SUBJECT: Science
GRADE: 7,8,9
GROUP SIZE: Large
TIME: 45 minutes
TYPE OF ACTIVITY: Teacher Demonstration
TEACHING STRATEGY: Guided Discovery
CONCEPTS: Pressurized Environment Boiling and Altitude
SKILLS: Observation Inference Hypothesizing
Objectives: To understand how air pressure affects the boiling temperature of water; to relate this concept to the boiling point of blood and the need for a pressurized environment for humans in space.
Materials: Pyrex flask; stopper for flask; ring stand; heat source; glass or sponge; cold water; hot pads; pan to catch water.
Procedure:
Fill flask with water 1/3 to 1/2 full. Bring the water to a brisk boil for at least 10-15 seconds. Turn off the heat source. Stopper the flask tightly (the water should stop boiling). USE HOT PADS to invert the flask on the ring stand. Pour cold water from the glass over the flask after placing a pan under the apparatus to catch the runoff. The water should begin to boil again. what may have caused the water to boil again? Does water always boil at the same temperature? Read the instructions on a cake mix box for high altitude baking. Why do you suppose it is necessary to cook for longer periods of time at high altitude? Could water boil at room temperature? How can you make a liquid boil without raising the temperature?
Safety Note: Do not use ice water. Cold water from the tap should work fine. Extremely cold water can cause such a pressure difference that the flask can implode. Do this as a demonstration and wear goggles.
Teacher Note: As the cold water is poured over the flask the steam inside will be cooled and will condense causing a lower pressure inside the flask. The lower the pressure the lower the boiling point of liquids. Relate the concepts learned in this lesson to the temperature of blood and the need for pressure suits or a pressurized cabin in a space craft. Pilots of aircraft which fly at high altitudes have used pressure suits for years. The “G” suits which some pilots use are to keep the blood in the upper part of the body (especially the brain) during high G maneuvers which might drain too much blood to the extremities (away from the brain) and thus cause a blackout. A space suit does not work in this way. It provides a pressurized atmosphere for the astronaut.
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