Vacuums and Pressure
Steps - Assemble apparatus to look like so:
Lesson ideas taken from Educational Innovations
Materials
10 Madgeberg hemispheres
10 bell jars with syringe and tubing and vial (from Ed Inn)
1 thermos hot water
10 balloons, slightly inflated
10 large marshmallows
Pieces of bubble wrap
10 suction cups
10 thermometers
5 bottles containing small marshmallows with fizz-keeper attached
10 scales
1 box of goggles (for all to wear)
Students and volunteers must wear goggles at all times
Investigating the Action of a Vacuum Pump
Divide the class into 2’s or 3’s. Give each group a Bell jar set.
Show students the handout of the apparatus. Point out that:
The check valves will allow air to flow in one direction but not in the other.
Ask students what is in the bell jar? Air is present - nothing is not the correct answer.
1. Tell one person in each group to push down on the bell jar to make certain that the bell jar is pressing
against the “O” ring. Another student should pull the syringe out to the 60 mark.
Ask students where does the air come from that fills the syringe?
The air comes from the bell jar (so there is now less air than in the bell jar than before).
How hard was it to pull out the piston? ( Not very).
2. Let go of the piston and watch what happens. Now quickly push the piston all the way back into the
syringe. Where did the air go (hint – listen for the sound of moving air at the end of the open tube).
Air moves out of the open end at point E.
3. Repeat the following steps five times in rapid succession:
a. Pull the piston out to the 60ml mark;
b. Let go of the piston , and see what happens;
c. Push the piston all the way in.
Ask students: what happened to the amount of force required to pull out the piston? (The force
needed increased.)
Explain why. (Less air pressure inside the jar, helping push the piston out)
4. Pull the piston of the syringe to the 60 ml mark and push it all the way back in. Do this 24 times in
rapid succession while listening to the air exiting the open end of the tubing. What has happened
towards the end of the 24 pull/pushes? What is in the bell jar now? (Very little air will be in the
jar = partial vacuum or vacuum).
5. Disconnect the hose D from the base of the bell jar. Note any sound you may hear. What does this
indicate? (Air entered the bell jar)
A. Decreasing Pressure
1. How can you increase the volume of an object by changing the pressure?
Use the equation PV=nRT if the students are adept at manipulation equations (ask teacher).
Ask students to hypothesize what will happen to the volume of the air (a gas) if the pressure is
decreased or increased?
A. Balloon – Make sure the balloon is slightly inflated (about 3-4 cm in diameter). The rubber
should be tight but not stretched.).
a. Place the balloon in the bell jar and start pumping the piston:
What happens to the balloon after a few pumps? (It grows larger.)
Why? (The pump removed air surrounding the balloon that had been pressing inward.
The air inside the balloon is still pressing outward, so the balloon expands.)
b. Loosen the connection between hose and the base until you hear the movement of air.
What happens?
c. Loosen the end connected to the bell jar. What do you hear, and what happens to the
balloon? (Air moves back into the bell jar and crushes the balloon.)
If you put a partly blown up balloon in a bell jar and then pump out the air from the bell jar the
balloon will slowly expand. This is because the air inside the balloon is at a room pressure and
when the air outside the balloon is removed there is a bigger pressure difference between the
inside and outside of the balloon. The balloon therefore expands to balance this difference.
Room pressure
Vacuum
B. Marshmallow and/or piece of bubble wrap – repeat with a marshmallow. Predict what will
happen. Note that the marshmallow now looks funky.
Marshmallows have small bubbles of air trapped inside them. These bubbles are at atmospheric pressure.
When the air inside the glass container is sucked out, the volume of the container remains the same
although there is much less air inside – so the pressure is reduced. The air bubbles inside the
marshmallows are therefore at a much higher pressure than the air surrounding the marshmallows, so
those bubbles push outwards, causing the marshmallows to expand. When air is let back into the glass
container, the surrounding pressure increases again, and the marshmallows deflate back to their normal
size.
This illustrates Boyle’s Law (as the pressure on a gas decreases, its volume increases).
C. The suction cup
Stick the suction cup firmly to the center of the bottom plate. Turn bell upside down.
a. Why does the suction cup stick to the plate?
Keep apparatus upside down and pump the piston several times until something happens to the
suction cup. (The suction cup eventually falls off because the pump has removed the outside
pressure that held the cup on the surface.) The pump removes the outside pressure that held the
suction cup on the surface.
D. The Effects of air Pressure on Boiling Water.
Tell students that water boils at 100 degree C (or 212oF or 373oK) at atmospheric pressure. The
boiling point will change if the pressure changes.
a. Fill vial halfway with HOT tap water. (You will need to have a thermos of water.)
Measure temperature with thermometer.
b. Place vial on bottom plate and assemble apparatus.
c. Hold bell jar so it does not tip. Have another person pump the piston vigorously, several
times. What happens?
d. Keep pumping. Then stop and observe water. Observe the inside of the bell jar water
vapor is condensing on the walls).
e. SLOWLY loosen connection between hose (D) and bell jar so that water does not spill.
f. Measure temperature of water. The temperature of the water has DECREASED. Boiling
is an endothermic process (needs energy). Energy is taken from the water, so its
temperature drops.
It will take up to a minute for the water to form bubbles.
Notice that there are 2 types of bubbles. The smaller bubbles that cling to the sides of the cup are dissolved
air that has been released from solution. If this were an astronaut’s bloodstream, those bubbles would
cause “the bends.” The larger bubbles are water vapor escaping from all parts of the liquid.
When you first turn on the pump, you might see a bit of moisture, a small cloud form inside the jar. This
because the air inside the jar is moist, and when the pressure drops, the air temperature cools. (This
principle is called Gay-Lussac’s Law). The cooled moist air forms a miniature cloud, but it may disappear
because even the moisture is evacuated from the jar by the pump.
E. Does Air have Mass?
Turn the scale on and make sure it is zeroed
Detach tubing where D tubing couples into syringe assembly. Do this by twisting gently.
Place bell jar apparatus with this remaining small piece of tubing on the scale. If you place the scale close
to the edge of the table, the tubing can hang over the edge (you do not want the tubing to rest on the
table.)
Mass the above set-up.
Reconnect the tubing and pump the air out as far as you can. Uncouple the D tubing – the vacuum will
stay in the jar.
Mass the apparatus again. Was there any difference (I found about .1-.3 lighter?
You can let the air back into the jar a mass it again to corroborate the initial reading.
B. Increasing Pressure
I. Balloon in a Bottle – to demonstrate that increasing the pressure on a gas will
decrease its volume.
Show students the bottle containing an inflated balloon and fizz keeper top.
Pump up the fizz keeper (40 times) and ask students what they have noticed about the size of the balloon.
The balloon will have decreased in size (this may not be easy to see).
Release the fizz keeper and observe that the balloon regains its original size (this is easier to see).
Explanation:
This illustrates Boyle’s Law (as the pressure on a gas increases, its volume decreases). We assume the
amount of gas and temperature remains constant.
As the fizz keeper is pumped more air is added to the bottle, which increases the pressure inside the
bottle. The added air cannot escape until the Fizz Keeper is unscrewed.
II. Marshmallows in a Bottle
Before repeating the above experiment on the bottle containing the marshmallows, ask the students to
predict what will happen.
The marshmallows will decrease in size as the pressure in the bottle is increased with successive
pumps. They will regain their size as soon as the pressure is released.
Magdeburg hemispheres
Overview:
In experiments, the Magdeburg hemispheres is a pair of copper hemispheres that can be sealed
together, by applying grease around the rim, and thereafter be connected to a vacuum pump so as to
actuate an artificial vacuum or near "perfect vacuum" inside of the sealed sphere. In this vacuumed-out
state, the pressure of the weight of surrounding atmosphere, piled upwards of 62-miles above the sphere,
as is the case for all bodies on the surface of the earth, to the Karman line earth-space boundary where
point weightlessness begins, acts to hold the spheres together tightly with great force by pressing inward
on the outer casing.
History:
The Magdeburg hemispheres were invented by German engineer Otto Guericke who some time near
the end of the Thirty Years’ War (1618–1648), became mayor of Magdeburg (hence the name), from
1646 to 1676, and in this period began to devote a considerable portion of his spare time to
experimentation and was especially fascinated with the nature of cold, becoming very curious on the the
question: [6]
Demonstration Predictions:
When a vacuum is induced within the sphere, the two hemispheres will be much harder to separate.
Demonstration Instructions:
1. Each student should pair up and each should have one hemisphere.
2. Align the two hemispheres together, keeping them snuggly fit.
3. Vacuum out the air by pressing the two hemispheres together.
4. Try to separate
Lesson written by Zach Ullmann, Frank Cai and Pat Tellinghuisen.