Free Energy
Diffusion
© 2009 Barbara J. Shaw Ph.D., Science A to Z
Permission is granted to make and distribute copies of this lesson plan for educational use only.
Background material retrieved on February 4, 2009 from:
http://en.wikipedia.org/wiki/Diffusion
umassk12.net/nano/materials/Diffusion.doc
http://www.btanj.org/demo/2001/toxic_jello.pdf
http://www.scienceteacherprogram.org/biology/kucine00.html
http://www.biologycorner.com/worksheets/diffusion.htm
http://www.tqnyc.org/NYC051293/acidbasesalt.html
Background
Diffusion
Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region
of higher concentration to one of lower concentration by random molecular motion. The result of
diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external
net forces acting on the particles, the diffusion process will eventually result in complete mixing
or a state of equilibrium.
Diffusion is part of transport phenomena. Of the mass transport mechanisms, molecular diffusion
is known as a slower one. Molecular diffusion is generally superimposed on, and often masked
by, other transport phenomena such as convection, which tend to be much faster. However, the
slowness of diffusion can be the reason for its importance: diffusion is often encountered in
chemistry, physics and biology as a step in a sequence of events, and the velocity of the whole
chain of events is that of the slowest step. For example, the rate at which a chemical reaction
progresses can be entirely limited by the rate of diffusion of reactants/products to/from the place
where the reaction occurs.
The speed of diffusion can be approximately illustrated as follows (at room temperature)
 In gas: 100 mm in one minute;
 In liquid: 0.5 mm in one minute;
 In solid: 0.0001 mm in one minute.
Transport due to diffusion is slower over long length scales: the time it takes for diffusion to
transport matter is proportional to the square of the distance. Conversely, diffusion can be quite
fast over small length scales; inside a living cell, chemicals are almost entirely transported by
diffusion.
The above numbers should be treated only as an illustration of the slowness of diffusion. Great
differences exist in the diffusion speed between particular systems, particularly in the solid state.
For example:
 Hydrogen gas in solid iron at 10 °C - diffusion coefficient of 1.66x10-13 m2/s;
 Aluminum in solid copper at 20 °C - diffusion coefficient of 1.3x10-34 m2/s.
When diffusion speed is proportional to the square root of the diffusion coefficient, then the
hydrogen in iron diffuses over 10 orders of magnitude faster than does aluminum in copper.
In cell biology, diffusion is a main form of transport for necessary materials such as amino acids
within cells. Diffusion of water is classified as osmosis.
Metabolism and respiration rely in part upon diffusion in addition to bulk or active processes.
For example, in the alveoli of mammalian lungs, due to differences in partial pressures across the
alveolar-capillary membrane, oxygen diffuses into the blood and carbon dioxide diffuses out.
Lungs contain a large surface area to facilitate this gas exchange process.
Acid, Base & Salt
 Acids: What do vinegar, lemons, and
sour milk have in common? They all
contain acids. An acid is a substance
that reacts with metals to release
hydrogen. Acids give vinegar, lemons,
and sour milk their sour taste.
Remember, however, that you should
never taste a substance to find out what
it is.
o Common acids: Acids are found in
many different substances. Citrus
fruit, such as lemons and oranges,
contain citric acid. Sour milk
contains lactic acid. Vinegar
contains acetic acid.
o Properties of acid: All acids
contain hydrogen. The hydrogen
in the acids causes the properties of
acid. When an acid is added to water, the acid forms positive and negative ions. For
example, hydrochloric acid forms positive hydrogen ions and negative chloride ions.
A hydrogen atom contains one electron and one proton. When the electron is lost to
form a positive, only the proton is left. Therefore, a hydrogen ion is the same as a
proton. For this reason, acids are sometimes called proton donors. The more
hydrogen ions in water, the stronger the acid. Acids react with metals to release
hydrogen gas.
 Bases: The early settlers in the United States made their own soap from animal fats and
ashes. Soaps are made by chemically combining fats or oils with a type of chemical
compound called a base. Ashes contain the base potassium hydroxide.
o Common bases: Milk of magnesia, ammonia, and soap are bitter tasting substances
that contain bases.
o Properties of base: A base is a substance that is formed when a metal reacts with
water. Bases taste bitter and feel slippery. The base it contains causes the slippery
feel of soap. Like acids, strong bases are dangerous. They can burn the skin. When
some metals are placed in water, a chemical reaction takes place. The reaction
produces a base plus hydrogen. For example, sodium metal reacts with water to
produce sodium hydroxide and hydrogen. The hydroxide ion, OH, is a negative ion
made up of one atom of hydrogen and one atom of oxygen. All bases release


hydroxide ions in water. Hydroxide ions can combine with hydrogen ions to form
water. Remember that a hydrogen ion is a proton. Therefore, bases are often called
proton acceptors. Note: Although bases have a higher concentration of hydroxide
ions, the pH of the solution is still based on the number of hydrogen ions in the
solution.
Salts: In chemistry, salt is defined as the product formed from the neutralization reaction
of acids and bases. Salts are ionic compounds composed of cations (positively charged
ions) and anions (negative ions) so that the product is electrically neutral (without a net
charge). These component ions can be inorganic such as chloride (Cl−), as well as organic
such as acetate (CH3COO−) and monoatomic ions such as fluoride (F−), as well as
polyatomic ions such as sulfate (SO42−).
o There are several varieties of salts. Salts that produce hydroxide ions when dissolved
in water are basic salts and salts that produce hydronium ions in water acid salts.
Neutral salts are those that are neither acid nor basic salts. Zwitterions contain an
anionic center and a cationic center in the same molecule but are not considered to be
salts. Examples include amino acids, many metabolites, peptides and proteins.
o When salts are dissolved in water, they are called electrolytes, and are able to conduct
electricity, a property that is shared with molten salts. Mixtures of many different ions
in solution—like in the cytoplasm of cells, in blood, urine, plant saps and mineral
waters— usually do not form defined salts after evaporation of the water. Therefore,
their salt content is given for the respective ions.
pH scale: pH means potential of H+, or hydrogen. The scale is an inverse log of the
hydrogen concentration in a solution.
Therefore, if the concentration of hydrogen atoms is high, as can be found in sulfuric
acid, the pH scale will be low. Water normally associates and disassociates, as can be
seen in the following equation:
H2O <—> H+ + OH-,
or more accurately,
H2O <—> H3O+ + OH-,
Even distilled water contains hydrogen and hydroxide ions. The concentration of the
hydrogen ions is approximately equal to the number of hydroxide ions, however there are
still hydrogen ions present in the water. The concentration of these ions in pure distilled
water is approximately 1x10-7 moles per liter, and the pH scale would be 7. By the way,
the concentration of hydroxide ions is the same, 1x10-7 moles per liter. If the
concentration of hydrogen ions is 1x10-3 moles per liter (much more concentrated), the
pH scale is 3, and the concentration of hydroxide ions would be 1x10-11 moles per liter.
o

Acids and bases can be strong or weak. Sulfuric acid and nitric acid are strong acids
that can burn the skin. Carbonic acid and boric acid are weak acids. Boric acid is
even in eyewashes. Sodium hydroxide and potassium hydroxide are strong bases.
Ammonium hydroxide is a weak base that is used as a household cleaner. Aluminum
hydroxide a weak base that is used as an antacid.
o The strength of an acid or a base depends
on the number of hydrogen ions in the
solution. Adding water will reduce the
concentration of ions and change the
strength of the solution. When water is
added to a strong acid or base, the acid or
base becomes weaker. Scientists have
developed a scale to measure the strength
of acids and bases. This scale is called the
pH scale. The pH scale indicates the
concentration of hydrogen ions in solution.
The pH scale is a series of numbers from 014. A neutral solution has a pH of 7. As
explained above, in pure distilled water,
there is still the disassociation, and
reassociation of hydrogen and hydroxide
ions that averages out to about 1x10-7
moles per liter. Taking the negative log of
1x10-7 gives us 7. A neutral solution is
neither acidic nor basic. Acids have a pH
below 7 (meaning that they have a higher
concentration of hydrogen ions in the
solution). Bases have a pH above 7
(meaning they have a lower concentration
of hydrogen ions in solution). Strong acids
have a low pH, while strong bases have a
high pH. Indicators can be used to help
find the exact pH of an acid or a base.
Indicators: Litmus paper can be used to
identify an acid or a base. Vinegar is an acid.
If you dip one end of a strip of blue litmus
paper into vinegar, the blue litmus turns red. If
you dip one end of a strip of red litmus paper
into soapy water, the red litmus turns blue.
Chemicals that change color in acids or bases
are called indicators. Litmus is an indicator
that turns red in acids and blue in bases.
Phenolphthalein is another indicator.
Phenolphthalein is colorless in acids and pink
to red in bases. The indicator methyl red is red
in acids and yellow in bases. Many common, everyday substances are indicators. Grape
juice is a good indicator. It is pink or red in acids and green or yellow in bases.
Hydrangeas have pink flowers in basic soil and blue flowers in acidic soil. Red cabbage,
beets, rhubarb, cherries, blueberries, and blackberries all can be used as indicators.
o We will be using red cabbage for our indicator. Red cabbage contains a pigment
called anthocyanin. The molecule changes conformational shape depending on the
number of hydrogen atoms that have bonded on this chemical. As this molecule
bonds with more hydrogen atoms (because more hydrogen atoms are available in
acids, they are actually “forced” onto this molecule), double bonds shift, and the
entire shape of the molecule changes. This in turn reflects and absorbs light
differently, and that is how the color changes to pink and red in our solution. By the
same manner, when this pigment is in a base, the hydrogen atoms are “forced” off the
molecule, double bonds shift again, and the conformational shape is different. This
reflects and absorbs light differently, and we see colors of blue and green. In a
neutral solution, the pigment remains in the lowest energy state, which is between the
two extremes of fully protonated or fully deprotonated.
The following pictures
http://www.chemistryland.com/CHM107Lab/Exp10_pHindicator/Lab/PreparingCabbageExtract.htm
http://www.ratlab.co.uk/indicators.htm
Activity 1: Is diffusion fast?
Materials:
 2 Petri dishes per pair of students
 2 pipettes per pair of students
 Clear metric ruler per pair of students
 Containers to mix gelatin, food dye, and tempura paint.
 Gelatin (plain - no colors or flavors)
 container to make gelatin
 distilled water
 way to heat water
 Food color.
 Tempera paint (diluted about 50/50)
 Glass pipette or glass eyedropper or
Razor blade (scalpel or craft knife)
several, and teams can share
Cheesecloth
Cup with
ammonia
Gelatin cube
The day before class:
Depending on your class, you can instruct them to do some or all of the following in preparation
of this experiment.
 Make the gelatin
1. Bloom gelatin at double strength (e.g., use 8 packets in cold 1 cup distilled water
(~240ml)
2. Heat 2.5 cups (~500ml) distilled water (e.g., microwave the mixture for 2 minutes)
3. Pour gelatin into the Petri dishes (2 per pair of students)
4. Allow the gelatin to cool overnight in the refrigerator
Procedures:
 With the glass pipette, punch a hole in the center of the gelatin about half-way down.
With the razorblade, remove the circle from the center of the gelatin.
 Using the pipette, add food coloring into the center hole of the gelatin, being careful not
to get food coloring solution on the top of the gel
 Put the name of your teams members on a sheet of paper and place the Petri dish on top
of the paper
 Dilute 50% tempera paint and 50% water.
 Using the pipette, add tempera paint solution into the center hole of the gelatin, being
careful not to get food coloring solution on the top of the gel
 Set aside each large Petri dish on the sheet of paper in a level place that will not be
disturbed for several days.
Collect Data
 Each day for one week, each person in the team measures from the edge of the hole
holding the food coloring or tempera paint solution to the leading edge of the color in the
gelatin
 Record these data and find the average distance the color has traveled



On the last day at the end of the experiment, pour out the food dye and tempera paint
solutions into containers that have been provided.
Use a ruler to measure the distance of penetration into the gelatin discs by looking at the
bottom of the dishes (The edges of the gelatin discs and the diffusion front will be clearly
visible. Both edges will be "fuzzy." Measure from the center of the “fuzzy” region for
both edges.)
Then place the Petri dishes containing the gelatin discs into a container that has been
provided.
Analyze Data
 The rate of diffusion is the length divided by the time.
 Compare the diffusion rate of the different dyes.
Questions
 Are the results expected?
 Which dyes penetrated better? Does that make sense?
 Conversely, does fast diffusion mean greater or poorer retention?
 How could diffusion and retention be optimized?
 Is this the intuitive result?
Activity 2: Why are the cells of Eukaryote organisms about the same size?
Materials
 9x9” pan
 Container to mix gelatin and dilute cabbage juice
 Plain gelatin - no colors or flavors
 Pot
 Element or burner
 Distilled water
 Small red cabbage
 Clear metric ruler
 Ammonia (dilute 25% ammonia 75% water)
 1 clear plastic cup per team
 Cheesecloth strips, large enough to hold 2x2x2cm gelatin, lower into the cup, and lift it
out again.
 Clock with second hand or one stop watch per team
The day before class:
 Make cabbage juice indicator:
1. Dice 1 small red cabbage.
2. Place in beaker or pot and heat with hot plate to boiling or about 10 minutes (do not
boil for 10 minutes, just bring to a boil).
3. Strain out the cabbage and reserve the cabbage juice.
4. After making the gelatin, label plastic water bottles with the permanent marker
“cabbage juice indicator” and fill with the left-over red cabbage juice. You can keep

this in your freezer. If teaching in the morning, take out of the freezer the evening
before. If teaching in the afternoon, take out of the freezer in the morning. Great for
all kinds of experiments.
Make the gelatin
5. Bloom gelatin at double strength (e.g., use 8 packets in cold 1 cup distilled water
(~240ml)
6. Heat 2.5 cups (~500ml) distilled water (e.g., microwave the mixture for 2 minutes)
7. Add between .5 and .75 cups of cabbage juice indicator, depending on the
concentration you prepared. The gelatin should be visibly purple and transparent.
8. Pour gelatin into 9x9” pan to 2cm high
9. Allow the gelatin to cool overnight in the refrigerator.
Procedure:
 Assign students the following tasks:
o Slice into one 2x2x2cm square and 1x1x1cm square for each team of students
o Cut cheesecloth into strips ~ 2½cm wide and 25cm long
o Pour dilute ammonia into cup ~2½cm deep
Collect data
 Measure the width, length and depth of each cube
o Find the surface area of the cube and record
o Find the volume of each cube and record
o Find the ration of surface area/volume and record
 With the 1x1x1cm cube, place on the cheesecloth and dip into the ammonia solution for
60 seconds, then remove
 With the scalpel, slice the cube directly down the middle, into two pieces. Measure the
pink from the edge of the cube to the leading edge of the color
 Record measurement
 With the 2x2x2cm cube, place on the cheesecloth and dip into the ammonia solution for
60 seconds, then remove
 With the scalpel, slice the cube directly down the middle, into two pieces. Measure the
pink from the edge of the cube to the leading edge of the color
 Record measurement
Analyze Data
 Graph the results of these data.
Questions
 What would you expect if you repeated the experiment with a cube 3x3x3?
 How does the surface area to volume ratio change?
 Why does the cube turn pink when you put it into the ammonia solution?
Alternative to Egg Osmosis activity. This is merely a demonstration rather than inquiry.
Students do not collect data, but make observations about
osmosis.
Activity 3: Semi-permeable membrane
Iodine is a known indicator for starch.
Materials
 1 baggie per team
 1 plastic spoon
 cornstarch
 1 dropper bottle with iodine
 1 clear cup with water
Directions
 Fill a plastic baggie with a teaspoon of cornstarch and a half a cup of water tie bag. (This
may already have been done for you)
 Fill the cup halfway with water and add ten drops of iodine.
 Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine
water mixture.
 Wait fifteen minutes and record your observations in the data table
Questions
 What happened to the cornstarch water?