Experiment 1: The indefiniteness of the boiling point
This video clip shows pure distilled water being boiled in an ordinary glass beaker over a Bunsen burner flame, with the temperature being monitored by an ordinary mercury thermometer and a digital thermometer, whose metallic probe is visible next to the bulb of the mercury thermometer (read more about the thermometers). Observe carefully the number, size, shape, and frequency of bubbles, and how they change as the heating progresses. (Note tiny bubbles coming out from very early on, which are most likely dissolved air being released, since the solubility of air in water decreases with temperature. Vapour bubbles also start forming from quite low temperatures, but for some time, these bubbles do not make it through to the surface of the water.) In this experiment it seems that something that looks vaguely like boiling begins around 97°C (not so far from where that point is marked on the Adams thermometer), and there is quite active boiling from around 98°C. After full boiling begins the temperature stabilizes around 100°C, then creep up slowly, eventually reaching nearly 101°C. By that point, the vapour bubbles arise from only a few spots on the bottom of the beaker.
Watch a video clip showing this experiment (2 mins.) (NOTE: At the end of this clip, the video-streaming will continue on to other experiments. You can simply press the "stop" button and close the video window, to return to the text. Most of the description above is repeated in the voice-over accompanying the video.)
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