Chemical Mystery #11
In Chemical Mystery #11, a hot metal sphere is placed in water. As expected, the water hisses and boils away, and the sphere cools. Next, the metal sphere is heated to a much higher temperature and is placed in water. Curiously, in this case the water does not hiss or boil away! What is going on here?
This unexpected behavior can be explained using the Leidenfrost Effect, which occurs when a liquid contacts a surface much higher than its boiling point. When this occurs, the liquid evaporates so quickly that it forms a protective, heat-insulating barrier forms between the surface and the liquid. In the case of the red-hot metal sphere immersed in water, a gaseous layer of water vapor surrounds the extremely hot sphere, insulating it from the water. Thus, the hot sphere does not make actual physical contact with the water, and no boiling occurs (Figure 1).
Figure 1 - A red-hot metal sphere immersed in water. Note the vapor barrier in between the liquid water and the hot sphere.
If you have ever flicked droplets of water onto a hot griddle to see if it’s ready to cook pancakes, then you have made use of the Leidenfrost Effect. A griddle is hot enough for cooking if water droplets form beads that persist, sizzle, and ride across the hot surface (Figure 2). On the other hand, if the griddle isn’t hot enough the droplets quickly evaporate away. On a very hot griddle, the formation of a vapor layer in between the droplet and the hot surface provides a protective barrier that slows the rate of vaporization. This prevents the droplets from quickly boiling away. In addition, the droplets “ride” on this vapor layer, much like an air hockey puck rides on a layer of air across an air hockey table.
Figure 2 -Water droplets on a stove surface at 475 K (205oC, 400oF). Water boils at 373 K (100oC, 212 oF). Note the gap between the bottom of each droplet and the surface.
The hot sphere experiment, along with explanations, can be seen in the video below. This video also contains several other experiments that demonstrate the Leidenfrost Effect, such as water droplets dancing on a hot surface in super slow motion.
I have a few more Leidenfrost Effect experiments up my sleeve that I hope to post in the future. If you have some favorite experiments that illustrate the Leidenfrost Effect, be sure to let me know about them.
Safety
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.
Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.