Experiment 3

Wow you can see right through me!!!

Glass Labs

Objectives:

The objectives of this lab are to form a low temperature glass, work with glass blowing and explore the conductive nature of glass.

Review of Scientific Principles:

Glasses are amorphous ceramic materials. The amorphous (or glassy) state of matter occurs when a substance has not been given sufficient time to crystallize. Glasses are most commonly made by rapidly quenching a melt. This means that the atoms making up the glass material are unable to move into positions which allow them to form the crystalline regularity. This may be attributed to the fact that each atom is strongly bonded to adjacent atoms while in the liquid state, and that the crystalline structures are very complex. The end result of all these factors is that the glass structure is disordered and therefore amorphous. One of the most notable characteristics of glasses is the way they change between solid and liquid states. Unlike crystals, which transform abruptly at a precise temperature (i.e., their melting point) glasses undergo a gradual transition. Between the melting temperature (Tm) of a substance and the so-called glass transition temperature (Tg), the substance is considered a supercooled liquid.

When glass is worked between Tg and Tm, one can achieve virtually any shape. The glass blowing technique is a fascinating demonstration of the incredible ability of glass to deform.

A chief advantage of the glass forming process is that the item remains one single piece with continuous molecular structure and without internal surfaces. That is why optical fibers are drawn from glass. No scattering of light at grain boundaries occurs. Certain glasses have non-linear optical properties that can be used for optical switches making the development of optical computers more likely.

Properly doped with polyvalent transition metals, glasses become semiconducting. But, their semiconducting properties can be altered by electrical fields, making these glasses suitable for information storage devices. Glasses of this kind are used for the coatings on the printing drums in laser printers or Xerox copiers.

Some glasses exhibit very high ionic conductivity, which makes them useful as electrolytes in batteries or sensors. One commercial example can be found in every chemistry laboratory, the pH meter. While crystalline ceramics, for the most part, have well defined chemical compositions, the compositions of glasses can be widely varied. Glass is made out of silica which has a very high melting point. In the attempt to lower the melting temperature, soda ash (a mixture of Na2O, sodium oxide, and Na2CO3, sodium carbonate), and limestone (CaCO3) are added as fluxes. Other glass fluxes might include lead oxides or lead carbonates (leaded glass or flint glass) or borax/borax oxides (borosilicate glass).

Borax is a naturally occurring mineral that is chemically hydrated sodium borate or Na2B4O7 10 H2O. The material is a white powder that is sold in super markets as a laundry aid. Borax is also used as a flux in working some metals because it coats and cleans the metal and allows soldering to take place. When Borax is heated, the water of hydration is driven off and the sodium, boron and oxygen form a non-crystalline glass. This glass is clear but will take a color from the various metal oxides such as cobalt or nickel. Thus the Borax beads can be used to identify some metal ions as well as demonstrate materials used to make colored glass. Borax Glass is also unstable in that it tends to absorb moisture from the air and revert back to a cloudy hydrated material.

This activity demonstrates the formation of a borax-based glass, the technique of glass blowing, and the electrical conduction properties of glass.

Time:

Part A: 20- 50 minutes

Part B: 40 minutes

Part C: 30 minutes

Materials and Supplies:

Part A:3 inch piece of nichrome wire
a small quantity of borax (0.5g) {sodium tetraborate}
Gas burner
Part B:lime or lead glass tubing (7 to 10 mm in diameter about 20 to 25 cm long)
2-3 cm rubber tube to fit glass tubing
gas burner
glass file
Part C:piece of lime glass rod or tubing (5 to 10 mm diameter )
piece of Pyrex (5-10 mm diameter)
ohmmeter
2 alligator clips
hook up wire
candle
ring stand
iron ring
gas burner

General Safety Guidelines:

* Nichrome is not a good conductor so you can hold one end of a 3 inch wire but remember, the other end is at 500 - 700 C .

* Remember, you will be using a gas burner. Perform this lab using all fire cautions.

* Hot glass and cold glass look the same. Be careful to check for hot glass.

* The glass bead will drip off if too large and it will be hot.

* The cool glass could break from the wire with very sharp edges.

Procedure:

Part A: Borax Glass

1. Obtain the nichrome wire and make a loop with a diameter of about 0.5 cm at one end.

2. Heat the loop over the Bunsen burner until the wire is red hot.

3. Dip the hot loop into the Borax powder.

4. Hold the loop with the powder stuck to it in the flame until the Borax becomes a clear, glassy drop. (Approximate time will vary depending on the temperature of the flame and the amount of Borax on the wire.). Add extra Borax if necessary by redipping the wire into the Borax.

5. With the Borax glass still in the wire, allow it to cool. After it has cooled, examine it.

6. Check the solubility of your "glass" by leaving it, still in the wire, in some water overnight.

Video Clip

Part B: Glass Blowing Bud Vase Procedure:

1. Preheat the glass tubing end by passing it back and forth in the flame.

2. Heat the end until it melts closed. Keep rotating the tube for even heating and to keep the glass from drooping.

3. When the end is very hot and completely closed, blow gently into the tube, watching the end at all times. When the bubble is about three times the diameter of the tubing, stop blowing. Cool the tube with the bubble.

4. When cool, cut the tubing about 2 inches from the bubble and fire polish the open end. Do not melt close. Allow the object to cool.*

*You now have a bud vase that will hold one rose bud or some other special flower from a special date. Some ribbon and a hot glue gun will dress it up. Have Fun but Play Safe.

Video Clip

Part C: Electrical Conductivity

1. Using the smoking flame from a candle, deposit two rings of carbon around a cool glass rod The rings should be about three centimeters apart.

2. Clamp an alligator clip to each of the carbon rings.

3. Attach the other end of the leads from the alligator clip to the ohmmeter

4. Clamp the glass rod so that it can be heated.

5. Heat the glass rod and record the electrical resistance every ten seconds.

6. Continue to heat until the glass softens, continue to record resistance

7. On a sheet of graph paper, plot resistance on the vertical axis and time on the horizontal axis.

Video Clip

Data and Analysis:

TIME
          
Start
 
10 Seconds
 
20 Seconds
 
30 Seconds
 
40 Seconds
 
50 Seconds
 
60 Seconds
 
70 Seconds
 
80 Seconds
 
90 Seconds
 

Questions:

Part A:

1. How is this glass like window glass? How is it different?

2. You are commissioned to make a mosaic picture of a rainbow using borax glass. What metal ions would you use for the colors?

Part B:

1. Why do professional glass blowers like Pyrex glass?

2. Why does glass just get soft and not melt suddenly and become a liquid?

Part C:

1. Why do different types of glass show different degrees of electrical conductivity?

2. If glass will conduct electricity under certain conditions, do you think it might conduct at room temperature if the voltage is high enough?

3. Do you think the distance between the alligator clips on the glass rod has any effect on the resistance? Why or why not?

4. What does this experiment tell you about the need to control temperatures in electronic devices like computers?

Teacher's Guide to Experiment #3
Glass Labs

Materials:

*Borax obtained from grocery stores may not work for this lab as some contain detergents or soaps).

General Safety Guidelines:

*Use a wire that is a poor conductor of heat and nonreactive at elevated temperatures. Handles could be added to the nichrome wire.

Procedure:

A-6: *If the "glass" comes out clear enough, you may suggest that they try to use them like a magnifying glass.

*If you want to, have them try a Borax Bead Test, either new wires will have to be handed out or have them make a new loop on the other end of the wire.}

Extension to Part B:

This is a procedure for making a glass swan. With lots of patience and practice, you or your students might want to try this.

Swan Procedure:

1. Place rubber tubing on one end of the glass tubing.

2. Light and adjust burner for a hot flame.

3. Preheat the center of the glass tube and then heat strongly while rotating the tube.

4. As the glass softens, gently push the tubing together just a little. (This allows extra glass for a strong bubble.)

5. Pinch the rubber tube and blow into the glass end until there is a bubble about twice the size of the tubing diameter.

6. While the glass is still hot, make the bubble shape not round by pushing both ends of the tubing at an angle, about 140 degrees relative to each other.

7. Heat either tube near the bubble (body), when soft push the tube back over the bubble (body) to make the neck of the swan. DO NOT HEAT THE BUBBLE (BODY).

8. Leave a section of the tube, a centimeter or so, at the end of the neck to form the head and heat the tube at this point. (This will make the beak.)

9. When the glass is soft pull it and melt it off.

10. Gently heat the bottom of the bubble (body) with the side of the flame. (This part of the bubble will then become flat.)

11. Holding the glass by the beak and the other end of the tube, preheat the tail area. (The tube opposite from the beak.) DO NOT HEAT THE BUBBLE (BODY).

12. When the glass is soft, pull upward and gently twist to form the tail.

13. Melt off excess glass tube and allow to cool.

14. HOW DO YOU MAKE A GOOD ONE ? THE SAME WAY YOU GET TO CARNEGIE HALL .... PRACTICE, PRACTICE, PRACTICE.

Video Clip


Part C Suggestions:

*You might have to seal a piece of Nichrome wire into each end of a 3 cm piece of glass tubing and use this.

*You may want different teams to try different materials such as Pyrex glass, lime glass, leaded glass and or different diameters of the same material. How about other ceramic materials? You may also want them to continue recording every 10 seconds while the glass cools and include that data in their graph.

*Electrical resistance should go from more than 20 megaohms at room temperature down to less than one megaohm at 700 C.

Sample Data and Analysis: Pyrex Tube

TimeElectrical Resistance
Start20 M
10 seconds 15.72 M
20 seconds 7.23 M
30 seconds 4.05 M
40 seconds 2.30 M
50 seconds 1.07 M
60 seconds 0.54 M
70 seconds 0.073 M
heat removed
 
80 seconds 0.23 M
90 seconds 0.95 M
100 seconds 1.85 M
110 seconds 3.17 M
120 seconds 8.22 M
130 seconds 14.78 M
140 seconds 20+ M

Sample Graph:

Answers:

Part A:

1. The glass is transparent and hard but not stable. 2. Answers will vary. Consult CRC Borax Bead Tests Table.

Part B:

1. Pyrex is tough and is heat resistant.

2. Glass is a mixture and does not exhibit long rang crystal structure.

Part C:

1. Different amounts and types of ionic and covalent bonds effect the degree of electrical conductivity.

2. Yes, at high voltages glass will conduct. A Tesla coil will show this or some adult toy like the plasma storm ball sold by stores.

3. Yes. It is analogous to resistance in wire. Resistance is directly proportional to the length and inversely proportional to the diameter.

4. Electronic devices need to be kept within their designated temperature range to operate as expected.

Next Topic:Electrical Resistance
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