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# Under The Dome: Demonstrations with a Vacuum Pump

Emmette Cox
Product Management Coordinator for Physical Science

### Background

Every physics and physical science student will eventually encounter gas laws or equations of state. These are the familiar Charles’s Law, Boyle's Law, the Ideal Gas Law and others. Understanding these concepts and solving problems based on these laws is a significant part of many physics and physical science courses. There are several activities and demonstrations designed to enrich students’ investigations into the properties of gases. Some of these activities are simple, and some are more involved. Some of the most effective demonstrations for teaching the relationships described in the gas laws can only be done properly with a good vacuum pump and a bell jar. Here are some of those activities.

### Equipment

For these classic demonstrations you will need:

1. A vacuum pump
2. A bell jar
3. A base plate
4. Connecting hose

### Safety

Follow the instructions for this activity closely and abide by established laboratory safety practices, including the use of appropriate personal protective equipment (PPE).

Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules.

Make sure that neither the pump nor the base nor the jar falls off the table from the motor's vibration. Read through the instructions carefully and be familiar with the various gauges and valves on each piece of equipment.

### Demonstrations

It is good idea practice the demonstrations before trying to perform them for your class even if you have done them before. These demonstrations require multiple pieces of equipment with lots of connections, valves, and parts. Practicing the demonstrations and becoming familiar with the equipment is the best way to ensure a safe, successful demonstration.

#### Balloon

A good place to start is with a simple balloon. Inflate a balloon just enough to fill the balloon with some air, but not enough to stretch the rubber too much. Tie off the end. Place the balloon on the base plate away from the hole, and place the bell jar on the base plate. Be sure there is good seal between the jar and the gasket. Use some vacuum grease if necessary. Check the hose connections and valves. Turn on the pump and watch as the balloon expands. This is a great example of a pressure deferential. The balloon starts out with equal pressure on the inside and outside, but as the pressure in the jar drops, the pressure inside the balloon causes the volume to increase. Release the appropriate valve and the balloon will deflate as air rushes in and pressure equalizes.

#### Marshmallow

A standard marshmallow or seasonal marshmallow based candy is made of a material with lots of little air pockets. Evacuating the air in the jar with one of these confections inside will cause the treat to expand to comical proportions. For a more dramatic effect, paint a face or shape on the marshmallow and observe how the features change.

#### Shaving cream

Shaving cream similarly has tiny pockets of air and an expandable volume. Place the shaving cream in a beaker or flask, lower the dome of the bell jar, and run the pump. To enhance the effect drizzle food coloring down the side of the beaker before adding the shaving cream. This can get messy, so be ready to shut the power before the mess gets too big. Do not allow the shaving cream to overflow the beaker. Avoid getting shaving cream in the hole in the base plate. Releasing the pressure valve collapses the cream to a different consistency.

#### Boiling water

Boiling water at room temperature drives home the relationships explained in the Combined Gas Law and Ideal Gas Law with an impressive visual spectacle. Fill a beaker 1/3 full of water and place on the base plate, under the dome. You can add a thermometer both inside the beaker and outside the dome for comparison. A digital camera and a projector can be useful to help students see the thermometer reading. Stronger pumps cause the water to boil at room temperature once the pressure is low enough. Some vacuum pumps boil water if the water is already hot, usually between 50º C to 80º C. Use caution when handling water at this temperature and use appropriate PPE. As the water boils there will be water vapor which can fog the glass or enter the pump through the hole in the base plate. Shut off the pump once the water starts to boil.

Hopefully these demonstrations will help add some excitement and interest to these topics. Below are a selection of vacuum pumps and equipment that can be used to perform these demonstrations: