More on Experiment 1, and related phenomena
When heating is done with an open flame, the "start of boiling" is a very ill-defined point. Vapour bubbles start coming up from the bottom when the temperature of the main body of water is well below the boiling point (this is called "subcooled" boiling in modern theories of boiling). In older terminology, what we have is "hissing (sifflement in French)", in which the vapour bubbles are collapsed, with a characteristic noise, before reaching the surface of the water. This was also known as the "singing of the kettle" to serious tea-drinkers -- the peculiar noise heard just before full boiling sets in.
We might say that real boiling requires a good number of bubbles to come through to the surface (instead of getting condensed back as they rise up). But even after this starts, often the temperature continues to rise slowly for some time, going up by at least 0.5°C, sometimes over 1°C. The maximum attainable temperature as the water boils vehemently is a more precise and stable point in each instance, but it is not constant across different circumstances, even under fixed pressure. This makes sense after all, since various factors would affect the rates of inflow and outflow of heat.
There seem to be clear differences of temperature associated with different "degrees" of boiling (ranging from simmering to vehement boiling), as various 18th-century observers reported. Some typical observations from my experiments: a range of 1.24° in a glass flask in a graphite bath (recorded on 5 July 2004); a range of over 1° in a glass beaker over a Bunsen burner flame (19 July); range of over 0.5° in a stainless-steel pot over Bunsen burner flame (19 July).
After the burner is turned off, the bubbling stops almost completely even before the temperature comes down to 100°C. It is quite an interesting asymmetry in the behaviour of the water while it is being heated and while it is being cooled. This probably has much to do with the effect of de-gassing that takes place with boiling, which is explored further in Experiment 5 and Experiment 6.
University College London - Gower Street - London - WC1E 6BT - +44 (0)20 7679 2000 - Copyright © 1999-2005 UCL