Vacuum Demonstrations

One of the most important concepts in understanding the operation of the early steam engines is the creation of vacuum by condensation. These demonstrations provide simple illustrations using a soft drink bottle and boiling water.

The important thing to recognize is that condensation can pull a vacuum. The vacuum can be substantial if all the air is removed from the vessel before condensation. If air is completely removed and the vapor is condensed, the final pressure will be the vapor pressure of water. At 25C, this is 0.032 bar, 24 mm Hg, or 29.0 inches of vacuum!

Experiment Setup

The setup consists of a pot of colored water on the left. The pot is not heated. The pan on the right holds colored water that is boiling during experiments where the bottle is heated.

Demo using dry bottle without heating. Click the image to watch the demo.

This demo shows an inverted bottle being submerged in colored water. The air keeps the colored water from entering the inverted bottle and verifies that water will not spontaneously enter. You may want to skip this one if you understand that concept.

Demo using dry bottle. Click the image to load the movie.

• A common PETE soft drink bottle is heated in boiling water for about 40 seconds.
• The air inside expands as it is heated.
• The bottle is rapidly inverted in colored water. Upon cooling the gas inside contracts, pulling some water into the bottle.
• Although some suction is created, it cannot create the same suction that condensation can at the same temperature as shown in the final demo below.

Demo using water in bottle. Click the image to watch the movie.

• A common PETE soft drink bottle has a few ml of water added to the bottom.
• The water in the bottom of the bottle is heated for about 40 seconds by submersion in boiling water. This causes vaporization of some of the water because of increased vapor pressure at the higher temperature. Some of the moles of air within the bottle are displaced by the water vapor.
• The bottle is removed from the heat, and quickly inverted in cool water. The water vapor condenses, undergoing an enormous volumetric contraction:
  vapor volume per gram (assuming that vapor is heated to 50C:
  V = nRT/P = (1 g)(8.314 cm³MPa/mol-K) (323K)/(0.1MPa)/ (18 g/mol) = 1492 cm³;
  liquid volume per gram: approx. 1 cm³ up to boiling point
• Upon cooling, condensation creates a vacuum in the bottle that draws water in. The bottle doesn't fill completely because the displacement of air from the bottle isn't complete during heating , and even after cooling, the remaining air is still in the bottle. Heating the water in the bottom of the bottle to the boiling point of water could potentially displace virtually all the air, but the entire bottle would need to be submersed when heating to keep all internal surfaces hot so that the vapors wouldn't condense on the cooler sides.
• The vacuum in the bottle draws liquid inside in a manner similar to Savery's pump.

[Return to Brief History of the Steam Engine]

Updated 9/22/21 ,Copyright © 2021, Carl T. Lira, lira@egr.msu.edu All rights reserved.
Prepared as a supplement to Introductory Chemical Engineering Thermodynamics.