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Boiling Point


Boiling point of a liquid depends on the pressure above its surface. Lower the pressure, lower is the boiling point. Thus you can boil water at a temperature much below the normal boiling point if the pressure above its surface is reduced. In this demo we do precisely this. The demo is very interesting and catches attention of everyone around.

You need a heater, a conical flask with tightly fitting stopper

Take some water in a conical flask (say half of its volume). The flask should be of good quality. I have done the experiment with Borosil flasks. Put the flask on a heater. Let water boil for about 5 minutes. Put off the heater. Now carefully hold the flask from its neck by using a handkerchief and put stopper in it as early as possible.

The water stops boiling because it is no more on the heater. Put some cold water on the flask, especially on the empty portion. The water starts boiling in the flask. Wait for about a minute and the boiling stops. Again put cold water on the flask. The water in the flask again starts boiling. You can repeat this several times.

Why does water boil when cold water is poured on the flask? When initially you boiled the water on the heater, there was no stopper. Vapour generated during boiling replaced air and there was largely vapour above the water in the flask. At this stage you fitted the stopper tightly which closed possibility of air entering the flask.

The water has cooled down a little by this time and there is no question of boiling. Even at the boiling temperature it boils only when heat is supplied to it which is used in conversion of water to vapour. You poured cold water at this stage. The vapour condensed to water as a result of cooling due to cold water. This greatly reduced the pressure inside the flask. At such a low pressure, the boiling point of water is quite low and the existing temperature of water is much larger than this new boiling point. Thus water starts boiling and coverts to vapour. The pressure inside again increases due to this vapour. Correspondingly the boiling point increases and the water stops boiling at a certain stage. Then you put cold water again. The same process repeats and water boils.

Please take care of accidents. The heater must be a good quality and you should check that there is no current in the body when the heater is put on. Handling hot water should be done with extreme care. Hold the flask from the neck only using sufficiently thick layer of cloth (handkerchief). While putting the stopper, the flask should rest on a firm surface. Though you put cold water several times, the lower portion of the flask remains hot. Don’t try to hold it from there without checking.

Once the demo is over you may like to open the stopper. This seems to be the most difficult task. Since the pressure inside remains much lower than the atmospheric pressure, it does not come out easily. Heat the flask a little and then take it out if it does not come out with mechanical effort.


  1. Experiments

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