TOPIC:Thermochemistry

Demo 008

Specific Heat of Lead

This is not for the faint of heart or for those who religiously adhere to OSHA guidelines. Lead has a very low specific heat; this demonstration uses this fact in a dramatic way when the demonstrator dumps hot lead pellets (100C) onto his or her open hand.

MATERIALS

25 g of lead shot
One large test tube (8 in)
Ringstand and clamp
Hot plate
500 mL beaker with 300 mL of distilled water.
Paper towels for cleanup
Tongs for holding hot test tube
Insulated gloves to handle the hot hotplate and water

PREPARATION

Place the beaker containing the water on the hot plate and heat to boiling. Place the lead shot in the test tube, clamp the tube in place so that the lead-containing portion dips well into the hot water.

PRESENTATION

This demonstration should follow a discussion of specific heat, the calculation of amounts of heat required to bring about given temperature changes, and a discussion of the magnitudes of the specific heat of various substances. The point to make is that water has an exceptionally high specific heat and that lead metal has a very low specific heat. This demonstration exploits these facts.

Compute the temperature change when 25 g of lead at 100C as added to 25 g of water at 25; it should be only 2-3 degrees. Ask the class if they believe the result of these calculations; more specifically, ask if they would be willing to have 25 g of lead at 100C poured into their open hand (which is mostly water).

With as much drama and flourish as you wish, remove the test tube from the boiling water and pour the hot lead shot into your hand. It will feel quite warm, but not painful. Ask whether anybody would want to repeat this with 25 g of boiling water. Be sure to demand an explanation for the expected negative response.

DISCUSSION

Since the specific heats of water and lead are 4.184 J/gC and 0.16 J/gC, respectively, the quantity of heat transfered from the lead to its surroundings will be only around 4% of the amount released by an equal weight of water.

HAZARDS

This works very well with 25 g of lead. Don't try to scale up the demonstration! There is no disposal to speak of. The lead can be recycled for the next class, and the water can be poured down the drain after it cools a bit.

REFERENCES

None for this demonstration that I know of...maybe a book on magic tricks.

 

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