Science fairs are almost a past-time of schools around the country, and possibly the world. Not only do they seem to stereotypically feature the mundane, but also the little gems that attract the crowd. What child wouldn’t have wanted to be that kid, the one with all of the eyes on them? To be the one getting the applause, and the “ooh”s and “ahh”s, along with the occasional startled gasps of delight. On top of it all, you would walk away, naturally, with that blue ribbon, and an “A+” on your project grade.
Ah, but I have to say, I was the one with the crowd. Well, one of the ones with the crowd. There was quite a competition going on, but I did have a crowd of my own. My experiment was quite simple, but demonstrated several very interesting physical changes, as well as some called-for spectacle and interest from the crowd.
Experiment: To demonstrate the high thermal conductivity of lead by melting, then flash-freezing, that is, pouring into cool water and causing an immediate state-change.
Oven Furnace (Mine had a maximum temperature of 2000 degrees Fahrenheit)
500 grams pure lead (granulated)
Clear 5 or more gallon jug or bucket (Jugs should be cut like a bucket, without a tapered rim)
Long steel tongs
If you would like to do this experiment multiple times in succession, it is best to have multiple crucibles filled with lead in the furnace at once.
1. Fill the clear bucket with cold water (not freezing, as the ice will present issues with the flash-freezing), to about one inch from the rim. Set aside and keep close.
2. Add 500 grams of lead into one 250mL crucible. Place the crucible into the furnace, and heat the furnace to approximately 900 Degrees Fahrenheit (482 Degrees Celsius).
3. Note that the lead may take some time to melt. While the lead is melting inside the furnace, put the gloves on, and light the torch. Use the torch to heat the bowl portion of the spoon, rotating while doing so, to heat the bowl completely. This will prevent any dangerous flying metal when stirring the molten lead, as the rapid temperature change can cause such. Once heated properly, set the spoon atop the brick, so nothing catches fire.
4. Once the furnace has heated to the proper temperature, the lead should be fully melted, its boiling point only being a meager 620 degrees Fahrenheit (326 degrees Celsius). If not, then proceed to wait until melted. If you feel that the temperature is not adequate for the time the experiment needs to take place, you may increase the temperature. It might be necessary to re-heat the spoon.
5. Once melted, remove the lead from the furnace using the tongs and gloves. Use the spoon to stir the lead, while holding it away from the face, to ensure that all of the lead is completely melted, if not, it may be prudent to put it back in the furnace for a short time. To cool the spoon, dunk it in the water. It will sizzle.
6. Have the audience stand away from the bucket of water. Using the tongs again, and while staying as far from the bucket as possible, slowly and carefully pour the lead into the center of the bucket, in a thin, constant stream.
7. If done properly, the lead will hit the water, and the instant transfer of heat will not only cause the water touching it to turn to steam, it will cool immediately, and sink to the bottom in a flowered mass of metal, or small, hollow droplets, (Depending on the speed of the pour) leaving the plastic bucket unscathed.
The droplets are the result of the lead bubbling from the temperature change as it hits the water, and the expanded, flowered mass is the result of a faster pour, causing all of the lead to remain together as it expands.
The instantaneous temperature change and change of state caused when the molten lead hits the cold water is commonly known as flash-freezing. This happens when the heat energy of the water is so much lower than the heat energy of the molten lead, causing it to immediately transfer its heat to the water, and change state instantly, or freeze. Because the amount of water is so much greater than the amount of lead, the water will absorb and dissipate the heat energy from the lead throughout itself. If anything, a slight temperature difference in the water can be noticed.
-Safety gear must be worn at all times. There are many chances to burn yourself, or do irreparable damage to your skin, body or eyes.
-This experiment is designed for lead. No other metals should be used, especially not those with higher melting points, as they may not have proper time to cool before hitting the bottom of the bucket.
-When pouring the lead into the water, the water will splash, possibly causing small droplets of lead to fly from the bucket. This is why it is not only important to have everyone stand away from the bucket, but to practice the experiment beforehand.
-If the lead is poured too fast, it will not have ample time to cool before reaching the bottom of the bucket. There is a chance that the bottom of the bucket could melt, the lead could burn the bottom exterior and cause the bucket to dump its contents and the molten lead, or at minimum cause the lead to become unsalvageable. This is another reason to practice the experiment beforehand.
-Practice the experiment beforehand.
-Extreme caution should be used whenever handling hot and/or volatile materials.
Among all of the noise and spectacle, the experiment is actually quite an amazing one, in and of itself. Not only does the lead lose approximately eight hundred and fifty degrees of heat in the blink of an eye, it changes state instantly, and, if done correctly, the lead is immediately cool enough to pick from the bottom of the bucket.
Having experience in metals, I felt safe enough to attempt some extra spectacle when trying this, and had someone hold out their hand before pouring the lead. Once the lead hit the bottom of the bucket, I would grab a piece, and toss it to the person holding out their hand. The person naturally either recoils, or catches the lead. Not only did this create some extra enthusiasm amongst the crowd, it did a fair job of demonstrating the immediate temperature decline of the material, and furthermore, proving the high thermal conductivity of lead.