“Demonstrations that don’t behave the way students expect facilitate both the learning of (science) and the retention of this knowledge.” George M. Bodner
Celebrate April Fool’s Day with the unexpected! Four discrepant event demonstrations engage students’ natural curiosity using the element of surprise. Students may be fooled, but as they predict, observe and try to explain the results, real learning takes place!
This set of four demonstrations includes:
- The Upside-Down Water Jar—An open jar of water is covered by a laminated card and turned upside-down. Air pressure holds the card in place and keeps the water in the jar. Imagine students’ surprise when the card is removed and the water still stays in the jar!
- Shish Keballoon—Surely the balloon will pop when a wooden skewer is poked through it! This “magic trick” takes advantage of the polymeric properties of latex.
- On the Level—The liquid in the U-shaped tube is not level! Have fun as students make suggestions to explain the phenomenon and try to even things out.
- Discrepant Balloons—Two identical balloons, inflated to different volumes, are connected. What will happen when the pathway is opened and air is allowed to flow between the two balloons? The outcome is surprising... but should it be?
The April Fool’s Day Demonstrations may be presented in a variety of ways. All four demonstrations may be performed during one class period, or a different demonstration may be performed each class period—preventing students from giving away the explanation to the next class. Optional worksheets are provided at the end of these instructions for each demonstration.
Additional classic discrepant event demonstrations, such as Potato Candle and Sewer Lice, may be obtained by contacting Flinn Scientific.
- Surface tension
- Air pressure
- Pressure of fluids
- Minimum energy state
- Mathematical models
- Observation and reasoning skills
The Upside-Down Water Jar
Surface tension is a force—a force powerful enough to prevent water from spilling out of an open jar when it is turned upsidedown! A fine mesh screen hidden inside the lid of the jar provides hundreds of tiny surface tension “membranes” that, in addition to air pressure, will support the weight of the water.
Inserting a skewer through the balloon is a fun and interesting way to demonstrate the unique properties of latex polymer molecules to your students.
On the Level
The liquid in the tube is unbalanced! How can this be possible? Amaze students with this simple demonstration of the density of liquids.
Learn how to build a simple device to demonstrate the relationship between surface area and energy. Two identical balloons, inflated to different volumes, are connected. What will happen when the pathway is opened and air is allowed to flow between the two balloons? The outcome may surprise you... although it shouldn’t!
(for each demonstration)
The Upside-Down Water Jar
Liquid detergent (optional)
Jar with screw-on ring lid*
Plastic tub or bucket (optional)
Balloons, 12" latex*
Cork, size 00*
Paper towel or cloth
Skewer, wooden, 12"*
On the Level
Food coloring, blue*
Sodium chloride, NaCl, 16 g*
Water, distilled or deionized, 80 mL
Beakers, 100-mL, 2
Clamp, double buret or 2 single buret
Funnel, small (optional)
Graduated cylinder, 50-mL
Stirring rods, 2
Stoppers, rubber, solid, size 00, 2*
Tubing, clear plastic, ½", 2½"*
Balloons, 12" latex, 2*
Clamp, adjustable plastic*
Stoppers, rubber, one-hole, #4, 2*
Tubing, soft glass, 5 mm diam., 2½", 2*
Tubing, latex, 3⁄16" diam., 8″*
*Materials included in kit.
Although The Upside-Down Water Jar activity is considered nonhazardous, please observe all normal laboratory safety guidelines. Although latex is not considered hazardous, not all health aspects of this substance have been thoroughly investigated. Latex may be an allergen. Keep a cork on the tip of the skewer when not in use to prevent accidental injury. If a balloon explodes, be careful of flying particles. Although the materials in On the Level are considered nonhazardous, follow all standard laboratory safety procedures. Make sure the ends of the glass tubing are fire-polished, with no sharp edges or chips. When inserting glass tubing, always protect your hands with a towel or leather gloves. Do not use the rubber stoppers or glass tubing for any other purpose in the laboratory. Laboratory equipment that may have been previously used with chemicals or solutions should be avoided. Wear safety glasses. Follow all laboratory safety guidelines. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
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. Dry and save all Upside-down Water Jar materials for future use. Dispose of latex balloons in the trash. Save other materials for future use. On the Level waste liquids may be disposed of down the drain according to Flinn Suggested Disposal Method #26b. The Discrepant Balloons apparatus may be stored for future use. Replace balloons as needed.
The Upside-Down Water Jar
Place the screen inside the lid of the jar, and screw the lid tightly onto the jar.
For best results, use a sharp skewer. If the skewer tip becomes dull, sharpen it by folding a piece of fine sandpaper around the tip of the skewer and rotating the skewer back and forth until the tip is sharp. Wipe the tip with a small amount of mineral oil on a paper towel or cloth.
On the Level
- Clamp the double buret clamp or two single buret clamps onto the support stand.
- Clamp the ½" plastic tubing into the double buret clamp to form a U shape (see Figure 1), with one side slightly higher than the other.
- Add a few drops of blue food coloring to 40 mL of distilled or deionized (DI) water in a beaker. Mix well with a stirring rod.
- Make a saturated sodium chloride solution by adding 16 g of NaCl to 40 mL of DI water in a second beaker. Stir thoroughly with a clean stirring rod.
- Add a few drops of blue food coloring to the saturated sodium chloride solution in the second beaker, matching the shade of blue to that of the water.
- Carefully and slowly pour the blue saturated sodium chloride solution down the higher side of the tube.
- Carefully and slowly pour the blue DI water down the other side of the tube to avoid mixing the liquids as much as possible. After the water has been completely added, the liquid levels should not be equal—the NaCl solution side will be lower than the DI water level.
- Carefully tip the setup toward the side with the NaCl solution to raise the level of liquid on that side of the tube.
- Place a rubber stopper in one side of the tube and gently set the apparatus upright.
- Stopper the other side of the tube. The completed setup will look like Figure 1 with the level of the NaCl solution slightly higher than the water level.
- Using a small amount of glycerin, carefully insert the glass tubing into a rubber stopper halfway from the bottom of the stopper (see Figure 2). Repeat for the second stopper.
- Stretch the neck of one balloon over the top of one stopper. Repeat for the second balloon over the other stopper (see Figure 3).
- Thread the latex tube through the adjustable plastic clamp until the clamp is in the center of the tube.
- Pinch the clamp tightly shut (see Figure 4).
The Upside-Down Water Jar
- Pour tap water through the screen until the jar is about three-quarters full.
- Place a laminated card over the top of the jar and hold the card down tightly with one hand. The water will form an adhesive seal with the laminated paper.
- Quickly invert the jar 180° over a sink or other container, such as a plastic tub or bucket. Note: A small amount of water may leak out if the card is not held tightly against the jar.
- While holding the jar steady, remove your hand from the laminated card. The card will remain in place over the mouth of the jar! The water forms a tight adhesive seal and external air pressure holds the card in place.
- Carefully slide the card out from under the jar with one hand while holding the jar steady with the other hand (see Figure 5). A little water may spill out, but most of the water will stay in the jar! The mesh screen provides a surface for the formation of hundreds of tiny surface-tension “membranes” that, in addition to air pressure, will support the weight of the water.
- Tilt the jar a few degrees to allow air to enter the jar (see Figure 5). The water will immediately spill out of the jar—gravity still works!
- (Optional) After performing the demonstration once for the students, ask for a student volunteer to repeat the demonstration. Dip a finger into detergent that is hidden from view and inconspicuously run the finger over the screen after the jar has been filled with water. When the student inverts the jar, the laminated card may stick for a short time due to the counter-force of air pressure acting on the outside of the card. When the card is removed, however, the water will rush out. The detergent interferes with the hydrogen-bonding network in water, which drastically reduces the surface tension of water and modifies its adhesive properties.
- Blow up the balloon to its full size. Release some of the air reducing the balloon volume to about two-thirds its full size. Tie the end of the balloon in a knot.
- Use a paper towel to coat the tip of the skewer with a small amount of mineral oil.
- Slowly insert the skewer through the end of the balloon where the latex is thickest, opposite the knot. Use a twisting motion on the skewer and apply only slight pressure. If the skewer does not slide easily, more lubrication is needed. Continue inserting the skewer through the balloon using a twisting motion until the skewer reaches the other side. Pierce the balloon again so that the skewer comes out of the balloon near the knot (see Figure 6).
- Withdraw the skewer from the balloon. The skewer will leave two small holes in the balloon since the latex does not make a perfect seal in those spots.
- Quickly throw the balloon into the air and pop it with the skewer so your students are not aware of the holes left in the balloon.
On the Level
- Show the setup to the students.
- Ask the students why the levels of liquid on each side of the tube are not even. It is possible to change the setup in some way to even out the levels?
- You can have some fun following the students’ suggestions, and they will learn to state their ideas clearly. A sample teacher/student dialogue follows.
S: Adjust the tube so the two sides are even.
T: Adjust as suggested. One level will still be higher.
S: Remove a stopper.
T: Remove left stopper. No change.
S: Remove the other stopper.
T: Replace the left stopper and remove the right. No change.
S: No, remove both stoppers!
T: Place thumb over opening to seal it and remove the left stopper. No change.
S: Leave both ends of the tubing open.
T: Remove thumb. Now the levels change, but the once higher level is now lower than the other, so they are still uneven as in Prelab Preparation step 7.
S: Could there be two different liquids in the tube?
T: “Well, I added food coloring.”
S: Besides that, could there be different liquids with different densities?
- Once the concept of density is suggested, ask for an explanation of density, which side of the tube contains the more dense liquid, and why this causes the levels to be uneven.
- Blow through the glass tubing to inflate one balloon nearly full.
- Carefully connect the inflated balloon to one end of the pinched latex tubing. Note: To keep the balloon from deflating while you are connecting it, use your finger to push the balloon to one side and cover the inside hole of the stopper through the balloon (see Figure 7).
- Repeat steps 1 and 2 with the second balloon, but inflate it only part way, about the size of a softball (see Figure 8).
- Show the setup to the students and have them predict on paper what they think will happen when the clamp is opened and air is allowed to flow between the two balloons.
- Open the clamp and observe as the air from the less inflated balloon flows into the more inflated balloon, making it larger still!
- In the Upside-Down Water Jar activity, the key to learning with discrepant events is prediction, observation and explanation. Before inverting the jar, ask students to predict what will happen. After the jar has been inverted, but before removing the card, ask students to describe their observations and then try to reconcile the conflict by explaining the result. Then ask students to predict what will happen when the card is removed. Be sure to hold the jar so students cannot see the screen. Even if you choose to reveal the screen in the end, having students observe and explain the result provides the opportunity for additional learning.
- If the jar is inverted quickly while the screen is uncovered, some of the water will pour out, but then will stop before the jar is empty.
- After asking if he or she minds getting a little wet, do the demonstration over a student’s arm. Have some towels handy in case the surface tension is broken!
- Test whether the demonstration will work with different amounts of water in the jar. Fill the jar completely, add only enough water to cover the screen, etc.
- A variation of this activity, Surface Tension Demonstration, may be viewed through the Flinn Scientific “Teaching Chemistry” eLearning Video Series at http://elearning.flinnsci.com.
- In the Shish Keballoon activity, this procedure is often used by professional magicians. Although not essential, clear tape can be used to keep the balloon from breaking and will provide the inexperienced demonstrator an extra margin of confidence. To do this, simply put a piece of clear tape on each end of the balloon where the skewer will pierce the balloon’s surface.
- Magicians often do a trick twice, purposely failing the first time to increase audience anticipation. Try to insert the skewer in the side of the balloon, where the latex is stretched the most. The balloon will pop. Indicate to the students that maybe the skewer was dirty and wipe it with the paper towel and mineral oil without letting the students see the mineral oil. Then repeat the demonstration as written.
- Show the students that the polymer chains are stretched less at the top and bottom of the balloon than at the sides. Using a new balloon that has not been inflated and a permanent marker, draw nine dots in an area about 1 cm2 on the end of the balloon opposite the opening. Do the same on one side of the balloon in the middle. Inflate the balloon and hold it closed. Show that the dots in the middle are spread out more than the dots on the end of the balloon.
- Wipe the skewer with a cloth or paper towel containing a small amount of mineral oil before storage. Store the skewer with a cork on its tip to prevent accidental injury. Insert the tip carefully into the cork so the tip does not break.
- This demonstration, titled Needle in a Balloon, may be viewed through the Flinn Scientific “Teaching Chemistry” eLearning Video Series at http://elearning.flinnsci.com.
- In the On the Level activity, buret clamps with plastic- or rubber-coated jaws work well in gripping the plastic tubing tightly enough to prevent slipping without crushing the tube. Both double and single buret clamps are available from Flinn Scientific, Catalog Nos. AP1246 and AP1034, respectively.
- If the tube is shaken up, the levels will even out.
- The On the Level demonstration may be viewed through the Flinn Scientific “Teaching Chemistry” eLearning Video Series at http://elearning.flinnsci.com.
- In the Discrepant Balloons activity, when starting the demonstration, you may want to let the students decide for you which balloon to blow up bigger than the other or use a coin toss. That way, students will not think it is some kind of trick that works because of some unseen difference in the balloons.
- Do not blow up the balloons too far in advance, for the stretch tends to affect the rubber and diminish its elasticity over time.
- The Discrepant Balloons demonstration may be viewed through the Flinn Scientific “Teaching Chemistry” eLearning Video Series at http://elearning.flinnsci.com.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Analyzing and interpreting data
Using mathematics and computational thinking
Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
HS-PS2.B: Types of Interactions
Structure and function
Stability and change
MS-PS1-1. Develop models to describe the atomic composition of simple molecules and extended structures.
HS-PS1-3. Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
Answers to Questions
Upside-Down Water Jar
- Predict what will happen when the water-filled jar covered with the card is turned upside-down.
The water will pour out of the jar; air pressure will hold the card in place, etc.
- Draw a sketch of the inverted jar filled with water and covered with the card. Use arrows to show the direction of the following forces acting on the water: gravity, external air pressure and pressure of air inside the jar.
- When the jar is inverted with the card in place, a small amount of water leaks out of the jar. Assuming that the card prevents air from entering the jar, how does the air pressure inside the jar change when water leaks out?
The air pressure inside the jar decreases, creating a partial vacuum.
- Predict what will happen when the card is removed from the jar. Give a reason for your prediction.
The water will pour out of the jar because the air pressure won’t be able to overcome the force of gravity.
- Draw the structure of a water molecule and show by means of a diagram the hydrogen bonds between water molecules. How many water molecules can be hydrogen-bonded to a central water molecule?
- Describe in detail what happened in this demonstration.
The first time the skewer was inserted into the balloon, the balloon popped. After the skewer was cleaned, it was carefully inserted all the way through the balloon and out the other side near the knot.
- Latex balloons are composed of rubber, a natural polymer. Using your textbook, define polymer.
Polymers are large, chain-like molecules composed of multiple repeating units of smaller molecules, called monomers.
- When a balloon is inflated, where are the polymer chains stretched the most? Where are they stretched the least?
The sides of the balloon are stretched the most when inflated, and the top (opposite the knot) and bottom (near the knot) are stretched the least.
- Give a possible explanation for how the skewer was able to pierce the balloon without popping it.
Student answers will vary. Note: See the Shish Keballoon Discussion section for the explanation.
On the Level
- Draw and label a diagram of the setup for this demonstration after the rubber stoppers have been removed.
- Water and saturated sodium chloride solution have different densities. One substance has a density of 1.00 g/mL and the other has a density of 1.2 g/mL. How can you infer from your diagram which substance is more dense?
Water must have the lighter density, 1.00 g/mL, because the level of the water was higher than the level of the sodium chloride solution. The saturated sodium chloride solution must have a greater density to exert a greater pressure on the water and cause it to rise.
- What would happen to the levels in the U-tube if you were to shake it and mix the water and the sodium chloride solution together? Explain why.
If the water and the NaCl solution were to mix and diffuse together, the density and therefore the pressure of the resulting combined solution would be uniform throughout the U-tube. Therefore, the level of liquid in the two sides of the U-tube would be the same.
- Isopropyl alcohol has a density of 0.785 g/mL. If isopropyl alcohol had been poured into the U-tube instead of the sodium chloride solution, which liquid would be higher in the tube? Why?
The isopropyl alcohol would be higher than the water in the U-tube because its density is less than the density of water.
- Predict what will happen when the air is allowed to flow between the two balloons.
Air from the larger balloon will flow into the smaller balloon until they are equal in volume.
- Describe what happened.
Air from the smaller balloon flowed into the larger balloon, making it larger still.
- Suppose the smaller balloon had been inflated to a volume of 1000 cm3 and the larger balloon to 3000 cm3. What would be the total surface area of the two balloons? The total surface area would be 1489 cm2.
- Which combination of two volumes totaling the same as the original two volumes in question 3 would result in the largest surface area? The smallest?
A ratio of 2000 cm3: 2000 cm3 would result in the largest total surface area, 1536 cm2. A ratio of 0 cm3: 4000 cm3 would result in the smallest total surface area, 1219 cm2.
- Use the answer from Question 4 to write a possible explanation for what happened when the air was allowed to flow freely between the two balloons.
If the balloons shared the gas inside equally, a larger surface area would result. Since the smaller balloon became smaller and the larger balloon larger, it seems that the balloon system tries to assume a state of minimum surface area.
The Upside-Down Water Jar
Two main questions are presented in this demonstration. What force holds the laminated card in place under the inverted jar (step 4)? What force prevents the water from spilling out when the card is removed (step 5)? In order to understand why this demonstration works, it helps to know also under what conditions the demonstration will not work! The demonstration does not work, for example, with alcohols, even though their properties are similar to water. The demonstration also does not work if the air pressure outside the jar is reduced (by placing the inverted jar inside a bell jar and applying a vacuum).
Water is a unique liquid—the surface tension of water is substantially greater than that of alcohols and other liquids. Surface tension is a net attractive force that tends to “pull” adjacent surface molecules inward toward the rest of the liquid. Surface tension is a result of uneven attractive forces experienced by molecules at the surface of a liquid versus those in the rest of the liquid. Molecules in the liquid are bound to neighboring molecules all around them. Molecules at the surface, however, have no neighboring molecules above them. Because the forces acting on the surface molecules are not balanced in all directions, the surface molecules are drawn inward toward the rest of the liquid.
When the jar is first inverted, a small amount of water probably leaks out from the jar. This creates a slight partial vacuum in the space above the water in the jar. The water in the jar also forms a tight adhesive “seal” with the card—in addition to forming strong intermolecular cohesive forces with other water molecules, water also forms strong adhesive forces to many other molecules or materials. External air pressure, acting in all directions, applies a net upward force on the card and the water and prevents the water from spilling out of the jar.
When the card is removed, the surface tension of water provides an additional force keeping the water in the inverted jar. The high surface tension of water arises because of strong hydrogen bonding among water molecules. As an analogy, the surface tension of water may be thought of as an invisible, “elastic” film that expands as needed to counteract the force of gravity and prevent the water from spilling out of the jar. The numerous tiny holes in the mesh screen provide a larger total surface area for the formation of thousands of invisible surface membranes.
When the jar is tilted, the forces become off-balanced and a greater pressure no longer exists on the outside of the jar. The surface tension “breaks” and the water spills out of the jar.
Polymers are long, chain-like molecules composed of multiple repeating units of smaller molecules, called monomers, that have been joined together by a chemical reaction. Latex balloons are composed of rubber, a natural polymer. The structure of natural rubber is shown in Figure 9.
The repeating unit is a five-carbon branched chain related to isoprene. The isoprene units are joined in a network structure and have a high degree of flexibility. Polymer chains are very long and, like pieces of string or yarn, become entangled easily as atoms rotate, bonds vibrate, and individual molecules slip past each other due to their thermal energy. Upon application of a stress to the balloon material, such as inflating it, the randomly oriented polymer chains undergo bond rotations allowing the chains to be extended and causing the rubber molecules to “stretch” out to their full length (Figure 10). Rubber stretches, but does not break, because cross-links between the polymer chains keep the molecules attached to one another.
If the skewer is sufficiently sharp and smooth, it will not tear the rubber, but will slide between the polymer chains (more easily if the chains are not fully extended), allowing them to stretch around the skewer. The skewer will leave two small holes where it pierced the balloon’s surface because the rubber does not make a perfect seal in those spots.On the Level
The U-shaped tube allows for free flow of a liquid, so it seems discrepant that the heights of the liquid in each side should differ. Pascal’s law states that pressure exerted by a contained liquid is transmitted undiminished in all directions and acts with equal force on equal areas. Since pressure is defined as force per unit area, the amount of pressure exerted by a liquid is proportional to the density of the liquid (density = mass/volume). In other words, a liquid with greater mass per unit volume would exert a greater force and thus exert more pressure than the same volume of liquid with less mass.
Saturated sodium chloride solution is more dense than water. For the same volume of liquid, the weight of the sodium chloride solution is 20% greater than the weight of the water. Therefore the sodium chloride solution exerts more pressure than the water, and the level of the water is raised on one side of the tube.Discrepant Balloons
Counterintuitive as it may seem, the air does not flow from the larger balloon to the smaller to equalize the volumes, but instead, it flows from the smaller to the larger, increasing the size discrepancy! This demonstration is not new; it has been around for many years. But whereas most explanations point toward the molecular structure of rubber to account for this illogical phenomenon, such explanations are unnecessary and can even be considered misleading. The phenomenon is not unique to rubber; in fact, just about any stretchable membrane will work, including soap films. Furthermore, the results, though perhaps counterintuitive, are in fact logical and completely predictable, given some well-known (though greatly underused) equations—mathematical, not chemical!
The fundamental equations for a sphere are as follows.
Using these two equations, one can determine the radii and surface areas for spheres of the volumes found in Table 1.
Assuming the balloons to be essentially spherical, it is now relatively easy to see why a 1000 cm3
two-balloon configuration (with a total surface area of 1,489 cm2
) would, if permitted, logically head toward the more lopsided 0 cm3
configuration (total SA = 1219 cm2
), and not toward the 2000 cm3:2000 cm3 configuration (total SA = 1536 cm2
). A system with minimum surface area has lower energy and is therefore more stable. The graph shown below may be thought of as a sort of potential energy “hill.” A ball placed at point X would logically move away from symmetry, not toward it, and it would accelerate as it moves. It is worth pointing out that the air flow in the two-balloon system does in fact accelerate during the demonstration—starting off slow at first, then growing gradually faster as the surface area gradient becomes steeper.
That a balloon membrane or system of balloon membranes will assume a state of minimum surface area may very well be a function of its molecular networking, but it is certainly nothing unique or remarkable. In essence, then, the outcome of this demonstration should be no more surprising than inflating a single balloon and then watching it deflate! Yet this demonstration is surprising. Even young children feel the balloons ought to “share” more equally. Perhaps this is because we are all to one degree or another believers in symmetry, and we automatically tend to equate symmetry with stability. This is easy to understand, for we see examples of stable symmetry over and over again throughout the natural world. (We also see many examples of stable asymmetry, but they are by their very nature not as noticeable in our mind’s eye.) And there is nothing inherently wrong with the favoring of symmetry, as long as it in no way interferes with our powers of observation or narrows in scope our capacity for logical thought. This two-balloon demonstration is indeed an eye-opener, but it opens our eyes not to any remarkable structuring in rubber, but to a remarkable bias in ourselves!
Special thanks to Bob Becker, Kirkwood High School, Kirkwood, MO, for providing the instructions for this activity to Flinn Scientific.