Introduction
Use this old parlor trick to teach about the incompressibility of liquids and Pascal’s law of equal pressure.
Concepts
- Pascal’s law
- Incompressibility of liquids
- Pressure
Materials
Food dye (optional)
Glass bottle*
Glass disposal container
Gloves, cotton (long enough to provide lower arm protection)
Rubber mallet*
Safety glasses
Safety shield (optional)
Transparent tape
Water
*Materials included in kit.
Safety Precautions
Use caution when striking the top of the glass bottle with a rubber mallet. Strike the top squarely so the lip of the bottle does not crack. If the lip cracks, but the bottle’s bottom does not fall out—DO NOT hit the bottle again. Throw the bottle away and use a new bottle. Students observing the demonstration need to wear safety glasses. Students need to stand at least 10 feet away when the demonstration is performed. The bottle may crack in areas other than the bottom and broken glass may fly several feet from the demonstration site. Wear thick cotton or Playtex®-type latex gloves, a long-sleeved shirt or lab coat, and safety glasses. Practice this demonstration several times before performing in front of the class. A safety shield should be used if students do not have safety glasses available.
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. It is recommended that this demonstration be done directly over a glass disposal container. Paper towels may be placed in the bottom of the container to absorb the water. If done over a garbage can, dispose of broken glass properly.
Prelab Preparation
- Obtain an empty glass bottle.
- Wrap transparent tape around the bottom of the bottle and half-way up the bottle (see Figure 1). This will help contain any shattered glass once the bottom cracks out.
{13972_Preparation_Figure_1}
- Fill the bottle about three-quarters to seven-eighths full with water so that the water level is in the bottle’s neck just above the wider “body” area (see Figure 1). Add food dye if desired.
- Allow the water to sit for several minutes or longer so that some of the trapped air can escape.
Procedure
- Obtain the rubber mallet, long cotton or Playtex-type gloves, safety glasses and the partially filled, taped bottle.
- Grip the neck of the bottle tightly with a gloved hand and hold it over a glass disposal container. Caution: Make sure everyone near the demonstration is wearing safety glasses!
- Firmly strike the top opening of the bottle with the rubber mallet. Make sure the end of the rubber mallet strikes the opening squarely (see Figure 2). (The bottom of the glass bottle should break and fall into the glass disposal container along with the water.) Caution: If the bottom of the bottle does not “fall out” with the first blow, but the bottle’s body or the lip cracks or chips—DO NOT strike the bottle again. Use a new bottle.
{13972_Procedure_Figure_2}
- Show the broken bottle to the class. Caution: DO NOT pass the bottle around to the class or allow the students to hold it. There will be many sharp edges that could easily injure the students.
- Dispose of or recycle the broken bottle appropriately.
Teacher Tips
- This kit includes enough bottles to perform the demonstration 12 times. Use a few bottles for initial practice.
- It is important to practice this demonstration several times before performing in front of the class to get a feel for how hard to strike the top of the bottle. It typically does not take a very hard hit to break the bottom out of the bottle.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models
Constructing explanations and designing solutions
Using mathematics and computational thinking
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
MS-PS3.A: Definitions of Energy
MS-PS3.B: Conservation of Energy and Energy Transfer
MS-PS3.C: Relationship between Energy and Forces
MS-PS4.A: Wave Properties
HS-PS3.A: Definitions of Energy
HS-PS3.B: Conservation of Energy and Energy Transfer
Crosscutting Concepts
Systems and system models
Energy and matter
Cause and effect
Performance Expectations
MS-PS2-2: Plan an investigation to provide evidence that the change in an object’s motion depends on the sum of the forces on the object and the mass of the object.
MS-PS4-2: Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motion of particles (objects) and energy associated with the relative positions of particles (objects).
HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.
Discussion
Blaise Pascal (1623–1662) is well known as a mathematician but he also performed many experiments involving pressure in fluids. One of the principles he developed with fluids became known as Pascal’s principle or Pascal’s law. Pascal’s law states that pressure applied anywhere to a fluid is transmitted undiminished in all directions. This law serves as a basis and exploration for much of what is now known as hydraulics.
In the Bottomless Bottle demonstration, the force of the rubber mallet striking the top of the bottle causes the air inside the bottle’s neck to compress slightly (because of inertia and the brief airtight seal around the bottle’s opening). The compressed air travels through the bottle’s neck as a shock wave (through compression and rarefraction) until the compression wave reaches the water level. At this point, the water will not compress. Instead, the force from the shock wave increases the pressure on the liquid. This pressure is then distributed equally to all points of the bottle holding the water. Pressure is equal to a force per unit area (P = F/A). Therefore, under constant pressure, a region of the container with a large surface area will experience a greater total force compared to a region with a smaller surface area. The small force that is applied to the water in the narrow neck of the bottle (from the compressed air) multiplies into a much larger force in the wider “body” portion. Depending on the bottle dimensions (neck diameter versus bottom diameter), this force may increase by 5 to 20 times at the bottom of the bottle. This large force causes the bottom of the bottle to “pop” out.
An alternative explanation for the force that “pops” the bottom out has to do with cavitation. When the top of the bottle is struck with the mallet, the bottle moves downward. The water inside the bottle, however, does not move down due to inertia. This briefly creates a vacuum at the bottom of the bottle. As a result of the low pressure area, tiny bubbles of water vapor form. When the water does move down, the bubbles collapse, creating a shock wave. This rapid formation and implosion of bubbles is known as cavitation. The combined force of all the collapsing bubbles is enough to break away the bottom of the bottle.
References
Special thanks to Todd Everson, Milwaukee School of Languages, Milwaukee, WI, for providing the cavitation explanation for this activity to Flinn Scientific.
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