High school (hard)

Experiment 3 – Crystal growth

Crystals are found in nature in different places. Crystal growth is generally associated with supersaturated solutions of a salt.

However, some elements form crystals  naturally, such as metals. Crystal growth can take thousands of years or only a few seconds depending on the degree of saturation and the type of material used. Understanding how to make the nucleation process by which crystals are formed is of vital importance in the manufacture of new materials and the calculation of the age that a geological region can have.

In this experiment you will see for yourself the process of nucleation, ie you will see how crystals form. This experiment is a particularly beautiful one, for which you need a microscope to see all visual effects. If you don’t have one, borrow one from your school’s laboratory, they should have one.

Material:

• A pair of polarizers (In Experiment 1 [see below] we show you how to get them)
• Microscope
• Naphthalene
• Benzene
• Mortar
• Heat resistant slide
• Metal pan
• Beaker (heat-resistant)
• Watchglass
• Ice

Procedure:

This experiment is divided into two parts. Remember that naphthalene is toxic and you could get poisoned if you ingest it.

The first part consists of observing the sublimation of naphthalene and the formation of crystals:

1. Pulverize naphthalene with a mortar (or, if you don’t have one, do it with a hammer) and place it in the beaker.
2. Put the glass on the fire until the naphthalene is melted.
3. At that moment, cover it with a watch glass, the beaker and place a pair of ice cubes on top. Continue heating the naphthalene until it begins to vaporize.
4. Wait a few seconds and turn off the heat.

You will see crystals forming in the watch glass little by little. If you expose them carefully in light of the sun, see how many colors are seen.

Uncover the beaker being careful not to breathe and see the crystals that are flying. You will see many colors in the sunlight. If you were patient, the crystals formed in the watch glass will be long enough for you to be able to see their structure.

In the second part of the experiment, you need to prepare a microscope.

1. Place a polarizer on the microscope and the slide above it. It will be double thick glass, so you’re at risk of breaking one of them, be careful.
2. Watch a hair under the microscope and make sure to leave it focused.
3. Melt naphthalene in the pan and mix it with benzene. Put as much benzene as naphthalene and heat it to keep it liquid, but don’t make it boil.
4. With a dropper take some of the naphthalene-benzene mixture and put it on the slide. Just put a drop and immediately observe it under the microscope, put another  polarized glass in front of the microscope eyepiece.
5. What will be observed is the growth of naphthalene crystals. You should view them in different colors and shades.

Questions:

What is the difference between sublimation by crystallization and the one made by solidification?
What is the geometric structure of naphthalene crystals?
Could you do the same with salt and water?
What crystalline materials you know?
What are the other non-crystalline solids?

Experiment 2 – Anemometer

An interesting device is the Pitot tube, or venturiometer, used in aircrafts to measure the speed of aircrafts with respect to the mass of air that surrounds them. This apparatus uses the venturi effect to measure the velocity of any fluid. The higher the speed, the lower the pressure it will exert a determined fluid in the directions perpendicular to the movement of the fluid.

Thus, if we can measure the pressure difference between a fluid that is static and one that is in motion, we can measure flow velocity in motion.

Using Bernoulli’s equation under the assumption that it is applicable to air (which is false, although a good approximation), it is possible to measure wind speed with a Pitot tube.

In this experiment we will show you how to make an anemometer (a device to measure wind speed) using the Venturi principle. It’s up to you to do the calculations to set the scale accordingly.

Material:

• 2 transparent  straws
• A glass or small container, also transparent.
• Food coloring
• Transparent tape.
• Plastic ruler.
• Acrylic marker
• Water.

Procedure:

1. Fill halfway with water or glass container.
2. Place food coloring in the water and mix well. Let it sit a bit.
3. Place one of the straws inside the glass, so as to not touch the bottom, but that it’s well submerged in water. Tape the straw to the glass with tape.
4. Using the ruler and the acrylic marker, make marks on the straw every half-centimeter.
5. One of the straws cut in half, you will just use that half.
6. Blow, with the straw you cut on the straw that is attached to the glass. Do it perpendicularly.
7. Now you can compete with friends to see who manages to get above the water. Does anyone manages to get the water out the cup?

Questions:

What wind speed you manage to reach with your breath?
What speed does a whistle reach?
Is a sharper note in the whistle blowing achieved with faster or slower wind?
Could you measure the wind speed that generates the sound?

Experiment 1 – Polarized Light and the different colors

Today polarized light has been used in a myriad of electrical appliances. The liquid crystal displays, projectors, 3D movies, lasers and many more devices. In this experiment we will show an artistic application of light and, in particular, polarized light.

Material:

• Transparent tape.
• White cellophane.
• Two polarizers or an old cell phone screen calculator you won’t use anymore.
• A vinyl acetate or a glass coverslip.

Procedure:

1. If you couldn’t get the polarizers, you can find an old calculator that’s no longer useful or an abandoned a cell phone. Disassemble the unit and remove the display, which consists of two polarizers, a pair of glasses bonded to the polarizers and the liquid crystal center with a nearly transparent circuit.
2. Separate both glasses very carefully. You must be very careful, because the glasses are fragile and should not be broken.
3. Once  glasses are separated, you have a couple of polarizers, with the disadvantage that the center will show the circuit. If you clean a lot with a paper towel, you can make the circuit disappear almost completely.
4. Check that you have indeed two polarizers. Put them against the light, facing each other and slowly rotate one clockwise until you reach 90 degrees. You have to see how light passing through them increases or decreases in intensity.
5. Now, put a layer of tape on the slide or vinyl acetate. If your acetate is letter size, we recommend trimming it: 5cm X 10cm is more than enough.
6. Choose a section where you won’t put any more acetate tape and cover the rest with a second layer. Repeat the procedure as many times as you have patience, leaving an ever larger section without tape. For example, you can put a strip of tape less at a time, in the beginning 8 strips, then 7, then 6, 5, 4, 3, 2, and finally just one. You’ll then have an object covered with different thicknesses of tape.
7. Put your object in the middle of the polarizers. If you like, you can paste one of the polarizers to the slide. As in Figure 1, watch against the light how does the microscope slide looks in between two polarizers and rotate one of the polarizers slowly to 90 °.

Questions:

What do you see in the experiment?

Find out how the light is polarized in a polarizer.

How do you think you could use this effect?

Does the same happen with transparent objects?