Solution to Chemical Mystery #11: Too Hot to Boil?

Leidenfrost droplet

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.