Demos: 3B-04 Boiling Water in Cup
A paper cup is filled with water and supported in a ring on a stand. A flame is put beneath the cup and eventually the water boils without burning the cup. This demonstration actually illustrates two concepts: (1) The thinness of the cup allows heat to pass through quickly (conduction); (2) Because water boils at well below the kindling temperature of paper, the paper will not burn. Also, the convection of the water keeps the temperature of the water fairly uniform throughout, even though water itself is a poor conductor.
Directions: Paper cups without thick ridges on the bottom are difficult to find. The kind we have are relatively free of these, but there are places on the cup that have double thickness. These areas of the cup are likely to flame up or char, but eventually these will burn off and the rest of the cup should stay intact. Move the flame around constantly to minimize these problems.
Suggestions for Presentation: Ask if it would be practical to try to boil water in a paper cup. Tell the students that youre pretty sure you can pull it off. After completing the demo, focus on the properties of both the paper and the water that allows you to do this.