Remaining questions and further work

1. Further investigation is required on the role of dissolved gases in facilitating ebullition.

  • Superheating might be taken further with the current de-gassing technique, with more refinement. I also need an appropriate thermometer to cover the range over 106°.
  • There may be better de-gassing techniques. De Luc reported that he shook his enclosed cylinder of pre-boiled water for 4 weeks as continually as he could, in order to get further air out. That is how he reached 112°, but I would like to achieve the same effect without replicating his method!
  • It would be useful to know how much further de-gassing is achieved by superheating itself. When temperature approaches 100°C, the solubility of air decreases rapidly; does that rapid decrease continue as the temperature goes past 100°C? I have not found the relevant data yet. If the data do not exist, it would be useful to produce them. (How is the solubility measurement done?)
  • Can modern theories provide useful insights into the mechanism of the solution of air in water (including its rate), and of the action of dissolved gases in facilitating vapour-formation? Can such theoretical insights suggest effective experimental methods? Is there a theoretical basis to De Luc's claim that the release of air is facilitated by the process of boiling, and also by mechanical agitation?

2. Further investigation could be made on the variation of boiling temperature depending on the character of the vessel.

  • There is a considerable body of recent and current work on this question, employing techniques that go well beyond my capabilities.
  • According to some 19th-century theorists, microscopic irregularities on the surface promote boiling by creating tiny pockets of vacuum that serve as sites of evaporation. This idea has been revived and elaborated in the modern theory. François Marcet in 1842 reported superheating beyond 105°C induced by treating glass vessels with hot acid, which presumably smoothed out such irregularities. An opposite effect would be expected from scratching the surface. My attempt using sulphuric acid showed no significant effect, but heat-treatment seemed to have some effect.
  • The observations made with the ceramic mug are intriguing, and deserve further investigation. It would be interesting to try a narrow-and-long-necked ceramic vessel in the graphite bath. (If ceramic is apt to produce superheating, then that would also help to explain the propensity for superheating in microwave ovens, in which water is typically heated in ceramic cups.)
  • Certainly worth examining further is the lowering of the boiling point to below 100°C by some metallic surfaces, and by the action of teflon (as a covering layer or as boiling stones). Marcet reported that a sulphur-rich covering brought the boiling temperature down to 99.7°C. The effect of teflon, as mentioned above, is much greater.

3. A study of the steam released by boiling would be instructive.

  • Cavendish asserted in the 1770s that the steam temperature was always fixed under fixed pressure and could never exceed or fall below the standard boiling temperature; he convinced the Royal Society, and this idea later found enough support to form the basis of all sophisticated 19th-century thermometry. However, the steam temperature cannot be as fixed as claimed, due to the possibility of supersaturation discovered by Aitken in the late 19th century.
  • Preliminary checks I made on the steam temperature were inconclusive. Getting more reliable results would require arrangements allowing a broad enough expanse of steam unmixed with air. It would also be interesting to check the temperature of large bubbles of steam arising within the water in superheated boiling, which would be possible with a small, accurate and fast-acting thermometer.
  • Various observations, particularly the lowering of the boiling point below 100°C by the action of certain surfaces or boiling stones, raise questions about the original definition of the "normal" boiling point (or saturation temperature) as 100°C. The cogency of the concept of the normal boiling point rests on vapour pressure being a well-defined function of temperature only, which is an assumption that may be open to doubt. E. E. Shpilrain states that "as a first approximation" the equilibrium vapour pressure is "a function only of the temperature and is defined by the Clapeyron-Clausius equation" ("Vapour Pressure", in Hewitt et al. 1997, 1262). And he goes on to say that "for most pure substances the equilibrium vapour pressures are defined experimentally and tabulated", though theory can predict the shape of the pressure-temperature curve. Therefore it may not be idle to inquire whether the vapour pressure depends on various circumstances other than temperature. Marcet thought that his observation of sub-100°C boiling was an indication that the numerical value of the normal boiling point had been misidentified; that possibility cannot be ruled out on the basis of what I have learned so far.

4. The temporal and spatial fluctuations in the temperature of boiling water could be monitored.

  • It seems that there is a body of sophisticated recent and current work on this subject, well beyond my capabilities.

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