Temperature Liquid Oxygen

Transcription

1 Temperature Liquid Oxygen Edited by Anne Starace ABSTRACT: We can sense temperature changes through our senses of touch, but what happens on a molecular level as temperature changes? This module demonstrates what happens to molecules as they change temperature. LAB MANUAL: Recipes for Science: Temperature and Kinetic Theory Citation: The Science Theatre, Recipes for Science, ed. By Jennifer Discenna (Michigan State University, Kalamazoo, Ml, 1993). Permission granted by Jennifer Discenna by The Science Theatre; reprinted with permission. Keywords: Demonstrations; Thermal Physics; Oxygen; Phases Of Matter; Molecular Model Of Matter; Kinetic Theory; Laboratory Manual Funded by the National Science Foundation and the University of Nebraska

2 Content Standards K History & Process Standards K Skills Used/Developed: 2

3 TABLE OF CONTENTS I. OBJECTIVES...4 II. SAFETY...4 III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTSError! Bookmark not defined. IV. LIST OF MATERIALS...Error! Bookmark not defined. V. INTRODUCTION...Error! Bookmark not defined. VI. PROCEDURE...5 VII. FREQUENTLY ASKED QUESTIONS...7 VIII. TROUBLE SHOOTING...7 IX. HANDOUT MASTERS...7 X. REFERENCES:...7 3

4 I. OBJECTIVES Students will: -understand that when molecules are cold the are closer to each other and move less than when they are warm. -observe that different elements change states at different temperatures. II. SAFTEY Do not do this experiment on a tablecloth! Setting the bowl on a metal plate may help the liquid Oxygen to collect. Do not touch liquid nitrogen with bare skin! Keep students away from open flame and liquid nitrogen. Have another pail of water available, just in case. III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS Level: This module is good for students grades Time required: 5-10 minutes Number of participants: Up to about 30. IV. Liquid Nitrogen Safety Equipment (see footnote 1) Shallow Metal Bowl Beaker Small Piece of Crumpled Paper Matches LIST OF MATERIALS V. INTRODUCTION The temperature in Kelvin, denoted with a T, equals the degrees Celsius plus 273. T= C Kinetic theory relates the temperature of an object to the motion of its molecules. The main equation of interest in this case, which is derived in many introductory texts (see for example, Physics by Cutnell & Johnson) is ½ mvrms2 = 3/2 Kt. 4

5 Where k is the boltzmann constant, T is the Kelvin temperature, m is the mass of the particle and vrms is the root-mean-square speed. This equation relates directly the Kelvin temperature and the average translational kinetic energy of an individual particle. As the kinetic energy of the particles increase, so must the Kelvin temperature and vice versa. When the temperature decreases, the equation above tells us that the kinetic energy, the speed of the molecules, must decrease as well. As the speed of the molecules increase, the molecules fill up more space. Molecules that are cold, vibrate very little and stay close together. Molecules that are warm, vibrate more rapidly and take up more space. A short temperature scale (Kelvin): Water boils 373 Room temperature 293 Water freezes 273 CO 2 freezes (dry ice) 194 Liquid Nitrogen 77 Looking at the temperature scale you can see that room temperature is pretty high on the temperature scale and the freezing point of water is a little bit lower, dry ice, which some students are familiar with is also on the list, but a little ways down. Liquid Nitrogen, however is all the way at the bottom of the list at 77K!! This is extremely cold. It is colder than ice, much colder than any winter day! VI. PROCEEDURE Execution: Place the crumpled piece of paper into the beaker and set aside. Pour some liquid Nitrogen into the bowl. Lift the bowl up with your gloves on. Show the audience that there is some frost on the outside of the bowl and some liquid dripping down. The liquid is Oxygen. To prove this, light the paper in the beaker with the match and drip the liquid oxygen onto the flames. You will see that the liquid does not drown the flames, but rather makes them brighter. Explanation: As discussed before, all liquids can boil and turn into gases if they are heated up to their boiling points. Likewise, all gasses can turn into liquids if they are cooled down enough. We already said that nitrogen gas turns to a liquid at 77K. Oxygen also turns to a liquid at a very low temperature, but it is higher than 77K. The liquid nitrogen inside the metal bowl cools the outside of the bowl. The bowl then cools the air on the outside of the bowl. The frost on the side of the bowl is frozen water vapor from the air. The liquid is oxygen gas from the air that has been cooled to a liquid. 5

6 For a fire to burn it needs oxygen. When the fire is burning in the beaker it is using oxygen gas from the air to burn. When the liquid oxygen hits the flame, the fire uses the liquid oxygen instead of the oxygen gas. The flames using the liquid oxygen burn brighter because there are more oxygen molecules in a drop of liquid oxygen than there are in a 'drop' of oxygen gas. This is because the molecules in a liquid are closer together, more dense, than the molecules in a gas. What to Say: When you are holding the bowl, show the audience what is going on. Ask them what they think the white frozen layer is. Most will guess correctly that it is frozen water. Make sure that you emphasize that it is frozen water vapor from the air. Now, ask them what they think the liquid dripping from the sides of the bowl is. Most will guess that it is water. Remind them that the frozen white layer is frozen water, so what is that liquid? The liquid is liquid oxygen. Remind them what air is composed of: Nitrogen, Oxygen, Carbon Dioxide, and water vapor. The water vapor froze and caused the layer of frost. The liquid is actually liquid oxygen. The liquid nitrogen is so cold that it froze the outside of the bowl and caused the oxygen in the air to liquefy. Ask the audience what a fire needs to burn. A fire needs heat, fuel and oxygen. When the fire is burning in the beaker, from where is it getting the oxygen? The oxygen it uses is oxygen gas that comes from the air. When the liquid oxygen is poured onto the fire, what will happen? The flames will be brighter. The flames now use the liquid oxygen instead of the oxygen gas from the air. This in itself is very interesting. The idea that you have liquid oxygen dripping from off of the bowl and that it doesn't drown out the fire. You can also bring up the fact that there are more molecules in a drop of liquid oxygen than there are in a 'drop' of oxygen gas. This is why the fire burns brighter, there are more oxygen molecules for the flames to use to burn. Remind the students of the difference between molecules in a liquid and molecules in a gas. When the molecules are in a gas they spread themselves out and take up as much space as possible. When molecules are in a liquid they stay closer together. So, if you look at any one little bit of space, there are more molecules in the liquid than in the solid. This might also be a good time to remind students how a scientist works. First, the scientist notices a phenomenon (liquid dripping) then they make a hypothesis (it's oxygen). Finally, they test their hypothesis (if it oxygen, the flame will burn brighter). Footnotes: 1 Liquid Nitrogen can be purchased on most campuses for a modest fee. Liquid Nitrogen is stored in a dewar which will keep it cold enough for long periods of time. When working with liquid nitrogen, you must use proper safety equipment: safety goggles and gloves must be worn. 2 To order contact: DaMert Company, Department T, 2476 Verna Ct. San Leandro, CA For More Information: TiNi Alloy Company th Street Oakland, CA

7 VII. FREQUENTLY ASKED QUESTIONS VIII. TROUBLE SHOOTING IX. HANDOUT MASTERS X. REFERENCES 7