Presentation of the apparatus:
-a bottle of cold water.
-a heat resistant recipient with a
-a small burner
Heat the water to boiling point.
Once it’s boiled take the container
off the gas...
... Seal it with a bung.
Now you have to pour some cold water on to the container !
It’s a miracle -the water starts boiling again.
- Once is not always! You need to acquire a few basics before the home made explanation. Let’s see.
- First of all you need to know that the pressure in a sealed container depends amongst other things on the number of gas molecules closed inside. If in one way or another the gas disappears, the pressure lowers.
- Next, remember that water boils at 100°c. Normal, yes at 1 bar of pressure that is the pressure in kitchens. If the pressure increases as in your mum’s pressure cooker, water boils at 105° and if pressure lowers as at the top of Mont Blanc, then water boils at only 85° C.
Finally you have already noticed the mirror in the bathroom after a shower : water vapour condenses in contact with the cold surface of the mirror and the water molecules change from gas to liquid.
Right, now we’re ready for the explanation:
As soon as you take the container of the gas the temperature of the water in it slowly diminishes. Above the liquid, water vapour is trapped. When the walls of the cylinder are quickly cooled by the shower, water vapour condenses and this is translated inside as the molecules of gaseous water are changed into liquid water - see the video opposite - This lowering of pressure brings about a lowering of the boiling point . This was initially 100°C!
and as the gas condenses, 95°C, 90°C, 85°C and when the boiling temperature becomes lower than the temperature of the liquid, well the liquid starts boiling! When boiling stops, you just have to start showering again to make the boiling point temperature below that of the liquid. You can make water boil in the palm of your hand when the boiling point goes below 50°C!
For the clever ones here’s a little graph to explain more precisely the different phases in the process.