Level: Elementary to H.S.
Time involvement: 5 to 10 minutes.
Sometimes called “The Love Meter” it is an eye-catching demonstration often sold in science stores or novelty stores. Delicately constructed of clear glass it must be handled carefully. The “boiler” lower bulb/chamber contains a colored fluid (highly volatile such as alcohol). Atop the boiler there may be a few coiled turns. Atop the coiled turns will be another bulb/chamber about the size of the chamber at the bottom. To operate the “boiler” gently cradle the lower boiler in your hand or hands. Within seconds the fluid will appear to begin boiling and promptly rise through the coiled section and begin bubbling in the top chamber much like a coffee percolator. By removing the hand(s) from the lower chamber and holding the mid-section the colored fluid will gradually return to the bottom chamber. The effect can be repeated indefinitely.
Discussion: Many chemical elements can exist in any one of the three states: solid, liquid, or gas. Although water is a compound (not an element) it exhibits the ability to exist as a solid, liquid or a gas. By heating or cooling the water can be converted to another state. The important transition that applies to this demonstration is the boundary between liquid and gas. If a glass of water were suddenly placed in the vacuum of space it would quickly dissipate into a gas. On earth our atmosphere slows the evaporation rate dramatically. If the water temperature is raised gradually evaporation will increase until a point is reached when the evaporation rate “explodes”. We call this temperature the “boiling point”. At sea level pressure that temperature is 212° Fahrenheit. In effect a tipping point is reached when the vapor pressure of the atmosphere is exceeded by the vapor pressure of the liquid. Interestingly, as an airplane climbs higher in the atmosphere the boiling point of water decreases until water (blood, saliva, urine, etc.) will begin to boil. The altitude where body temperature (98.6°F) is sufficient to cause boiling is about 63 thousand feet.
At a molecular level there is constant activity. A closed bottle of soda or water is constantly evaporating but simultaneously gaseous molecules are dissolving into the liquid. Because the air above the water is saturated (humidity 100%) the net effect is stability. In the Hand Boiler heat is added by the hands’ temperature above room temperature. With the addition of body heat the special highly volatile liquid crosses the vapor pressure barrier and begins to boil. The boiling would stop immediately if the glass breaks because in the manufacture of the hand boiler the air pressure is largely removed and the glass is melted shut.
Notes: Younger students may not grasp the full science behind the explanation but will probably remember the amazing effect of hand temperature causing a liquid to boil. Understanding the basic physics of water and evaporation will be very helpful when learning the science behind the formation and dissipation of clouds.