skip navigation
» McREL: Mid-continent Research for Education and Learning, delivering research and practical guidance to educators
     
 

Whelmer #21: Balloon Vacuum


Illustration of two flasks with a balloon over the opening of each. One balloon is slightly inflated above flask #1, and the other balloon is being sucked into the flask #2. Description:
A balloon is mysteriously "sucked" into a flask.

Science process skills:

  1. observation
  2. communication
  3. interpreting data
Complex reasoning strategies:
  1. induction
Standards:
K-4:
  • Materials can exist in different states--solid, liquid, and gas. Some common materials, such as water, can be changed from one state to another by heating or cooling (Standard B.1.3).
5-8:
  • Unbalanced forces will cause changes in the direction of an object's motion (Standard B.2.3).
  • Energy is a property of many substances and is associated with heat (Standard B.3.1).
  • Heat moves in predictable ways, flowing from warmer objects to cooler ones until both reach the same temperature (Standard B.3.2).
9-12:
  • Solids, liquids, and gases differ in the distances and angles between molecules or atoms and therefore the energy that binds them together...in gases molecules or atoms move almost independently of each other and are mostly far apart (Standard B.2.5).
  • Heat consists of random motion and the vibrations of atoms, molecules, and ions. The higher the temperature, the greater the atomic or molecular motion (Standard B.5.3).
Above Standards from the National Science Education Standards.

Content topics:
  1. air pressure
  2. the relationship between heat and the volume of a gas
You will need:
  1. 250 ml or 500 ml Florence or boiling flask
  2. large balloon
  3. water
  4. heat source
  5. hot pad
Instructions:
This activity requires the use of a flask made of borosilicate glass, such as Pyrex® or Kimax® brand. It should be spherically shaped, similar to a Florence flask or round bottom boiling flask. The spherical shape is much stronger and resistant to the pressure stress involved in this activity.

Select a balloon that is large enough to attach to the neck of the flask. Pre-stretch the balloon by pulling it in several directions.

Place 25 to 50 ml of water in the flask. Heat the flask until the water boils. Remove the flask from the heat. Immediately stretch the balloon over the neck of the flask, leaving the body of the balloon on the outside of the flask. Place the flask on a hot pad and observe. As the flask cools, the balloon is drawn into the flask.

Picture of goggles. Goggles should be worn by all who are in the vicinity of this activity.

Presentation:
This activity provides an outstanding opportunity for students to experience analytical thinking. Give students plenty of time to analyze the phenomenon.

Before you add water to the flask, ask students to describe the contents of the flask (air molecules). Ask them what is on the outside of the flask (air molecules).

As the water is boiling, ask students the same questions. (Water vapor replaces some of the air molecules inside the flask). As you attach the balloon, ask students to predict what will happen. Many may suggest the balloon will inflate. It will, if you attach the balloon before you heated the flask.

As the balloon starts to be drawn into the flask, ask students to explain what is happening. If any suggest that it is being sucked into the flask (most do), ask them to explain exactly what is pulling the balloon into the flask. Ask them to look for clues and to analyze. What is happening to the water vapor as the flask cools? How much air is in the sealed flask? What role does the air on the outside play?

Content:
Air molecules can not pull, only push. It is the air on the outside of the sealed flask that pushes the balloon into the bottle. Nothing pulls it in.

When the water boils, water vapor forces a portion of the air molecules out of the flask. As the flask cools, the vapor condenses back to liquid water, creating a partial vacuum. As the air molecules return to the interior of the flask, they push the balloon.

Assessment:
Type: individual.
Content/Process: air pressure, relationship between heat and volume of gases.
Age/Level: all.

Using a whistling tea kettle and an electric hot plate set up this demonstration. Put a cup of water in the tea kettle and heat it until it whistles. Have students explain what causes the teapot to whistle. Discussions should include the movement of water molecules during the cold and the heated stages, and the pressure of the heated water vapor against the inside of the tea kettle and the whistle on the top of the kettle. Students should focus on the change of water from a liquid to a gas and the expansion of gases when heated.

Notes:





  
     
 

back