Particle Size Lab 1

Particle Size Lab 1

Particularly Potatoes

Original lesson at

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.

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:

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

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.
Put the potato in the iodine.
Observe that nothing happens.
Ask the students for explanations.
Explain that the starch is all inside the potato. (If you leave a potato in iodine for 10-15 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.)
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.
Have students cut their half potato as follows:
One should remain whole.
One should be cut in half.
Another potato should be cut into fourths.
Another potato should be cut into sixteenths.
Have students place 100 ml of water and 20 drops of iodine in each beaker.
All at one time, students should drop all of pieces of potato into their beaker of iodine.
Have students record the sequence of rate of color change, which should be from most surface area to least surface area.
Discuss their results.
Pour 2 Tbsp. of potato flakes into your hand.
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.)
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