[Paul Anderson]

Paul Anderson  asked the Naked Scientists:
   
I was listening to one of your podcasts yesterday in which the question was:  "Which is heavier, a pot of cold water or a pot of hot water?", and the answer given was a pot of hot water because of extra energy.

How was this measured?

A pot of cold water has a certain amount of dissolved oxygen. If you boil the water the oxygen will leave the water (bubbles in a boiling pot). Based on that I would have thought that a pot of hot water would be lighter than a pot of cold water.

However to ascertain this, there are other problems. If the pots are open, when you boil the water, there will be steam loss, which will upset the calculation. The other suggestion is to try boiling the water in a sealed container, which might run the risk of an explosion. However, if it didn't, and the water boiled, then there would be no oxygen loss, so the difference then would be the energy supplied to the pot. Is that the way the experiment was done? Of course I am assuming if this were done with two containers, that both empty containers were identical in weight, or if only one container were used, that the container was weighed cold and hot and any weight differences deducted from the weight of the cold and hot water.

What do you think?

[Vern]

A pot of cold water has a certain amount of dissolved oxygen. If you boil the water the oxygen will leave the water (bubbles in a boiling pot). Based on that I would have thought that a pot of hot water would be lighter than a pot of cold water.

I have to agree with you Paul. The increased mass from heat energy would not be enough to offset the mass loss due to evaporation etc. E = MC^2 gives mass a powerful edge.

[syhprum]

The gain in mass from heating one liter of water from 0°C to 100°C is 4.64 pico grams, quite difficult to measure in the school lab.

[lightarrow]

Paul Anderson  asked the Naked Scientists:   
I was listening to one of your podcasts yesterday in which the question was:  "Which is heavier, a pot of cold water or a pot of hot water?",

But you have to specify if the volume of the water or the mass is the same before the experiment. If they have the same initial mass, there is no loss of water due to vaporization and you can neglet gases dissolved in the water (for example boiling the water and then cooling it down before the experiment) then the hot water is heavier because of the energy you have given to it, but it's impossible to measure the difference, as syhprum said.

[lyner]

Go on syphrum.
I love it when you talk numbers.

[syhprum]

I don't understand maths much past day one of the calculus course but I was brought up having to use slide rules and log tables and I find it so easy to do arithmetic with modern aids like calculators and BASIC programs.

[lyner]

So many of the questions on these fora just need a back of fag packet calculation to tell you whether something's possible or not.
It gives you a comforting feeling to 'know' the numbers.

Slide rules rule, too. Respect my son.

[LeeE]

...I find it so easy to do arithmetic with modern aids like calculators and BASIC programs.

Try using a spreadsheet prog instead; dealing with series of numbers is a breeze, as is plotting curves from the results.  Much more useful than simple calculator progs or writing something in BASIC.

[swansont]

What do you think?

I think that physicists love ideal cases and are really pedantic, so one looks at the pots of water that are otherwise identical.  No differences in dissolved oxygen, no evaporation, etc.  IOW, it's not meant to be a trick question.

You won't be able to measure the mass difference of a pot of water because no scale exists that can.  But scientists have measured the mass difference of atoms in excited states vs. the ground state, which is conceptually the same thing.  An isomer of Fe with a different mass than the ground state was discovered in a Penning trap.  The energy difference between the states is 402 keV

http://blogs.scienceforums.net/swansont/archives/278

[yor_on]

Nice one Swansont:)

Reading the question makes me think of another guy.
I'm sure he would have been up to try to test it.

Wasn't he doing some kind of experiments involving 'reducing a mass into another substance' in a closed system, and then finding the weight of it to be the same after finishing.

I wish I could remember his name?
Nicolas??
Or was it Leonard?

And it was some time ago, I'll admit...