More on experiment 5, and related phenomena

What is shown in Experiment 5 is quite typical of all the trials I made with slow heating in volumetric flasks. Boiling in this circumstance is very prone to superheating, the temperature easily exceeding 102°C, in one experiment reaching over 105°C. Sometimes there is one spot (or two) on the inner surface of the flask, or on the thermometer bulb, from which a continuous stream of bubbles arise (I will call this "streaming"). Temperature can be quite steady if there is continual streaming, and tends to get only up to 102°-103°C. If there is no streaming and the boiling is unsteady, the temperature is quite variable. During bumping, the temperature dips visibly after the production of a burst of bubbles, and climbs slowly back during periods of no activity. During puffing, the temperature usually creeps up in the long run (despite noticeable dips when large bubbles are produced), going up to 104°-105°C.

It is important to note that the superheating observed here is not a state of unstable equilibrium that is reduced to the "normal" stable equilibrium on some stimulus. Boiling by streaming can be sustained, apparently indefinitely, at 102°-103°C. During bumping and puffing, the production of a burst of bubbles or a large puff does bring down the temperature, but never close to 100°C. There are various stable temperatures depending on the particular circumstances, and from my experiments there is no evidence that 100°C is generally speaking any more stable than any other point.

When water is boiled in a this way, even the use of anti-bumping granules fails to bring down the temperature near 100°C. The granules do facilitate smooth boiling rather than bumping or puffing. However, even as the smooth boiling takes place, the temperature creeps up: in an initial trial, a small number of granules produced steady streaming, but the temperature easily reached 101.50°C; in a later trial a generous amount was used, to cover the base of the flask almost completely, but still the temperature went up to 100.65°C, where it seemed to stabilise (cf. the maximum of 100.00°C in wide-open beakers with anti-bumping granules). It would be interesting to observe the effect of the Teflon boiling stones in this circumstance.

At the end of several of the trials, I confirmed that the water had not acquired impurities or any other strange properties that might affect the boiling temperature, by pouring it out into a beaker and boiling it with the bunsen burner. In each case the boiling behaviour was "normal", with maximum temperature around 101°C.

On the whole, my observations confirm the results reported by De Luc in 1772.

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