Purpose: Students will explore how our sense of smell helps us interpret the world around us, and how those interpretations may vary. Students will practice analyzing data and determine how temperature affects vapor pressure and thus the intensity of scents.
Procedural overview: Students will record the scents they detect from smelling various samples at room temperature and after warming in a water bath. Comparing data with others in the class will encourage students to think about what factors affect the intensity of a scent and a person’s perception of it. After reading the Science News article “People who lack olfactory bulbs shouldn’t be able to smell. But some women can,” students will answer final questions about how nerve cells in the nose and brain are responsible for scent perception.
Approximate class time: 1 class period to complete the experiment, activity questions, calculations and graphing.
Supplies:
Test tube racks (1 per group)
Test tubes (4 per group)
Stoppers (4 per group)
Hot plates to create a water bath (1 per group)
Beakers for the water bath (2 per group)
Boiling stones for the water bath (3 or 4 per group)
Permanent markers (1 per group)
Thermometers (1 per group)
Room temperature water
Samples including methyl salicylate (a maximum of 10% solution), ethyl cinnamate, hexyl hexanoate (a maximum of 10% solution) and benzyl salicylate, which can be purchased from Sigma-Aldrich, Fisher Scientific or another laboratory supplier
Labeled waste containers
Computers, graph paper or graphing calculators
A projector, chalkboard or white board for displaying the class’s recorded results
Directions for teachers:
During Parts 1 and 2 of this activity, students will record the scents of different samples, determine how those scents and their intensity changes after the samples are heated, create graphs of the data and analyze the results.
During Parts 1 and 2 of this activity, students will record the scents of different samples, determine how those scents and their intensity changes after the samples are heated, create graphs of the data and analyze the results.
Teachers should consider running through the smell tests at least once on their own before students embark on the activity. This will allow teachers to check the strength of the scents and dilute the samples accordingly.
Teachers may also want to set up and turn on the warm water baths before class begins to reduce the waiting time. To set up the water baths, add about 50 to 75 milliliters of water to a beaker at each station and place three to four boiling stones into the beaker. Turn on the hot plate to a medium setting, and place the beaker on the hot plate. Monitor the temperature until it reaches about 60° to 70° Celsius, adjusting the hot plate temperature as needed.
Once the lab is set up, direct students to form pairs or small groups for the activity and assign each team to a station. Each station should be equipped with a thermometer, test tube rack, test tubes, beakers, stoppers, a warm water bath (hot plate and boiling stones, if the teacher hasn’t set up the bath), and a permanent marker. You will also distribute the chemicals to each group, put the stoppers on their test tubes and provide room temperature water for the students’ “cold” water baths.
Instruct all students to wear laboratory goggles and gloves during this experiment. You should also explain to students how to waft scents from the test tubes toward the nose in order to smell the samples. Students should hold each test tube in one hand at least 20 centimeters away from the faces, and use the other hand to wave or draw the air above the test tube toward themselves four times; ensuring consistent distance and number of waves will help limit variables between tests. Demonstrate this method and explain that smelling directly from test tubes in a laboratory is never done, as it is not safe and can cause injuries.
Students are now ready to begin the activity. After each set of smell tests, the students will record their data in their data table and then provide their data to you. You will need to record it in a table like the one below. Once the lab is complete, display all of the data via projector or on the board for students to see.
After completing Parts 1 and 2, students will read the Science News article “People who lack olfactory bulbs shouldn’t be able to smell. But some women can” and answer the accompanying questions.
Additional notes on safety
This experiment uses volatile chemicals, which should be handled in a well-ventilated area. Students who are particularly sensitive to smells should be placed in groups closest to ventilation, where there will be the least accumulation of scents. These students should also hold the test tubes farther away from their faces (>30 centimeters instead of 20 centimeters).
The chemicals used in this experiment are:
Methyl salicylate — a chemical commonly used to treat minor aches and pains and as a flavoring agent. According to the Material Safety Data Sheet for methyl salicylate, exposure to a solution greater than 99% in concentration may cause eye or skin irritation, respiratory tract irritation if inhaled and can cause nausea and vomiting if swallowed. The boiling point for methyl salicylate is 222° Celsius at 760 mm Hg.
Ethyl cinnamate — a chemical commonly used as a flavoring agent in foods and in air care products. According to the MSDS for ethyl cinnamate, it may cause eye or skin irritation and respiratory tract irritation if inhaled. The boiling point for ethyl cinnamate is 271° C at 760 mm Hg.
Hexyl hexanoate — a chemical commonly used as a flavoring agent in foods and in air care products. According to the MSDS for hexyl hexanoate, it may cause mild skin irritation. The boiling point for hexyl hexanoate is 246° C at 760 mm Hg.
Benzyl salicylate — a chemical commonly used in soaps and cosmetics as a fragrance, as well as sunscreens and deodorants. According to the MSDS for benzyl salicylate, exposure to a solution greater than 98% in concentration may cause eye or skin irritation, respiratory tract irritation if inhaled and can cause nausea and vomiting if swallowed. The boiling point for benzyl salicylate is 320° C at 760 mm Hg.
While all of the materials used in this experiment are commonly found as additives for flavor and odor in foods, cosmetics and other products, concentrations are stronger in the lab. It is always best to practice laboratory safety when working with chemicals. As with all chemicals in the laboratory, MSDS for the chemicals used in this experiment should be clearly marked and available in the classroom during the experiment for easy reference by students and for safety personnel during emergencies.
Test tubes in the warm water bath should not have stoppers. Leaving the tubes open will prevent the vapor pressure within the test tubes from building up and breaking the test tubes or launching the stoppers.
Instruct students on how to safely dispose of chemicals after the experiment is completed.
Directions for students:
During the activity, you will work with a partner or small group to determine what factors affect the intensity of a scent and a person’s perception of it. You and your partner(s) will record the scents that you each smell from different samples, first at room temperature (or “cold”) and then after warming in a water bath. You will compare your results with each other and with your class, so it is important that you record your own observations before discussing them with your partner(s).
Notes on safety
Because this activity involves laboratory equipment and chemicals, it is also important that you wear lab safety gear (goggles and gloves) and that you follow your teacher’s additional instructions. Your teacher will show you the method for wafting scents from a sample toward yourself. This is the method that scientists use so they aren’t overwhelmed by powerful scents and don’t expose themselves to corrosive or toxic chemicals. While the chemicals you are using in this activity are commonly found as additives for flavor and odor in foods, cosmetics and other products and are therefore relatively safe in small amounts, you should not be directly touching, tasting or smelling laboratory chemicals. The chemicals used in this experiment are:
Methyl salicylate — a chemical commonly used to treat minor aches and pains and as a flavoring agent. According to the Material Safety Data Sheet for methyl salicylate, exposure to a solution greater than 99% in concentration may cause eye or skin irritation, respiratory tract irritation if inhaled and can cause nausea and vomiting if swallowed. The boiling point for methyl salicylate is 222° Celsius at 760 mm Hg.
Ethyl cinnamate — a chemical commonly used as a flavoring agent in foods and in air care products. According to the MSDS for ethyl cinnamate, it may cause eye or skin irritation and respiratory tract irritation if inhaled. The boiling point for ethyl cinnamate is 271° C at 760 mm Hg.
Hexyl hexanoate — a chemical commonly used as a flavoring agent in foods and in air care products. According to the MSDS for hexyl hexanoate, it may cause mild skin irritation. The boiling point for hexyl hexanoate is 246° C at 760 mm Hg.
Benzyl salicylate — a chemical commonly used in soaps and cosmetics as a fragrance, as well as sunscreens and deodorants. According to the MSDS for benzyl salicylate, exposure to a solution greater than 98% in concentration may cause eye or skin irritation, respiratory tract irritation if inhaled and can cause nausea and vomiting if swallowed. The boiling point for benzyl salicylate is 320° C at 760 mm Hg.
If at any time during the activity you feel light-headed, nauseous or dizzy, tell your partner(s), put the test tube you are holding in the rack and go with your partner to sit in a well-ventilated area. Partner(s) should be sure to turn off the hot plate and notify the teacher immediately.
Do not pour chemicals down the drain, as this can contaminate animal habitats. Follow your teacher’s instructions on disposing of chemicals when the activity is completed.
Part 1: Experiment
Preparing the equipment
Your teacher will assign your partner(s) and station. Each station will be equipped with a thermometer, test tube rack, test tubes, beakers, warm water bath (or hot plate and boiling stones for creating your own) and a permanent marker. Your teacher will provide the samples and room temperature water.
To prepare your laboratory equipment for the activity, use the permanent marker to label your test tubes 1 through 4 and place them in your test tube rack. Your teacher will come to your station and add 1 milliliter of each solution in separate test tubes (methyl salicylate in test tube 1, ethyl cinnamate in test tube 2, hexyl hexanoate in test tube 3 and benzyl salicylate in test tube 4) and then place a stopper on the top of each of the test tubes.
You will need to ensure that the contents of your test tubes are all the same temperature for your “cold” smell test. Your teacher will have a jug of room-temperature water for you to use. Fill one beaker with about 50 to 75 milliliters of water and place it on your station. Place the test tubes in the beaker with the thermometer and wait several minutes for the test tubes and samples to reach the same temperature before recording the thermometer reading.
Finally, to warm the test tubes for the warm smell test, you will need a water bath. A water bath helps distribute heat evenly to all the samples. To set up your water bath (if your teacher hasn’t done it for you), add about 50 to 75 milliliters of water to a beaker and place three to four boiling chips into the beaker. Turn on the hot plate to a medium setting, and place the beaker on the hot plate. Monitor the temperature until it reaches about 60° to 70° Celsius, adjusting the hot plate temperature as needed. While this water is warming, complete the cold smell test portion of the experiment.
Cold smell test
Remove the stopper from each sample one at a time and, using the wafting technique your teacher showed you (holding test tubes 20 centimeters from your nose and waving the air above the test tube toward your nose four times), take turns with your partner(s) smelling the contents of each of the test tubes. After everyone on your team has smelled the contents of a test tube, place that test tube, without its stopper, in the warm water bath so that it will warm while you continue your experiment.
1. Create a data table and record your observations. Your data table should include the type of test (cold), the test tube number, the chemical name, what you smell (a description of the smell and a rating of its strength) and what your partner(s) smell (a description of the smell and its strength). Rate the strength of the smell on a scale from 0 to 5, where 0 is no smell, 3 is a medium smell and 5 is a very strong smell. Once you have recorded your data and that of your partner(s), share the data with your teacher.
See Teacher Answer Key for sample student data table.
2. What is the temperature of the “cold” water bath?
The temperature of the “cold” water bath is 20° Celsius.
3. Did you and your partner(s) always describe the scents in the same way? Explain.
No, sometimes we did, but sometimes we did not.
4. Could something affect the way the samples smell to you and your partner? What could that be?
My partner and I have different noses, so maybe that affects the way things smell to us. Also, if we had a cold or an allergy, that could affect our ability to smell because our noses would be stuffed up.
5. Did you and your partner(s) ever use different words to describe the same smell?
Yes, we used different words to describe the smell of methyl salicylate.
6. Why do you think the words you chose to describe the scents might differ? Use an example from your table, discuss with your partner(s) and explain your reasoning.
My partner uses mint-flavored toothpaste, so the smell of mint makes my partner think of toothpaste. We don’t use mint toothpaste in my house, so mint scents don’t make me think of toothpaste. This is why my partner described methyl salicylate as toothpaste, but I did not.
Also, we eat a lot of spicy food in my family, so I am used to smelling spices. My partner does not eat spicy foods so may not know how to describe the spicy smell of ethyl cinnamate and may have used the closest smell she could think of, which was plums.
Warm smell test
Move your thermometer to the water bath and leave it there while you complete the warm smell test. Using the same wafting technique used for the “cold” smell test, take turns with your partner(s) smelling the contents of each of the warmed test tubes.
7. Record your findings and those of your partner(s) in a data table. Your data table should include the type of test (warm), the test tube number, the chemical name, what you smell (a description of the smell and a rating of its strength) and what your partner(s) smell (a description of the smell and its strength). Rate the strength of the smell on a scale from 0 to 5, where 0 is no smell, 3 is a medium smell and 5 is a very strong smell. Once you have recorded your data and that of your partner(s), share the data with your teacher.
See Teacher Answer Key for sample student data table.
8. What is the temperature of the warm water bath?
The temperature of the warm water bath is 60° Celsius.
9. Look at both of your tables. List all of the variables in this experiment.
The variables in this experiment are the temperatures of the test tubes, the difference between me and my partners’ noses and differing experiences and vocabulary that affect how we describe scents.
Part 2: Data analysis and graphing
Review the class data, then answer the following questions.
10. For which of the cold test tubes did the class have the most different descriptions of smell? What were those descriptions?
The test tube of ethyl cinnamate had the most different descriptions. It was described as fruity, fruit, plums, spicy, spices, berries, raspberries, sweet and powdery.
11. For which of the cold test tubes did the class have the most similar descriptions for the smell? What were those descriptions?
The entire class agreed that the test tube containing methyl salicylate smelled like mint, but we used different words for mint including “minty,” “toothpaste,” “gum” and “spearmint.”
12. Were there any cold test tubes where some members of the class were able to detect a scent while others were not?
Many people in the class couldn’t smell the benzyl salicylate when it was cold. Also, one student (who has a cold) couldn’t smell any of the samples except for the methyl salicylate.
13. Use graph paper, a computer or a calculator to create your own graph of the data from Parts 1 and 2 of your assessment of the strength of each sample’s scent versus the temperature of the test tubes.
Sample student graph:
14. What trend do you see in the graph? How did heating the test tubes affect the scents?
The trend of the lines is a positive slope for all of the samples except ethyl cinnamate, which stayed the same. Heating the test tubes made many of the scents stronger.
15. Why do you think this occurred? What changed by heating the test tubes?
Heating the test tubes caused more of the solutions to evaporate. This increased the concentration of gaseous molecules of the samples in the air, which increased our ability to smell the odors.
16. When the rate of evaporation/vaporization equals the rate of condensation of a liquid at a certain temperature in a closed container, the vapor pressure is defined as the pressure exerted by the gaseous molecules above the surface of the liquid. As the temperature of a liquid changes in a closed container, the number of gaseous molecules above the surface of the liquid changes, which in turn changes the vapor pressure.
The vapor pressures of three liquids at different temperatures are listed in the table below. Use graph paper, a computer or a calculator to graph the data from the table of the vapor pressure versus temperature.
Sample student graph:
17. What trend do you see in the graph of vapor pressures?
The vapor pressures increase as the temperature increases.
18. Does this trend make sense to you? Why or why not?
Yes, it makes sense because more gaseous molecules will exist above the surface of the liquid the warmer the liquid is.
19. Is the trend in this graph consistent with the data from your experiment? Explain your reasoning.
Yes, the trend is consistent with the data from my experiment. The warmer a liquid is, the more gaseous molecules of the sample there are in the air above the liquid. This is why the scents were stronger when the temperature of the test tubes was increased.
20. Based on what you have learned about vapor pressure, why was it important to leave the stoppers off the test tubes when you were warming them?
As the test tubes got warmer, more gaseous molecules would go into the space above the liquid in each test tube, increasing the pressure. If the stoppers had been on the test tubes, the pressure inside the test tubes might have broken the test tubes or launched the stoppers into the air.
Part 3: Article-based questions
Now, read the Science News article “People who lack olfactory bulbs shouldn’t be able to smell. But some women can” and answer the following questions.
21. According to the article, how are scents detected?
The article says that there are structures in the brain called olfactory bulbs. Each bulb is made up of about 5,500 nerve clusters that receive signals about scents from the nose and relay them to other parts of the brain to be interpreted.
22. What is the function of the nose in this process? Is it just a tube that funnels scents directly to the brain, or does it serve another function?
The nose does not just funnel scents directly to the brain. There are sensory cells in the nose, and these detect the scents. Then signals about the detections are sent to the olfactory bulbs in the brain.
23. Does what you found in your experiment support this explanation for how smell works? Why or why not?
My experiment supports this explanation. The sensory cells in the nose pick up the gaseous molecules from the sample, and the more molecules that are sensed, the stronger the smell. Since heat increases the number of gaseous molecules above the surface of a liquid, there were more molecules in the air above the warm samples, so the cells in my nose sensed more molecules for the warm samples and sent stronger signals to my brain, which told me the smell was stronger.
24. How does the research in this article challenge this understanding of how people detect scents?
There are two women in the article who do not have the olfactory bulb structures in their brains, but the women are still able to detect scents.
25. How do researchers hope this research will be helpful to people?
The scientists are hoping that they will be able to determine how the women are able to smell, and then use that knowledge to help other people with smell disorders.
26. Why is a sense of smell important? Think of ways that people use scents every day.
Smell is important for safety. Smell can tell us when food is ready to eat or when it has spoiled. Smells can also tell us when something is burning or toxic and so alert us to danger.
27. Based on your experience with your experiment, what advice might you give to the scientists in the article to help them reduce the variables in their experiment?
I would suggest providing a list of words for participants to choose from in describing scents. This would ensure that the descriptive words are all the same, and that smells aren’t categorized as different when they are really just different words for the same smell, like how we used many different words for “mint” when describing methyl salicylate.