Particle Size Lab 1
Particularly Potatoes
Original lesson at http://www.nisenet.org/catalog/programs/surface-area
Modified by Jeanne Nye, Lake Mills Area Schools, Institute for Chemical Education and Nanoscale Science and
Engineering Center, University of Wisconsin-Madison
Surface Area
Objectives
• Nano means working at super small size scales to manipulate materials to exhibit new
phenomena. It's different down there!
• Nanotechnology exposes much more surface area, allowing new reactions to take
place.
Background
Nanotechnology is the development of artifacts and features about 1-100 nanometers long.
At this scale, strange new properties can emerge. Materials that are not combustible
suddenly can become flammable. Metals change color, insulators become conductors.
One of the reasons that properties change at the nanoscale is the amount of available
surface area. Chemical reactions take place between the atoms on the surface of objects. A
particle 10 nanometers wide contains dozens of atoms, instead of the millions or billions in a
larger object, but most of the atoms of a nanoparticle are on the surface, available to react.
By engineering nanoparticles of a substance, we can drastically increase the amount of the
material that is available to react. This can both increase the reaction, and save on material
costs, because much smaller amounts of material can be used effectively.
For example, silver has antibiotic properties. Several companies use nanoparticles of silver
in bandages, socks, cutting boards, and more to prevent the growth of bacteria. Iron can be
used to clean some toxic waste, and nanoparticles of iron are much more effective,
traveling further in underground aquifers than larger particles.
In nature, geckos stick to walls using nanoscale “hairs”. The hairs on a gecko’s toe are so
small that the electrons in the hairs stick to the atoms in the wall with a kind of static
electricity called Van Der Waals forces. This is only possible because the enormous number
of hairs on the toe actually increases the surface area of the toe available to stick to the
wall.
Medical implants are another example of surface area creating new properties. Thomas
Webster and others at Brown University have found that nanobumps of titanium dioxide on
the surface of metal joint implants can reduce infection and increase bone growth. The nano
bumps increase the surface area of the joint tremendously, and make it easier for bone to
grow around the implant. http://www.primidi.com/2003/11/05.html
At the nanoscale, aluminum oxide becomes flammable. This is also partly due to the
increased surface area.
One dramatic example of new properties at the nanoscale is the false negative of
nanotubes in a toxicity test:
http://www.newscientisttech.com/channel/tech/mg19025546.600-how-toxic-nanotubeswere-faking-it.html
Materials
2 potato per team of four students, each student starting with half of a potato
knives
four beakers per group
iodine
water
four 250 ml beakers per group
potato flakes
1.
2.
3.
4.
5.
Hold up the potato and iodine.
Explain that iodine is red, but turns blue/purple with starch.
Remind the students that potatoes are full of starch.
Put 100 mL of water and 20 drops of iodine in a 250 mL beaker.
Ask the students to predict what will happen when they put the potato in iodine
solution.
6. Put the potato in the iodine.
7. Observe that nothing happens.
8. Ask the students for explanations.
9. Explain that the starch is all inside the potato. (If you leave a potato in iodine for 1015 minutes small blue dots appear on the surface of the potato. If this happens in the
demo, you can cut the potato in half to expose new surface.)
10. From now on, use only half of a potato for each sample, so that the total volume
remains the same in each sampling; only the surface area will vary. Cut two
potatoes in half. Distribute these halves among each team of students, one half per
student.
11. Have students cut their half potato as follows:
a. One should remain whole.
b. One should be cut in half.
c. Another potato should be cut into fourths.
d. Another potato should be cut into sixteenths.
12. Have students place 100 ml of water and 20 drops of iodine in each beaker.
13. All at one time, students should drop all of pieces of potato into their beaker of
iodine.
14. Have students record the sequence of rate of color change, which should be from
most surface area to least surface area.
15. Discuss their results.
16. Pour 2 Tbsp. of potato flakes into your hand.
17. Ask students to predict what they believe will occur when you drop these flakes into
a beaker containing iodine. (The flakes are potato as well, but they’re small particles
instead of a big potato. All the starch in the flakes is on the outside, none is trapped
inside. The flakes have much more surface area.)
18. Put the flakes in the iodine and stir. Behold the color change.
Note: If you don’t have time for this lab, you might wish to do a demo with only the potato,
quartered potato and potato flakes.
Particle Size
Student Lab 1
Particularly Potatoes
Materials
2 potato per team of four students, each student starting
with half of a potato
knives
four beakers per group
iodine
water
four 250 ml beakers per group
Procedure
1. Cut your half potato in one of the following ways so that one member of your team
has prepared a different sample:
a. One should remain whole.
b. One should be cut in half.
c. Another potato should be cut into fourths.
d. Another potato should be cut into sixteenths.
2. Place 100 ml of water and 20 drops of iodine in each beaker.
3. When everyone is ready, all four samples, a-d above, should be dropped into your
beakers of iodine solution at the same time. Yes, all the pieces, all at once.
4. Record your observations. Which changes color first? Second?
Observations
Size pieces of potato half
whole
half
quarters
eighths
Rate of color change of iodine solution
1. Why did the solutions of iodine change color in that sequence?
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2. What do you predict will be the rate of change of potato flakes?
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3. Why do you think that this is a realistic prediction?
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