Lab 11. How to Apply Gas (Laws) Can water boil at room temperature? How can you use baking soda and vinegar to pop a stopper?

Transcription

1 Lab 11. How to Apply Gas (Laws) Can water boil at room temperature? How can you use baking soda and vinegar to pop a stopper? Temperature or pressure or both determine the state of matter (solid, liquid, or gas) of a substance. At room temperature, the state of matter depends on the chemical forces between atoms or molecules. A phase change occurs by changing the temperature or pressure or both. In this lab, you ll apply gas laws to measure the pressure inside popcorn to make it pop, make water boil with ice, figure out how to propel a rubber stopper with a solid and liquid, and see how much water a penny holds. Part 1. How Does Popcorn Pop? Students: bring popcorn (unbuttered) 1. a. Describe how popcorn pops. Draw a picture of popcorn popping from the time a bag of popcorn is popped into a microwave oven to the time the popcorn pops. Your picture should answer the following questions: What is the gas inside a popcorn kernel that makes popcorn pop? Must the pressure of the gas inside the popcorn kernel be less than, equal to, or greater than atmospheric pressure to make the popcorn pop? 2. a. A popcorn kernel looks dry. But, popcorn kernels, like all animal and plant tissues, contain water. How can you experimentally determine whether popcorn contains water? How can you determine the amount of water in popcorn? b. What physical quantities must you know to calculate the pressure of gas inside a popcorn kernel to make it pop? Write a mathematical equation that shows the relationship among these quantities. (i) describe how pressure, volume, and temperature affects a gas You ve rented a video (who does that any more?). You ve got a beverage. You ve got popcorn. You put the unpopped popcorn bag into the your microwave oven and turn on the oven. You wonder, What s happening inside that bag? I bet gas laws are involved here. Popcorn consists of three components: the germ (embryo), endosperm (mainly starch), and pericarp (hull). Popcorn needs about 14% water to pop. The water is found in starch granules in the endosperm. Compared to other types of corn, the popcorn hull is thicker, has a denser arrangment of cellulose fibers, conducts heat several times faster to the endosperm, and can withstand higher steam pressures. As the kernel heats up, the water evaporates (at what temperature?), the steam turns the starch into a gelatin, and the pressure. Popcorn pops best at about 400 o F ( o C). At this temperature, the pressure is high enough to burst the hull. When the hull breaks, the pressure and the steam expands along with the soft starch. As the starch cools, it solidifies and forms your light and crispy and delicious popcorn. There are three common ways to pop popcorn: in a microwave oven, in a pot on a stove with oil, and with an air popper. In this experiment, you will make popcorn by air popping and by heating oil. The, you ll use gas laws to estimate the pressure of steam inside a popcorn kernel to make it pop into a healthy, nutritious snack. Students: bring popcorn (unbuttered) Determine the pressure of gas inside an unpopped popcorn kernel to make it pop 1. You will make popcorn using the hot oil method and hot air method. Using each method, design an experiment to measure the following:

2 the mass of one kernel of unpopped popcorn. Is the mass of one kernel the same as every other kernel? If not, how can you determine the average mass of one kernel? The volume of one kernel of unpopped popcorn. Is the volume of one kernel the same as every other kernel? If not, how can you determine the average volume of one kernel? The mass of water in one kernel of unpopped popcorn. The temperature at which popcorn pops best is described in the. How will you heat the popcorn kernel to this temperature? How will you measure this temperature? 2. Start with a known number of kernels, e.g., 20, of unpopped popcorn to determine this information. Before you start your experiments, show your experimental design to your lab instructor. Make any modifications as needed. Have fun! Waste Disposal: solids in trash. Solutions in sink. 3. Record your data and results in Table 1. Table 1. Popcorn data and results. What goes in this table? 4. a. Ask your instructor to tell you the pressure at which a popcorn hull will burst. (Note: your instructor will tell you this pressure after you do your experiment so as not to influence you in any way.) Compare the pressure you calculated to this pressure. Calculate % error. b. Calculate the % water from your data. Compare to the 14% water described in the. 1. Clearly show the data you collected and results you calculated. 2. Describe the gas laws involved in popping corn. 3. Good popped popcorn depends on the rate of heating. The popcorn hull is hard and not porous. The tip of the kernel where it grew from the cob is not pressure tight and will gradually equalize internal pressure with its surroundings. If the rate of heating is not right, a few things can happen: The starch at the center of the kernel does not turn to gelatin and does not soften. The rise in pressure inside the hull is slower than the loss of steam from the tip of the kernel. The starch at the outer edge reaches the required temperature and causes the hull to rupture but the uncooked starch at the center of the kernel does not expand and forms a small hard partially popped kernel of corn. Kernel does not pop (called an old maid in popcorn circles). a. If the rate of heating is too fast, which two things happen? b. If the rate of heating is too slow, which two things happen? Part 2. Make water boil with ice. In a beaker, boil water. a. What chemical force(s) are breaking when water boils? b. Draw a picture of liquid water boiling. c. From your picture, what prevents the liquid water molecules from escaping into the gas phase? d. Is there a way to lower or decrease the thing that prevents liquid water molecules from escaping into the gas phase?

3 (i) explain how you can make water boil at room temperature using a phase diagram ice Water boils at 100 o C at 1 atm pressure. Does the boiling point change if the pressure changes? You perform the following demonstration. You heat some water to boiling in an Erlenmeyer flask. You remove the flask from the heating source and close the flask with a rubber stopper. Next, you make the water in the flask boil simply by rubbing an ice cube on the outside walls of the flask. (Reference: See Chang, General Chemistry: The Essential Concepts, 5 th ed., 2008, p. 423, Problem ) Report Do this demonstration in front of your instructor. Explain why the water boiled using your knowledge of gas laws, states of matter, and phase diagrams. Part 3. SLAC Students: bring some vinegar to lab. Add baking soda to vinegar. a. What happens? Does the reaction get hot or cold? How can you tell? b. What happens to the ph as the reaction goes? Use litmus or a ph probe. c. What happens to the mass as the reaction goes? Use a balance if you wish but don t spill anything on the balance. (i) apply a gas forming reaction to do work Each year, the Physics Olympics is held at Hartnell College. During the Spring of 1999, the Hartnell College Chemistry Club designed a chemistry event: Salinas s Linear Accelerating Corks (SLAC). In this event, the reaction between baking soda and vinegar is used to propel a rubber stopper from a container toward a target. The score is based on distance and accuracy. We ll focus on distance. Students: bring vinegar 1. Design your SLAC. Use your investigation of the vinegar-baking soda reaction to help your design. a. You can use a maximum of 0.5 g of baking soda. You can use as much vinegar as you need. b. You can use one of the pieces of equipment in your Chem 1A locker as your reaction chamber. Check out a rubber stopper that fits your container from the stockroom. c. Use your knowledge of gas laws to design an apparatus that will propel the rubber stopper as far as possible. Show your calculations of your design. Your calculations should include: the amounts of baking soda and vinegar that you use, the pressure of gas (which gas is present?) that propels the stopper, a comparison of at least two container sizes and gas pressures, any other relevant aspects to your design.

4 d. Identify the data you will collect, equipment to measure the data, and results to calculate results. Summarize your data and results in Table 3. Give this table a title. e. Do your experiment. Table 2. (title). What goes in this table? Waste Disposal: Vinegar solutions in sink. Solids in trash. 1. a. Show your Table 2. b. Discuss the conditions that made the stopper travel the farthest. Include numbers! What gas laws are involved? c. Discuss any relationship between distance and pressure. Include numbers! Part 4. How Much Water Can a Penny Hold? Students: bring some pennies to lab. Take a 50 ml beaker. Fill it with water. Using a dropper, add water until you see a mound of water above the beaker a. What chemical force between water molecules is responsible for forming this mound? b. What is this liquid property called? Is this liquid property of water higher or lower than other liquids, e.g., ethanol? c. If you used ethanol instead of water, would you see the same thing? (i) relate liquid properties to chemical forces You ve seen these before: water droplets beading up on a leaf (or your clean car), bugs floating of the surface of water, water dripping from a faucet, separation of oil and water (you saw this when you made a lava lamp). These phenomena are due to a specific property of a liquid. This liquid property is due to chemical forces between liquid molecules. Students: bring pennies Water ethanol dropper We will do Part 5 together as a lab class. 1. Find a penny. Make sure it is dry. 2. a. Calibrate a dropper. How many drops is in 1 ml of water? b. Repeat your calibration. Calculate an average and % difference. 3. You will measure the number of drops of water you can add to the surface of a penny until the water spills over. Can your group use more drops than the other groups? a. Predict how many drops the penny can hold without the water spilling over. b. Add one drop of water to the surface of the penny. Did it spill over? If not, continue adding drops of water until the water spills over. How high is the mound of water on the penny? c. Record your data in Table 3.

5 Table 3. Penny drops data and results. What goes in this table? d. Using the class data for all groups, calculate the average number of drops and % difference. e. Compare the pennies for the groups with the highest number of drops and lowest number of drops. Is there any difference between these pennies? 4. You will add one drop of ethanol to the water to see if the liquid property changes. a. Add one drop of ethanol to 3 ml of water. b. Predict how many drops on the penny without the ethanol-water solution flowing over. c. Repeat Step 3 using your ethanol-water solution from Step 4a. d. Is the number of drops of the ethanol-water solution greater than, equal to, or less than the number of drops of water until the water spilled over? If the number of drops is different, what account for this difference? 5. You will compare the liquid property of water to the same liquid property of ethanol. Repeat Step 3 except use ethanol instead of water. 6. a. Take another dry penny. b. Add 20 drops of water to the surface of this penny. How high is the mound of water? c. Find a substance in the lab, besides ethanol, so that adding one drop of this substance to the water will make the water spill over. Hint: do you want this substance to increase or decrease the liquid property of water? d. Add one drop of this substance to the water. Record your observations. e. Explain how this substance increased or decreased the liquid property of water. 1. a. Show your Table 3. b. Look up the numerical values of the surface tension of water and ethanol. Cite the reference where you found this information. c. What chemical force accounts for the difference in surface tension of water and ethanol? d. Based on your observations, is water more attracted to other water molecules or is water more attracted to the penny? e. How is your answer to Question 1d related to the meniscus of water in a straw? References: 1. D. D. Holmquist and D. Volz, Chemistry With Computers, 2nd ed., 2000, Vernier Software and Technology, p. 6-1 to 7-5.