September 2013

# Shout-out: Neat little trick for gas laws

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Emil Lozanov, Wayne State University, Detroit MI

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The small balloon in a syringe for demonstrating Boyle’s law can be substituted with an air-containing, cut and knotted finger of an exam glove. The glove’s “finger balloon” is pushed into the open end of a 60-mL plastic syringe, and adhesive tape is attached to its opening to work as a removable stopper, Fig. 1. Decreasing the air pressure in a syringe is achieved by moving the plunger.

Figure 1.

Figure 2.

Figure 3.

The position of the plunger (decreased pressure, increased balloon volume) can be secured by making a hole in the plunger for a screw/nail, and the syringe can be handed to students for closer observation, Fig. 2.

Taking off the tape will increase air pressure on the balloon, decreasing its volume, Fig. 3. It's interesting because people usually do the experiment in a closed syringe, increasing the air pressure by pushing down the plunger. In this case, by taking off the tape, air rushes inside, and the volume of the balloon decreases.

A 60-mL syringe with a closed end (heated, softened and pressed with pliers) can also be used to demonstrate Charles’s law. If 40 mL of air is heated by a hair dryer, the volume of air will increase by 5-6 mL. At room temperature, the plunger of the syringe returns to the starting position — the volume decreases. The change will be faster if the syringe is cooled with running tap water or an ice bath with dissolved table salt. The volume changes are even faster if oil is applied on the rubber part of the plunger. In addition, there is an opportunity to discuss condensation and pressure change when the water vapour in hot air condenses to form droplets inside the syringe.

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