Boyle's Law with Marshmallows (Brent Shano)
Author
Brent Shano
Principles Illustrated
Boyle's Law P1V1=P2V2
There is an inverse relationship between Pressure and Volume
Temperature must be held constant in order for the law to work
Life application to breathing, scuba-diving, being an astronaut in space, etc.
Standards
HS-PS3-5 Develop and use a model of two objects interacting through electric or magnetic fields to illustrate the forces between objects and the changes in energy of the objects due to the interaction.
Cross Cutting Concept: Cause and effect relationships can be suggested and predicted for complex natural and human designed systems by examining what is known about smaller scale mechanisms within the system.
Questioning Script
Prior knowledge & experience:
Student's will be discussing Gas Laws. Depending on how the subject is taught this might be the first gas you cover or perhaps one might work backwards and discuss the combined gas law and then derive Boyle's, Charles's, and Gay-Lussac's laws accordingly. Irrelevant of how one goes about it the relationships between pressure (P), volume (V), and temperature (T) are all constant. When manipulating the variables be sure to know what law you are discussing so that the students do not get confused as to which are the variables and which are to remain constant. A general knowledge of gas particles and their random movement in a confined space is to be known as well as the fact that gases can be compressed due to their general nature in that space.
Root question:
What will happen to the marshmallow when:
a) the pressure is increased?
b) the pressure is decreased?
c) the volume is increased?
d) the volume is decreased?
Target response:
Student's will see that although there are four situations in which they will make an observation they will find only two outcomes. The marshmallow will get bigger when you increase the volume aka decrease the pressure AND that the marshmallow will get smaller when you decrease the volume aka increase the pressure. They will learn that there is an inverse relationship between pressure and volume when the temperature is held constant.
Common Misconceptions:
Students generally understand that when you increase the pressure that the marshmallow will shrink. They have a harder time understanding when the pressure is decreased that the marshmallow gets bigger. Students will often point out that since this is a gas law it doesn't make sense to use a marshmallow. Here I will throw a penny into the syringe and try the same thing but nothing happens. The students then navigate their way to understanding that the reason a marshmallow works is due to the AIR inside the marshmallow and not the marshmallow itself. Due to the general "gooey/stretchiness" of the marshmallow is what makes it a great tool to show Boyle's Law.
Photographs
