stopper in C so that the jet end of the longer glass tube is above the water
level.
Fill jar B with water.
2. Invert jar C while keeping the ends of the rubber tubes in the jars.
Water in jar C. starts to run
down the rubber tube into jar A, increasing
the volume of air in jar C. and decreasing the air pressure
in jar C. Water
in jar B. is pushed up the rubber tube by (atmospheric pressure - air pressure
in jar C). The water shoots out through the jet end of the longer glass tube
until the levels of water
in jar B. and jar C. are the same.