Air Pressure Differentials - marshmallows in syringe (Mark Rothenay)
Author
Mark Rothenay
Principle(s) Illustrated
The overall purpose of this demonstration is to have students observe how pressure differentials can change the size of an object. The higher the pressure in an enclosed system the greater the amount of force being exerted on the marshmallow and therefore will shrink in size. Students can create a model to demonstrate what is occurring.
Standards
Science & engineering standards
Asking questions and defining problems
Develop and use of models
Engaging in arguments from evidence
2. Cross-cutting concept standards
Cause and effect: Mechanism and explanation. Events have causes, sometimes simple, sometimes multifaceted. A major activity of science is investigating and explaining causal relationships and the mechanisms by which they are mediated. Such mechanisms can then be tested across given contexts and used to predict and explain events in new contexts.
Systems and system models. Defining the system under study—specifying its boundaries and making explicit a model of that system—provides tools for understanding and testing ideas that are applicable throughout science and engineering.
Questioning Script
Prior knowledge & experience:
Students may have knowledge and experience in pressure and volume relationships -Boyle's Law
Root question:
Explain what you predict will happen to the marshmallow when the plunger is pushed and pulled?
Target response:
When the plunger is compressed in the syringe, the air molecules are compressed and the tiny air pockets inside the marshmallow will also compress causing the marshmallow to decrease in size. When the plunger is pulled, the air pressure will now decrease inside the syringe causing a vacuum. Under this reduced pressure, the expanding air will fill the syringe and cause the marshmallow to increase in its size.
Common Misconceptions:
The marshmallow will stay crushed.
The marshmallow will prevent the plunger from being compressed.
Photographs and Movies
Applications to everyday live
Ever shake a soda can? Well, this is a great everyday example of Boyles law.
"All is fine and dandy, until you shake the bottle up. Shaking up the bottle causes that neat pocket of carbon dioxide gas in the top to mix in with the soda. Now, pop the cap off. Suddenly all of these excess gas bubbles within the soda want to expand and escape their high pressure environment as well. Rather than being able to expand and shoot out of that neat pocket of air with a "pffffffft," they expand while they're still in the soda. As it tries to muscle its way out, it pushes the soda along with. Pressure in the bottle goes down, volume of the gas goes up, and you have yourself a mess to clean up." - Steven Pearson