Laboratory Safety Essentials

Introduction

It is important to teach students essential safety information before beginning laboratory studies. Show students how important it is to follow all safety precautions when working in the laboratory, and how to apply their safety knowledge. Because these safety demonstrations involve intentionally creating hazardous conditions on a very small scale, it is essential to practice all the demonstrations beforehand. You (the demonstrator) do not want to be surprised or possibly disoriented by the observations. Follow all safety precautions and do not attempt to scale-up the demonstrations or substitute alternative materials.

The set of four demonstrations includes:

1. Acid in the Eye—Demonstrate the irreversible damage caused by strong acids to human tissue by adding hydrochloric acid to egg white.
2. Grease Fire—A sudden fireball leaves a lasting impression of what can happen when water is added to a grease fire.
3. Flaming Vapor Ramp—You have to see it to believe it! Labels on several household chemicals warn not to use them around heat or open flame. Demonstrate why even invisible vapors can be dangerous by igniting hexane vapor!
4. SDS Challenge—Teach students how to apply the information given on Safety Data Sheets (SDSs) by using the data to identify a mystery chemical.

Concepts

• Goggle safety
• Reactivity of strong acids
• Density
• Combustion
• Surface area
• Flammable liquids
• Fire safety
• Safety Data Sheets
• Laboratory safety
• Chemical properties

Experiment Overview

Acid in the Eye
Use this simple demonstration to show students the physical effects acids have when they interact with proteins of the eye.

Grease Fire
Each year hundreds of people are injured, some fatally, by grease fires when they attempt to extinguish such fires with water. This demonstration shows the dramatic and frightening results when water is splashed on burning candle wax.

Flaming Vapor Ramp
Vapors from volatile liquids are generally heavier than air and can travel along a countertop to an ignition source. Once vapors have been ignited, the flames will quickly follow the vapor trail back to the vapor source, resulting in a larger fire or explosion.

SDS Challenge Inquiry Demonstration
Verify your class can accurately use an SDS with this inquiry-based activity where each student must correctly identify a mystery chemical.

Materials

Safety Precautions

Hydrochloric acid is toxic by ingestion or inhalation; it is severely corrosive to skin and eyes. All food-grade items that have been brought into the lab are considered chemicals and are for lab use only. Do not taste or ingest any materials after they have been used in the lab. Do not remove any remaining food items from the lab after they have been used in the lab. Practice the Grease Fire demonstration before performing it for students. Wear chemical splash goggles and a lab coat to protect against flames. Due to the nature of the demonstration, have a dry powder fire extinguisher available. It is recommended that the demonstration be performed outdoors. Make sure all students are at least three feet away from the demonstration. All observers should wear safety goggles. Make sure all students are at least three feet away from the demonstration. Be very careful while performing the Flaming Vapor Ramp demonstration. Hexane is a flammable liquid and may be irritating to the respiratory tract. Do not use more hexane than is specified in the procedure—the flames may become too large and it will also increase the fire hazard should the flask fall and break. Do not substitute a more volatile liquid; many are dangerously combustible and the vapor trail may enter the flask and lead to an explosion. Ether (diethyl ether) or methyl alcohol for example, are far too volatile to use anywhere near an open flame or ignition source. Hexanes are flammable liquids, a dangerous fire risk, and a respiratory irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The egg whites should be rinsed with water and disposed of in the trash. The rinse solution should be neutralized according to Flinn Suggested Disposal Method #24b. Any excess acid should be stored in a dedicated acid cabinet for future use. Any remaining candles may be disposed of according to Flinn Suggested Disposal Method #26b. The hexanes solution may be disposed of according to Flinn Suggested Disposal Method #18a.

Prelab Preparation

Acid in the Eye

1. Draw a large eye on the bottom of a Petri dish using a permanent marker.
2. Gently crack an egg open and separate the egg white from the egg yolk. Place the egg white in the Petri dish.
3. Place the Petri dish on the overhead projector stage.

Flaming Vapor Ramp

1. Clear off a countertop before starting. Remove all combustible materials such as paper from the demonstration area.
2. Prepare a vapor ramp by elevating one end of the aluminum angle bracket using a support stand and clamp. The ramp should be at a 20° angle or about 20 cm elevation (see Figure 1).
   {12776_Preparation_Figure_1}
3. Place an unlit candle on the countertop directly below the lower end of the vapor ramp (see Figure 1).
4. Pour about 2–3 mL of hexanes into the 1-L Erlenmeyer flask.
5. Place a one-hole stopper on top of the flask and swirl the flask to evaporate the hexanes. Allow the flask to sit for a few minutes to allow hexanes vapors to fill the flask. Set the flask aside.

Procedure

Acid in the Eye

1. Briefly discuss the similarities between an egg white and the human eye. See Discussion section.
2. Using a Beral pipet, place several drops of 6 M hydrochloric acid on the egg white. The egg white will immediately become opaque.
3. Attempt to reverse the damage by gently rinsing the egg white with water. Collect the rinse water in a beaker for neutralization prior to disposal. The egg white will not become transparent again—the damage to the “eye” is permanent.
4. Place the lid on the Petri dish and tape it shut with transparent tape. Pass it around for the students to see that the egg white is permanently damaged, as their eye would be. Caution: Rinse the Petri dish before passing it around check there is no unreacted acid present. Advise students not to open the Petri dish or touch the egg white.
5. Discuss with students how this damage might affect their vision.

Grease Fire

1. In a suitable spot (preferably outdoors) where no combustible materials are overhead, spread out some wet newspaper to catch any splashed wax that may result from the demo. (A 12" x 12" area is sufficient.)
2. Place the support stand with a ring clamp on the center of the wet newspaper. Place the wire screen on the ring and the evaporating dish on the wire screen.
3. Break the candle in half.
4. Light the top half of the candle. Explain to students what is burning in a candle flame. Heat from the flame melts the wax, which is absorbed by the wick. The liquid wax vaporizes and it is the wax vapor that burns.
5. Place the bottom half of the candle in the evaporating dish. Place the portable burner under the ring.
6. Light the burner and slowly heat the candle wax until it starts to smoke freely.
7. Attempt to light the wax vapors with a match or safety lighter. When the wax vapors ignite and continue to burn on their own the wax temperature is high enough to complete the demonstration.
8. While the wax is burning on its own, stand or squat at least three feet off to one side and use the wash bottle to squirt a small stream of water into the evaporating dish. A fireball will erupt from the pan as the water hits the burning wax and turns to steam! Caution: Do not stand over or near the burning wax!
9. Extinguish any residual burning wax in the evaporating dish by adding a small handful of baking soda, NaHCO₃ to the dish.

Flaming Vapor Ramp

1. Have the entire class put on their safety goggles.
2. Light the candle and position it so that the flame is even with the bottom of the vapor ramp.
3. Remove the stopper from the flask containing the hexanes. Gradually pour the hexane vapors down the ramp for about three seconds. Tip the flask slightly and do not allow any liquid to pour out. Be prepared to have the vapor catch fire. Stopper the flask and remove from the ramp area immediately. Remember to only pour the vapor.
4. Nothing will happen for a few seconds—be patient. The vapor will soon ignite and then race up the ramp. Take the flask away from the top of the ramp before the flames reach the top.
5. After the flames race up the trough and have extinguished due to the absence of a fuel, the demo can be repeated.

SDS Challenge Inquiry Demonstration

1. Using a Beral pipet, add 2 mL of the mystery chemical (hexanes) to a clean 500-mL Erlenmeyer flask.
2. Once students arrive, fill the flask containing the mystery chemical with tap water.
3. Using a butane safety lighter, light the solution on fire. Explain to students that there is another chemical which was added to the water and which causes the liquid to burn.
4. Place a watch glass over the top of the Erlenmeyer flask to extinguish the fire.
5. Distribute copies of the SDS papers for the following chemicals—ethylene dichloride, hexanes and isopropyl alcohol.
6. Inform students to use the information on each SDS to determine the identity of the mystery chemical and to justify their answers in writing. Hint: Depending on the level of the class, you may want to advise students to consider the following physical and chemical properties: flammability, specific gravity and solubility.

Student Worksheet PDF

12776_Student1.pdf

Teacher Tips

• The Acid in the Eye experiment may be repeated using other concentrated acids and bases. Concentrated nitric acid turns the egg white brilliant yellow. Strong solutions of sodium hydroxide do not discolor the egg whites but instead solidify them.
• Acid solutions less concentrated than 6 M will work but the effects are not as dramatic.
• A video of this demonstration is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://eLearning.flinnsci.com for viewing information. The video is part of the Start with Safety—Safety Demonstrations video package.
• Paraffin wax, in the form of a candle, is used instead of animal lard or vegetable oil in the Grease Fire demonstration to reduce the amount of smoke produced and aid in the cleanup. The effect is the same.
• This experiment may also be done in a hood as opposed to outdoors. If the demonstration will be performed in a hood, make sure there are no flammable chemicals stored in the hood and remove any combustible materials (e.g.. paper, cardboard).
• If a portable burner is used, cover the tank of propane or butane with several layers of soaking wet paper and then a layer of aluminum foil. The foil and wet paper prevent the possible overheating of the liquefied petroleum gas if the burning wax should spill. A regular burner may be used as long as the experiment is performed in a safe area such as a hood.
• The steam splatters the wax and, since the wax is already above the flash point, the increase in surface area of the wax droplets results in rapid combustion. Use only a small amount of wax in this demonstration. This ensures that most of the material will be consumed and little will remain to continue a fire. Do not scale up the demonstration.
• Do not attempt the same demonstration with the same evaporating dish unless it is totally free of any water prior to heating the wax.
• A lab coat provides some temporary protection against fire, particularly when compared against synthetic fibers in many types of regular clothing. A lab coat can also be easily removed in the event of a fire.
• A video of this demonstration is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Start with Safety—Safety Demonstrations video package.
• Hexane, hexanes and petroleum ether (not ethyl ether) are all similar materials and will work well in the Flaming Vapor Ramp demonstration. Do not substitute any other flammable liquids for this demonstration. (“Hexanes” denotes a mixture of isomeric hydrocarbons having the formula C₆H₁₄.)
• If the ramp cannot be adjusted using a support stand and clamp, hold the ramp with one hand. Using an oven glove or a fire resistant welder’s glove, hold the ramp from beneath, open-side up. Keep fingers and gloves away from any flames.
• Practice this demo beforehand to understand how long to pour the hexanes vapors. The flask should not be near the trough when the flames ignite. If you are still pouring when the flames start, take the flask away from the top of the vapor ramp to prevent the fire from going back into it. (If the flame does make it back into the flask, it is okay; it will just burn there for a while at the mouth of the flask, unable to burn the entire sample due to inadequate oxygen.)
• A video of this demonstration is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Start with Safety—Safety Demonstrations  video package.
• A video of the 
  SDS Challenge Inquiry Demonstration demonstration is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://elearning.flinnsci.com for viewing information. The video is part of the Teaching Safety to Students video package.

Answers to Questions

1. What caused the egg white to turn white once it was exposed to hydrochloric acid?

   The protein structure of the egg white is highly specific and when broken down by the acid it turned white.

2. Why did the egg white not return to its original consistency or appearance after it was rinsed with water?

   Proteins need very specific three-dimensional structures to perform their biochemical functions. Destruction of the protein structure changes the properties of the protein. This change is frequently irreversible.

3. Why does adding water to a grease fire cause the fire to spread?

   The grease is less dense than water so if water is added to the fire it sinks to the bottom and immediately boils. This causes the already burning grease to splatter out of the pan and spread.

4. Describe two effective means of putting out a grease fire.

   A grease fire may be extinguished by covering it with a pan lid to eliminate the oxygen as fuel. It can also be extinguished with copious amounts of sodium bicarbonate.

5. What physical properties allowed the hexanes solution to evaporate so easily?

   The hexanes mixture probably has a low boiling point that is closer to room temperature than most other liquids, including water. Because of this, the liquid probably also has the ability to evaporate at lower temperatures than other solutions.

6. Why did the hexane vapors (C₆H₁₄) travel down the ramp when they were poured out of the flask, instead of just dispersing into the air? Hint: Keep in mind that the molecular weight of oxygen is 32 g/mol.

   Hexanes have a higher molecular weight, at 86 g/mol, than oxygen (32 g/mol). Therefore its vapor density is greater, as well. This forces hexane vapors to sink in air and travel along surfaces rather than rising into the air and dispersing.

7. What does this demonstration teach you about using flammable liquids in the laboratory and at home?

   Using flammable liquids such as hexanes is dangerous in the lab for a number of reasons. First of all, many organic flammables have low boiling points and evaporate easily. These vapors can travel all over a lab or even a building. The same precautions should also be taken at home when working with certain aerosols. Should they encounter an ignition source, such as a furnace or an electrical switch, they will light on fire and spread quickly.

8. Given the fact that the mystery chemical was ignited after the flask had been filled with water, which chemical could be immediately eliminated based on its density?

   Given that the mystery chemical is mixed in water which has a specific gravity of 1.0 and this chemical was ignited from the top of the flask, it must be less dense than water and therefore have a smaller specific gravity. Since ethylene dichloride has a specific gravity of 1.3 it can be eliminated as an option.

9. What additional information included on the SDS allowed the identification between the last two possible choices for the mystery chemical? Hint: Remember that only 2 mL of the mystery chemical was present in 500 mL of water.

   Both hexanes and isopropyl alcohol are flammable and less dense than water and would therefore ignite at the top of the flask. Hexanes are insoluble in water and isopropyl alcohol is soluble in water. If the mystery chemical was soluble the concentration would be much too weak to ignite at the top of the flask. Therefore the mystery chemical must be hexanes.

Discussion

Acid in the Eye
Egg whites and human eyes contain an abundance of proteins. Proteins are natural polymers (also called polypeptides) formed by linking amino acids together. Proteins, when subjected to strong acids, first undergo a process known as denaturation, in which they lose their native three-dimensional structures. Concentrated acids may further break down proteins to amino acids via hydrolysis reactions. Proteins need very specific three-dimensional structures to perform their biochemical functions—denaturing of the protein structure due to changes in pH changes the properties of the protein and is frequently irreversible. Hydrolysis of the protein destroys the protein.

This demonstration should convince students the importance of wearing chemical splash goggles any time chemicals, heat or glassware are used. During the school year, a gentle reminder of “remember the egg white” should bring back students’ vivid memories of why they want to wear their goggles.

Grease Fire
This demonstration clearly shows using water is not only an ineffective method but a dangerous method of extinguishing a grease fire. Wax and grease (vegetable oils or animal fats) are less dense than water. When water is added to a grease fire it sinks to the bottom of the container. Once the water touches the surface of the hot pan it immediately boils. This causes the hot droplets of grease to shoot out of the pan and the fire to spread.

There are much more effective ways to extinguish a grease fire. Oftentimes the fastest method of smothering a small grease fire is to cover it using the lid of a pan. Grease fires may also be extinguished by smothering with copious amounts sodium bicarbonate, NaHCO₃.

Flaming Vapor Ramp
Many organic solvents have very low boiling points and hence are highly volatile at ambient temperatures. For example, the hexanes solution has a boiling point of 68–70 °C (154–158 °F) and a vapor pressure of 150 mm Hg at 25 °C. Most organic compound vapors are also colorless and therefore nearly impossible to see. Hexanes (C₆H₁₄) have a molecular weight of 86 g/mol.

This gives hexanes vapors a density of nearly three times that of air (M.W. of oxygen is 32 g/mol). Thus hexanes vapors (and most other organic vapors) are heavier than air and will sink in air. Heavier-than-air vapors are also easy to pour. When the hexanes vapors are poured down the vapor ramp and make their way to the lit candle, all three necessary ingredients for a fire are present—air containing oxygen, hexanes fuel, and a source of ignition or heat. The flame travels back up the ramp, leaving an impressive trail of fire as it goes.

What makes this an especially valuable demonstration is the safety lesson to be conveyed. Using flammable liquids indoors can be a fire hazard even if you are nowhere near an open flame. Indeed, as any firefighter can attest to, volatile fumes can travel along the floor, even down steps and find an ignition source, such as the pilot light of a furnace or hot water heater or an electric switch. Outdoors, ignition sources are less common, and winds generally cause the fumes to dissipate and not reach combustible levels.

SDS Challenge Inquiry Demonstration
Students are given Safety Data Sheets for ethylene dichloride, hexanes, and isopropyl alcohol. An ideal category to begin searching is in the Hazards Identification section. All three chemicals are very flammable.

Students know that the water is not flammable. Therefore, the flammable mystery chemical must be less dense than water since the liquid catches fire at the top of the Erlenmeyer flask. Water has a specific gravity of 1.0. The mystery chemical could not be ethylene dichloride as its specific gravity is 1.3 and it would be at the bottom of the flask.

Hexanes and isopropyl alcohol are both highly flammable and have a lower specific gravity than water. It is not visible that there are two layers near the top of the flask so one might think that the mystery chemical is soluble in water. However, if the mystery chemical is soluble in water there would be 2-mL of the flammable mystery chemical in 500-mL of nonflammable water. This concentration is much too low to cause the water to catch fire at the top of the flask. Therefore the mystery chemical must be hexanes as it is flammable, has a lower specific gravity than water and is insoluble in water. The hexanes will remain at the top, capable of being ignited.

References

Becker, R. Twenty Demonstrations Guaranteed to Knock Your Socks Off!, Vol 2; Flinn Scientific: Batavia, IL, 1997, pp 59–60.