Experiment 6: The boiling of de-gassed water

This clip shows the effect of de-gassing on boiling. First, the de-gassing procedure: initially the water is boiled for a long time in a covered pot, to sweep out as much of the air as possible; then the boiled water is poured carefully into a long-necked flask, and placed on a hotplate; boiling in this degassed water is very bumpy, and the temperature goes well beyond 100°C, resulting in further degassing. At high degrees of superheating the insertion of a thermometer excites violent boiling, as the roughness at the tip of the thermometer serves as a site for bubble-formation, or “nucleation”. After a while the flask is removed from the hotplate, and allowed to cool slightly. Then it is inserted into a bath of graphite, for gentler heating, with the graphite temperature at only about 250°C. The temperature of the water in the graphite bath is monitored by inserting the thermometer occasionally. For the most part the water is absolutely still, although its temperature is very high, easily reaching 107-108°C. Inserting the thermometer prompts very active boiling, bringing the temperature down. When there are higher degrees of superheating, the water will explode on contact with the thermometer, or sometimes spontaneously. If the surface area of the water is relatively large, the fast evaporation that happens at the surface of the superheated water can cause heat-loss that matches the rate of heat input from the graphite, so we can easily have water superheated to 105-106°C sitting there indefinitely with no bubbling. These behaviors match De Luc's report very well.

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