Ice In Boiling Water?

  • #1

Main Question or Discussion Point

"Ice In Boiling Water?"

If you nearly fill a test tube with cool water and then take a piece of ice and press it down on the bottom of the tube with a small weight, heat the test time with a flame that licks only the upper part of the tube, the water will start to boil sooner or later.But the ice at the bottom will not melt.

What's going on?
 

Answers and Replies

  • #2
SpectraCat
Science Advisor
1,395
1


If you nearly fill a test tube with cool water and then take a piece of ice and press it down on the bottom of the tube with a small weight, heat the test time with a flame that licks only the upper part of the tube, the water will start to boil sooner or later.But the ice at the bottom will not melt.

What's going on?
Well, first of all I don't think your last statement is strictly correct, I mean, the ice will melt eventually if you just leave it sitting out at room temperature .. you don't need the flame. However, assuming that you have a long enough tube to observe the effect you mention, all that is going on is that it takes time for the different regions to reach thermal equilibrium with each other. Thermal transport in gases and liquids takes place largely through the phenomenon of convection, where less-dense, warmer "packets" of material rise up while more-dense, cooler packets fall down, leading to mixing. In a narrow test tube, it could easily take more time for this to occur than for the water near the top of the tube to reach the boiling point.
 
  • #3


Yeah I took it straight out of a book and didn't understand...

So basically it just takes more time for it to melt because heat rises and the ice is at the bottom. Yeah?
 
  • #4
SpectraCat
Science Advisor
1,395
1


Yeah I took it straight out of a book and didn't understand...

So basically it just takes more time for it to melt because heat rises and the ice is at the bottom. Yeah?
Yes ... the situation would be different if you locally heated the bottom of the tube and had the ice at the top .. in that case, the convective mixing would be faster, since the heat transfer is going in the "favored" direction for convection .. sorry if that wasn't clear from the last post.
 

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