Mass the Gas

In this demo, baking soda and vinegar are mixed in a plastic storage bag. The reaction container is massed before and after the reaction for students to see the Law of Conservation of Mass in action.
NaHCO3 (s) + CH3COOH (aq) →CO2 (g) + H2O (l) + CH3COONa (aq)

Pressure Cooker

boiling water in a low pressure syringe

In this demonstration, warm water is placed in a plastic syringe, the syringe is sealed, and the plunger is pulled back causing the water to boil. The water boils because the action of pulling back the plunger increases the volume, thus decreasing the pressure. The boiling point of a liquid is dependent on the pressure of the system, so a decrease in pressure leads to a decrease in boiling point.

Marshmallow Magic

When a marshmallow is placed in a large capped syringe and the plunger is pushed in, the air in the marshmallow contracts from the pressure. Conversely, if the plunger is pulled back, the pressure decreases causing the air in the marshmallow to expand.

Mushroom Cloud

dry ice mushroom cloud bubble

When dry ice is placed in warm water it sublimes very quickly forming a large amount of carbon dioxide gas. When a bubble is placed over this the bubble grows from the pressure.

Chemistry Pop Guns

This demo harnesses the reaction of baking soda and vinegar to blow a cork off a test tube.
NaHCO3 (s) + CH3COOH (aq) →CO2 (g) + H2O (l) + CH3COONa (aq)