Vacuum Pump and Bell Jar (Mara Desso)

Author

Mara Desso

Principles

• Combined gas law

Standards

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.

Cause and Effect

Empirical evidence is required to differentiate between cause and correlation and make claims about specific causes and effects. (HS-PS2-1), (HS-PS2-5)

Systems can be designed to cause a desired effect. (HS-PS2-3)

Materials needed

• Bell jar

• Vacuum pump

• Outlet

• Balloon

• Marshmallow

• Bag of chips

• Beaker of water

Procedure

1. Attach the vacuum pump hose to the base of the bell jar.

2. Plug the vacuum pump into a functioning outlet. Turn it on to ensure it is working properly.

3. Place an object on the base of the bell jar and put the bell lid over the top, ensuring that all edges are sealed.

4. Turn the vacuum pump on and observe the difference in size.

5. Turn the vacuum pump off.

6. Carefully undo the vacuum hose from the base.

7. Observe what happens.

8. Lift the lid and begin again with a new object!

Explanation

The bell jar creates a sealed chamber while the vacuum pumps all the air out of the chamber. as the air is pumped out, there are fewer and fewer particles of air pushing on the object inside, so there is less and less pressure pushing on the object. If the object is composed of gas particles, those particles will begin to have enough energy to push out and occupy a greater volume when there is less pressure pushing in on them. For example, a marshmallow will grow because there are tiny pockets of air inside it. As the external pressure on the marshmallow is decreased, the air particles inside the marshmallow are able to overcome the forces pushing on them, and they expand, causing the marshmallow to also expand. When the vacuum is released, there will be a sudden influx of air from the atmospheric pressure, oftentimes causing the object to fly around in the bell jar until the pressure equalizes. The object should return to relatively the same size because the normal atmospheric pressure is once again pushing in on it.

Questions

1. Why did the marshmallow get bigger when the vacuum pump was turned on?

   1. A marshmallow will grow because there are tiny pockets of air inside it. As the external pressure on the marshmallow is decreased, the air particles inside the marshmallow are able to overcome the forces pushing on them, and they expand, causing the marshmallow to also expand.

2. Why did the marshmallow return to its original size once the vacuum was released?

   1. When the vacuum is released, there will be a sudden influx of air from the atmospheric pressure, oftentimes causing the object to fly around in the bell jar until the pressure equalizes. The object should return to relatively the same size because the normal atmospheric pressure is once again pushing in on it.

3. What do you think would happen to the size of a bag of sealed chips, if it was brought from sea level to 6000 ft above sea level for a snowboarding trip? Why?

   1. The bag of chips would expand because there is less atmospheric pressure at 6000 ft above sea level. There are fewer particles pushing in on the bag of chips so the gas inside the bag is able to expand.

Everyday examples of the principles illustrated

• Bag of chips on an airplane or as you drive up a mountain

• Balloon as you drive up a mountain/ release it into the air

• The bends/ decompression sickness and diving

Photos

Movies

References

ChemTeam Combined Gas Law Explanation

NASA and Boyle's Law