Atmospheric Pressure and Volume Differences Between Vapor and Liquid

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Updated on Nov 15, 2010

The Idea

This is a nice attention-getting demonstration that produces a fountain-like spray in an inverted flask. If you are doing this as a demonstration—especially for younger children—be prepared to be asked to do it again.

What You Need

  • ring stand with a small 2-inch diameter ring
  • flask (250 ml works)
  • 1-hole rubber stopper to fit the flask
  • approximately 12-inch section of glass tubing that can be inserted into the stopper
  • hotplate
  • oven mitt
  • beaker of equal or larger volume as the flask
  • water (with food coloring optional)
  • safety glasses


  1. Put on safety glasses.
  2. Carefully slide the glass tube through the stopper, so approximately 1 inch protrudes through the narrow end of the stopper. Use proper techniques for handling the glass tubing (including wearing eye protection, protecting your hands as you push it through using a towel, and lubricating the edge with a bit of Vaseline, so you don't have to force it through the hole).
  3. Insert the stopper into the flask.
  4. Attach the ring on the ring stand high enough so the entire length of the tubing is supported about ½ inch above the base of the stand.
  5. Fill the beaker close to the top with water. (Food coloring can temporarily stain your fingers.)
  6. Assemble the apparatus, as shown in Figure 40-1. Make sure everything fits and is secure.
  7. An air-pressure fountain.

  8. Take the flask out of the ring stand and remove the stopper with the tubing.
  9. Put (a few tablespoons of) water into the flask. Set the stopper on a table.
  10. Place the flask on the hotplate (Figure 40-2).
  11. When the water starts to boil and the flask fills with steam (using the oven mitt), remove the flask from the hotplate and attach the stopper.
  12. Quickly, but carefully (still using the oven mitt), reassemble the apparatus. One convenient way to do this is to note the position of the ring, remove the ring, and then place the ring over the collar of the flask. Then, with the stopper inserted, invert the flask, and (with the flask supported by the ring as it is transferred) reattach the ring on the stand. It wouldn't hurt to choreograph this a little bit before doing it (and have a second person help you). The idea is to do the transfer quickly (so you don't lose all of your steam), but safely (because you are working with glass and hot liquids). Placing an ice cube on top of the flask may further accelerate the process.
  13. An air-pressure fountain.

  14. With the flask in the ring stand and the glass tubing close to the bottom of the beaker, observe what happens.

Expected Results

At first, the water starts to rise up the tube. This begins slowly at first. As the water works its way up the tube, it begins to pour into the flask. Once the water touches the interior of the flask, it begins to spray, forming a fountain that increases in intensity until the water is completely drawn out of the flask (Figure 40-3).

If positioned just right, the fountain ends in a gurgling effect. While many observers may expect the rise of the liquid up the tube, the surge of the fountain catches many people off guard.

An air-pressure fountain.

Why It Works

As the steam inside the flask begins to cool, the air pressure inside the flask drops. This is primarily the result of the phase change of the steam from vapor to liquid water, which occupies a much smaller volume. The cooling air inside the flask also contracts, adding to the reduced pressure. Atmospheric pressure pushes down on the liquid in the flask, driving up into the glass tube. The cooler the flask gets, the lower the pressure. This process feeds on itself in an accelerating manner, producing the fountain effect.

Other Things to Try

The mechanism that drives the liquid up into the flask is the basis for what is known as a Torricelli barometer. Air pressure is measured by how high a column of water can be supported by air pressure with a vacuum in the flask. Mercury is used instead of water because standard air pressure can support a mercury column roughly 30 inches high, compared with a much-higher column for water. Because of potential difficulties in working with mercury in academic settings, it is probably best just to read about this one.

The Point

This project works because of the volume differences between vapor and liquid, and the force exerted by air pressure.