2018-03-11 04:58:42 UTC
James McGinn says that water vapour cannot exist as a gas under 100 deg C, going by steam tables.
He gets much ridicule for taking this viewpoint from all in this newsgroup, and this arouses my interest for unfortunately I too face a similar situation, although for other subjects - such as holding that e=mcc=hv is nonsense, Lorenz force has no opposite reaction, unlimited acceleration is thus possible, etc etc.
James is correct up to a point, going by the experiment my science teacher performed right before the whole class when I was in Std 4, in the year 1965.
Those were the great days of science. e=mcc=hv mumbo-jumbo had not reduced physics to the status of the most ridiculous, worthless and cantankerous liberal arts subject imaginable, as is the situation now.
So it was, that right on the teachers' table was mounted a gas burner, and after lighting it, was placed upon it a metal oil can of rectangular cross section, and filled up to a third or so with water.
Soon the water heated up, and started boiling. The teacher allowed a good few minutes for the water to boil, and the steam to come out from the narrow round opening in the can. Then, very carefully so as not to get scalded, he secured the can shut by tightly screwing in the oil can cap.
After that, it was pure fun for us 8-9 year olds. As the can cooled, the most weird noises were heard as the rectangular metal walls of the can caved in. The shape it finally took was peculiar. What enjoyment, well remmebered after 53 years!
As a few here may have guessed, our teacher was showing us through an experiment, the existence of atmospheric pressure.
According to him, the steam in the can, rising up, forced the air molecules out of the can, so there were not so many air molecules left in the can after a while - the can was full of hot steam and its pressure matched the air pressure outside, so the walls of the can did not cave in so long as the water was boiling.
But after the burner was shut off, the temperature fell, the steam in the can cooled down and became water. (This is the interesting part for McGinn - when there are no air molecules to stop the interaction of water molecules joining each other to form water, then yes, water vapour does become water below 100degC as this experiment shows.)
Now as the hot steam became water, with not too many air molecules left in the can, there was less pressure from air within the can. There was more pressure outside the can. This greater air pressure pushed the metal walls in, making all the enjoyable sounds.
This experiment certainly proves the existence of air pressure, or rather, what happens when there are air pressure differentials - forces come into play! It also proves that given a closed space, with not too many air molecules to obstruct, water vapour gas below 100degC unites to form water.
In the open region, another story. The steam that escaped, was monomolecular and diffusing upwards. Some of it while cooling would interact with each other to form water. The rest would rise up to form clouds. If the surroundings were too muggy, they would add to it by combining with the extra water vapour around, thus causing more fog, etc.