Leidenfrost Effect Demonstrations

Leidenfrost Effect Demonstrations

This is a diagram of the Leidenfrost effect.
In the Leidenfrost effect, a droplet of liquid is separated from a hot surface by a protective layer of vapor. Vystrix Nexoth, Creative Commons License

There are several ways you can demonstrate the Leidenfrost effect. Here is an explanation of the Leidenfrost effect and instructions for performing science demonstrations with water, liquid nitrogen, and lead.

What Is the Leidenfrost Effect?

The Leidenfrost effect is named for Johann Gottlob Leidenfrost, who described the phenomenon in A Tract About Some Qualities of Common Water in 1796. In the Leidenfrost effect, a liquid in proximity to a surface much hotter than the liquid's boiling point will produce a layer of vapor that insulates the liquid and physically separates it from the surface. Essentially, even though the surface is much hotter than the boiling point of the liquid, it vaporizes more slowly than if the surface was near the boiling point. The vapor between the liquid and the surface prevents the two from coming into direct contact.

The Leidenfrost Point

It's not easy to identify the precise temperature at which the Leidenfrost effect comes into play -- the Leidenfrost point. If you place a drop of liquid onto a surface that is cooler than the liquids boiling point, the drop will flatten out and heat up. At the boiling point, the drop may hiss, but it will sit on the surface and boil into vapor. At some point higher than the boiling point, the edge of the liquid drop instantly vaporizes, cushioning the remainder of the liquid from contact. The temperature depends on many factors, including atmospheric pressure, the volume of the droplet, and the surface properties of the liquid. The Leidenfrost point for water is about twice its boiling point, but that information can't be used to predict the Leidenfrost point for other liquids. If you are performing a demonstration of the Leidenfrost effect, your best bet will be to use a surface that is much hotter than the boiling point of the liquid, so you'll be sure it's hot enough.

There are several ways to demonstrate the Leidenfrost effect. Demonstrations with water, liquid nitrogen, and molten lead are the most common...

Overview of the Leidenfrost Effect
Water Droplets on a Hot Pan
Leidenfrost Effect with Liquid Nitrogen
Immersing Your Hand in Molten Lead

Water on a Hot Pan - Leidenfrost Effect Demonstration

This water droplet on a hot burner is displaying the Leidenfrost effect.
This water droplet on a hot burner is displaying the Leidenfrost effect. Cryonic07, Creative Commons License

The simplest way to demonstration the Leidenfrost effect is to sprinkle droplets of water on a hot pan or burner. In this instance, the Leidenfrost effect has a practical application. You can use it to check whether or not a pan is hot enough to be used for cooking without risking your recipe on a too-cool pan!

How To Do It

All you need to do is heat up a pan or burner, dip your hand in water, and sprinkle the pan with water droplets. If the pan is hot enough, the water droplets will skitter away from the point of contact. If you control the temperature of the pan, you can use this demonstration to illustrate the Leidenfrost point, too. Water drops will flatten out on a cool pan. They will flatten near the boiling point at 100°C or 212°F and boil. The droplets will continue to behave in this fashion until you reach the Leidenfrost point. At this temperature and at higher temperatures, the Leidenfrost effect is observable.

Overview of the Leidenfrost Effect
Water Droplets on a Hot Pan
Leidenfrost Effect with Liquid Nitrogen
Immersing Your Hand in Molten Lead

Liquid Nitrogen Leidenfrost Effect Demos

This is a photo of liquid nitrogen. You can see the nitrogen boiling off into the air.
This is a photo of liquid nitrogen. You can see the nitrogen boiling off into the air. David Monniaux

Here's how to use liquid nitrogen to demonstrate the Leidenfrost effect.

Liquid Nitrogen on a Surface

The easiest and safest way to demonstrate the Leidenfrost effect with liquid nitrogen is to spill a small amount of it onto a surface, such as a floor. Any room temperature surface is well above the Leidenfrost point for nitrogen, which has a boiling point of −195.79 °C or −320.33 °F. Droplets of nitrogen skitter across a surface, much like water droplets on a hot pan.

A variation of this demonstration is to throw a cupful of liquid nitrogen into the air. This can be done over the audience, although it's generally considered unwise to perform this demonstration for kids, since young investigators may wish to escalate the demonstration. A cup of liquid nitrogen in the air is fine, but a cupful or larger volume thrown directly at another person could result in serious burns or other injuries.

Mouthful of Liquid Nitrogen

A riskier demonstration is to place a small amount of liquid nitrogen in one's mouth and blow out puffs of liquid nitrogen vapor. The Leidenfrost effect isn't visible here -- it's what protects tissue in the mouth from damage. This demonstration can be performed safely, but there is an element of risk, since ingestion of liquid nitrogen could prove fatal. The nitrogen is not toxic, but its vaporization produces a giant gas bubble, capable of rupturing tissue. Tissue damage from the cold could result from ingestion of a large amount of liquid nitrogen, but the primary risk is from the pressure of nitrogen vaporization.

Safety Notes

None of the liquid nitrogen demonstrations of the Leidenfrost effect should be performed by kids. These are adult-only demonstrations. The mouthful of liquid nitrogen is discouraged, for anyone, because of the potential for an accident. However, you may see it done and it can be done safely and without harm.

Overview of the Leidenfrost Effect
Water Droplets on a Hot Pan
Leidenfrost Effect with Liquid Nitrogen
Immersing Your Hand in Molten Lead

Hand in Molten Lead Leidenfrost Effect Demonstration

Lead is a soft metal with a low melting point.
Lead is a soft metal with a low melting point. The low melting point makes it possible to perform a Leidenfrost effect demonstration. Alchemist-hp

Putting your hand in molten lead is a demonstration of the Leidenfrost effect. Here's how to do it and not get burned!

How To Do It

The set-up is quite simple. The demonstrator wets his or her hand with water and dips it into and immediately out of molten lead.

Why It Works

The melting point of lead is 327.46 °C or 621.43 °F. This is well above the Leidenfrost point for water, yet not so hot that a very brief insulated exposure would burn tissue. Ideally, it's comparable to removing a pan from a very hot oven using a hot pad.

Safety Notes

This demonstration should not be performed by kids. It's important that the lead be just above its melting point. Also, keep in mind lead is toxic. Don't melt lead using cookware. Do wash your hands very thoroughly after performing this demonstration. Any skin not protected by water will be burned. Personally, I'd recommend dipping a single wetted finger into the lead and not a whole hand, to minimize risk. This demonstration can be performed safely, but does entail risk and probably should be avoided altogether. The 2009 "Mini Myth Mayhem" episode of the television show MythBusters demonstrates this effect quite nicely and would be appropriate to show to students.

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Helmenstine, Anne Marie, Ph.D. "Leidenfrost Effect Demonstrations." ThoughtCo, Oct. 12, 2017, thoughtco.com/leidenfrost-effect-demonstrations-604259. Helmenstine, Anne Marie, Ph.D. (2017, October 12). Leidenfrost Effect Demonstrations. Retrieved from https://www.thoughtco.com/leidenfrost-effect-demonstrations-604259 Helmenstine, Anne Marie, Ph.D. "Leidenfrost Effect Demonstrations." ThoughtCo. https://www.thoughtco.com/leidenfrost-effect-demonstrations-604259 (accessed November 17, 2017).