Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: Sagren Moodley on 07/02/2011 07:30:03

Title: How can boiling water evaporate immediately?
Post by: Sagren Moodley on 07/02/2011 07:30:03

Sagren Moodley  asked the Naked Scientists:
   
Hi Chris

I hope you are well, Im from south africa and i listen to your show with Redi Tlabi every friday, but i can never get through to ask you any questions.

In this youtube video (

the boiling hot water evaporates immediatly when the lady thows it our of the glass, how is this possible and is it true?

I hope that you can respond to my questions and that they don't seem outrageous, I love your show and have only today discovered your website, keep up the good work, looking forward to listening to you on Friday

Thanks

Sagren Moodley
South Africa

What do you think?

Title: How can boiling water evaporate immediately?
Post by: CliffordK on 07/02/2011 09:47:52

I suppose you could fake the phenomenon with something like dimethyl ether. 

However, if I had to venture a guess of what is happening...  It likely has to do with humidity and the dew point.

From Wikipedia:
http://en.wikipedia.org/wiki/Dew_point
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F9%2F91%2FDewpoint.jpg&hash=c7bf8a84746c76337a323377ee2de10f)

Here are some more charts.

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html#c2

So, at -30°C, the water carrying capacity of air is very low.
At 100°C the dewpoint, or water carrying capacity of the air is relatively high.

So, if you could locally change the air temperature from -30°C to 100°C, it might be like putting the water in a high vacuum, and it might evaporate very quickly.

At least...  in theory.

It looks like a fun experiment...  if one actually had -30°C weather. 

Title: How can boiling water evaporate immediately?
Post by: lightarrow on 07/02/2011 10:16:31

<<How can boiling water evaporate immediately?>>

When you throw the hot water in the air, it disperses in many little drops, so the total area of interface liquid/air increases greatly. For this reason the vaporization rate increases abruptly. Then the water vapour and the remaining smaller liquid drops, in contact with the very cold air, solidify immediately forming the "snow" cloud.
As CliffordK wrote, all the process is facilitated from the fact that at -30°C the air is very dry (this increases further the vaporization rate).