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Old 01-02-2005, 06:38 PM   #1 (permalink)
Mjollnir Incarnate
 
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BrainQuest!

Anyone remember this game? It was a fairly early educational series that made being smart "cool". Anywho, I was cleaning out the closet and found some cards, with questions I disagree with. What's your take?

True or false? Adding salt to a pot of water makes it reach the boiling point faster. True

Erm, adding solutes does affect the boiling/freeezing point, but by making them higher/lower respectively. Which would technically make it reach the BP later, right?

In which of these states is matter heaviest: solid, liquid or gas? Solid

Weight is determined by mass*gravity. In transition between phases, matter is not lost. So if both matter and gravity are constants, then so is weight.
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Old 01-02-2005, 06:44 PM   #2 (permalink)
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Aaah, so we were more ignoarant back then
Not sure about the first one, way too rusty on that.. but the second definetely seems untrue to me.. and pretty obviously, too...
*Hoping I didn't just miss an opportunity to shut up*
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Old 01-02-2005, 06:56 PM   #3 (permalink)
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OK , I tested the boiling question. Without salt. it took my electric stove set on high. 2,54 mintues to boil 2 cups of water. using a different burner, pot, the same amout of water (2 cups) added one tsp of salt. It took 2.4 mintues to reach boiling point. Not a great differenace. but still a differance

as for the mass question. That's a little bit more tricky. the mass is the same. this is true. however the particles are so spaced out in the container that it effects the weight. so the truth should hold the same for liquid. I would have to do more research to confirm it though.

nice idea for a thread. thanks
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Old 01-02-2005, 07:04 PM   #4 (permalink)
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Quote:
Originally Posted by ICER
as for the mass question. That's a little bit more tricky. the mass is the same. this is true. however the particles are so spaced out in the container that it effects the weight. so the truth should hold the same for liquid. I would have to do more research to confirm it though.

nice idea for a thread. thanks
A pound of ice weighs the same as a pound of water, no? (Or, if you're not an American, use the far superior metric system)

As far as your experiment, that's pretty interesting. Although did you make sure the burner cooled off completely before boiling again? Whatever, my Chem teacher is gonna hear about this tomorrow.
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Old 01-02-2005, 07:16 PM   #5 (permalink)
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Quote:
Originally Posted by Slavakion
A pound of ice weighs the same as a pound of water, no? (Or, if you're not an American, use the far superior metric system)
Now, let's not start that again

Correct. a pound of water should weigh the same. But if that water was steam? how would it weigh? Gas have a very low density because the particles are so spaced out in the container.

Quote:
Originally Posted by As far as your experiment, that's pretty interesting. Although did you make sure the burner cooled off [i
completely[/i] before boiling again? Whatever, my Chem teacher is gonna hear about this tomorrow.
And yes. I used a completely different burner, pot, water, everything.
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Old 01-02-2005, 10:50 PM   #6 (permalink)
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Well, I found the reason for the boiling point question:

Quote:
Salt (or other solutes, like sugar) can easily dissolve in liquid water. However, taking the solute out of the water and putting it in the gas phase (air) requires a lot of energy. At temperatures around the water boiling point, these solutes stay in the liquid.

Now the total pressure in the liquid and the air at the boundary are the same- otherwise one would push the other into a smaller space. Part of the pressure in the liquid comes from the solutes, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapor pressure, that is, the pressure of water vapor that would stay in equilibrium with the liquid, is reduced by the same amount because of the solutes.

Water boils when the vapor pressure of the water gets to be as big as the pressure of the atmosphere. At that point, vapor bubbles in the water can grow. You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmospheric pressure, so it has a higher boiling point.

A very similar argument explains why solutes also lower the freezing point. Since the solutes are almost completely excluded from the solid (like from the gas) they stabilize the liquid. A search of this site will turn up some answers about freezing salt water.

http://van.hep.uiuc.edu/van/qa/secti...0912061445.htm
still working on the weight/mass thingy
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Old 01-03-2005, 03:36 AM   #7 (permalink)
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Quote:
Originally Posted by ICER
Correct. a pound of water should weigh the same. But if that water was steam? how would it weigh? Gas have a very low density because the particles are so spaced out in the container.
It shouldn't make the slightest bit of difference. Gravity is constant, right? And when the water boils into steam, matter is conserved, right? So we have constant mass and constant gravity. There are only three terms in the formula weight=mass*gravity. So weight must be constant as well. Just because it's a gas doesn't mean that it has any less weight. The particles just have too much kinetic energy to settle into a solid/liquid.

Oh, and I knew why solutes raised/lowered BP/FP, I was wondering how they could make something boil faster. That quote agrees with me that it would take more energy. So it boils faster because it needs more energy to boil?

Last edited by Slavakion; 01-03-2005 at 03:38 AM..
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Old 01-03-2005, 06:18 AM   #8 (permalink)
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Bah, I can imagine some copy writer somewhere going "Hey WTF? I was just paid to type this shit!" Salt would raise the boiling point, boiling it slower. The mass of a material is the same, whatever state it is in. The apparent weight might be different if you take into account it's bouyancy and relative densitiy to air (assuming you are weighing it in air, steam might be apparently lighter than water/ice due to it's relative density) Strange thing about water of course, is that in it's solid form, it's actually less dense than it is in it's liquid form (the reason ice floats), so if relative bouyancy is at issue, and if the material is water, then the answer would be liquid. But like I said, hack writer + one too many pulls on the bottle = crappy questions
 
Old 01-03-2005, 08:13 AM   #9 (permalink)
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What's heavier? A pound of feathers, or a pound of bricks?
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Old 01-03-2005, 08:36 AM   #10 (permalink)
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Quote:
Originally Posted by fckm
What's heavier? A pound of feathers, or a pound of bricks?
neither, a pound is a pound. which is what the whole debate about which is heavier, solid, liquid, or gas.
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Old 01-03-2005, 08:50 AM   #11 (permalink)
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In the presence of a constant gravitational force, constant mass implies constant weight. Weight has nothing to do with density. Nothing. None.
Boyance is a separate issue from weight.

EDIT: quoted for great justice
Quote:
You have to heat the liquid with solutes up more to get the vapor pressure in it to equal the atmospheric pressure, so it has a higher boiling point.
from above.
The boiling temperature increases. High school chemistry tells us that solutes increase boiling point and lower freezing point. That's why cooks put salt in their water when they're boiling stuff, and why homeowners go out and buy bags of salt for their driveways.
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Last edited by fckm; 01-03-2005 at 08:53 AM..
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Old 01-03-2005, 09:14 AM   #12 (permalink)
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Quote:
Originally Posted by Slavakion
In which of these states is matter heaviest: solid, liquid or gas? Solid
Of course, they were thinking "density" when they wrote the question, not "mass", but even if they wrote it correctly, the question remains a bad one. Ice (solid) floats on water (liquid), and is therefore less dense.

EDIT: zen_tom already mentioned this in his post, but I missed it on my first read, so I'll keep my post up here to make the point more obvious.
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Old 01-03-2005, 11:57 AM   #13 (permalink)
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Quote:
Originally Posted by Slavakion
True or false? Adding salt to a pot of water makes it reach the boiling point faster. True

Erm, adding solutes does affect the boiling/freeezing point, but by making them higher/lower respectively. Which would technically make it reach the BP later, right?
Right, adding salt increases boiling point, and ICER gives a good explanation.

However, the question is only talking about reaching the bp, not what the bp itself is. And I'd agree with you that with a higher bp, it would take a longer time to heat the water. Which then confuses me, because when cooking pasta, you sometimes add a pinch of salt, which people say makes the water boil faster. I think this is where this question comes from.

I've googled it, and one possible answer is that the salt provides nucleation sites for the water to start boiling, but this seems unlikely because salt is soluble in water, and you would need larger insoluble particles for nucleation sites. My only other guess is that the salt affects the specific heat but i don't know enough about those thermodynamics to say anything definite, and my instinct says that it can't affect it by much...

Last edited by Amano; 01-03-2005 at 08:34 PM..
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Old 01-03-2005, 12:32 PM   #14 (permalink)
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Just as a follow-up, I asked my Chem teacher today. She confirmed that all phases weigh the same, and the BP should logically take longer to reach. She did say something about salt and bubbles that might affect the BP, but I didn't really catch it. And she said it was unlikely. So there, teach says so.
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Old 01-03-2005, 12:52 PM   #15 (permalink)
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Quote:
Originally Posted by Slavakion
Just as a follow-up, I asked my Chem teacher today. She confirmed that all phases weigh the same, and the BP should logically take longer to reach. She did say something about salt and bubbles that might affect the BP, but I didn't really catch it. And she said it was unlikely. So there, teach says so.
Good enough for me

So, what have we learned.

1: Don't buy a Brain Quest for your kids
2: a pound is a pound.
3: Always listen to the teach (unless you want to fail)

And I goofed on the boiling test. Although I used exact amounts in both pots. The second pot was a few inches smaller and had a wider bottom. allowing the water to heat up faster. (hate it when that happens)
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Old 01-03-2005, 03:59 PM   #16 (permalink)
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Quote:
Originally Posted by ICER
1: Don't buy a Brain Quest for your kids
Eh, there were hundreds of cards, each with 6 questions. Typos (while annoying) happen.
Quote:
And I goofed on the boiling test. Although I used exact amounts in both pots. The second pot was a few inches smaller and had a wider bottom. allowing the water to heat up faster. (hate it when that happens)
Blargh. Oh well, that's what the scientific method and structured experiments are there to prevent, right? No hard feelings.
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Old 01-07-2005, 07:18 AM   #17 (permalink)
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In which of these states is matter heaviest: solid, liquid or gas? Solid

If we assume that we are talking about the same material, wouldn't the gas weigh the most and the solid the least, with the liquid in the middle? Given that the mass is the same the only difference between them would be the amount of energy they contain, and the gas would have the most.
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Old 01-07-2005, 07:35 AM   #18 (permalink)
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Phage, keep your terms consistent.

Mass
Volume
Weight: force, or mass times gravity (which can be considered a constant, so weight is another way of saying mass)
Density: mass divided by volume
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Old 01-07-2005, 07:55 AM   #19 (permalink)
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Quote:
Originally Posted by Redlemon
Phage, keep your terms consistent.

Mass
Volume
Weight: force, or mass times gravity (which can be considered a constant, so weight is another way of saying mass)
Density: mass divided by volume
My terms are consistent; when I said weight I was talking about the weight, and when I said mass I was talking about mass. I did not mention volume or density because they have no bearing on weight, which was what the question was about. My point is that the extra energy required to change the solid or liquid into a gas would result in an increase in mass and thus weight.
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Old 01-07-2005, 08:13 AM   #20 (permalink)
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Hmm, missed that, sorry. But, I don't think that adding heat energy increases the mass; does it? It's been quite a while since I've taken either chemistry or physics.
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Old 01-07-2005, 08:08 PM   #21 (permalink)
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Quote:
Originally Posted by Phage
My point is that the extra energy required to change the solid or liquid into a gas would result in an increase in mass and thus weight.
No. The energy that changes the liquid into a gas or any other phase change goes into making the molecules move faster. Einstein's E=mc^2 (where mass and energy can be interconverted) has no bearing on this question.
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Old 01-07-2005, 08:15 PM   #22 (permalink)
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You are correct in both analyses. It would take longer for the water to boil, and the weight = m*g or the mass alone would be constant regardless of the phase of the compound. I have a Ph.D. in chemistry.
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Old 01-07-2005, 08:17 PM   #23 (permalink)
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Quote:
Originally Posted by roderickpsu
You are correct in both analyses. It would take longer for the water to boil, and the weight = m*g or the mass alone would be constant regardless of the phase of the compound. I have a Ph.D. in chemistry.
Ok...so not until May...but good enough
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Old 01-11-2005, 05:44 AM   #24 (permalink)
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Quote:
Originally Posted by roderickpsu
You are correct in both analyses. It would take longer for the water to boil, and the weight = m*g or the mass alone would be constant regardless of the phase of the compound. I have a Ph.D. in chemistry.
These two links refer to what I am speaking about specifically:
http://dbhs.wvusd.k12.ca.us/webdocs/...ss-Energy.html
http://van.hep.uiuc.edu/van/qa/secti...0923112019.htm

...while this one is just good reading.
http://en.wikipedia.org/wiki/Conservation_of_mass

"The law of conservation of mass states that the mass of an isolated system will always remain constant, regardless of the processes acting inside the system."

Suppose that we use our handy-dandy e=MC^2 formula on an isolated system which is one cubic foot in size, and convert 1 gram of a 5 gram block of iron directly into energy. What you have just suggested is that the mass of the closed system will be reduced by this change, which is clearly in violation of the Law of Conservation of Mass. By extension we must conclude that because energy has mass (even though a gram of energy would be quite a lot) adding it into a closed system would increase the mass of that system.
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Old 01-11-2005, 02:31 PM   #25 (permalink)
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I read your links... and I have to say that this is the first time I have ever heard of energy having mass.
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Old 01-11-2005, 03:02 PM   #26 (permalink)
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Quote:
Originally Posted by Slavakion
I read your links... and I have to say that this is the first time I have ever heard of energy having mass.
yeah, that's a new one on me too. Interesting concept though. Does energy exist as mass at the atomic level, or is it nothing more then a radiation that excite particles to become energy.
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Old 01-11-2005, 07:03 PM   #27 (permalink)
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Well Energy itself doesn't have mass per se , but as the first link points out, it is possible under certain circumstances to interconvert mass and energy. That's where E=mc^2 comes in.

When the US was testing out its atom bombs in WW2, they showed that some mass was lost, and transformed into energy (just a little, not all the energy of the bomb). They did the calculations and the data fit E=mc^2 rather well
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Old 01-11-2005, 07:37 PM   #28 (permalink)
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Quote:
Originally Posted by ICER
And yes. I used a completely different burner, pot, water, everything.

hehe. You changed too many variables. How do you know the 2nd burner gets to the same temperature? How do you know the 2nd pot conducts heat the same as the first one. What you needed to do was to turn the burner on for say 15 minutes to guarantee it was at max temperature. Then put the pot with the fresh water and time it. Then empty the pot, and wait for it to cool to room temperature. Then put saltwater in it and put it over the same burner, which you have either not turned off or have turned on 15 minutes previously.

That would be much more accurate, though still poor in accuracy because electric stoves don't maintain a constant temperature unlike gas.
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Old 01-11-2005, 11:56 PM   #29 (permalink)
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Quote:
Originally Posted by Amano
Well Energy itself doesn't have mass per se , but as the first link points out, it is possible under certain circumstances to interconvert mass and energy. That's where E=mc^2 comes in.

When the US was testing out its atom bombs in WW2, they showed that some mass was lost, and transformed into energy (just a little, not all the energy of the bomb). They did the calculations and the data fit E=mc^2 rather well
As I have said before mass must be conserved, so if it is possible to convert matter into energy then it must follow that the energy released has mass. Yes, it per se has mass. The testing by the US would have found out that some matter was converted to energy, instead of showing a mass loss and refuting basic concepts of thermodynamics.
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Old 01-12-2005, 04:16 AM   #30 (permalink)
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Quote:
Originally Posted by Phage
As I have said before mass must be conserved, so if it is possible to convert matter into energy then it must follow that the energy released has mass. Yes, it per se has mass. The testing by the US would have found out that some matter was converted to energy, instead of showing a mass loss and refuting basic concepts of thermodynamics.
Correct me if I'm wrong, But don't we covert matter into energy everytime we set a log on fire? I mean the fire consumes the log, giving off heat energy right?
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Old 01-12-2005, 04:32 AM   #31 (permalink)
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Quote:
Originally Posted by ICER
Correct me if I'm wrong, But don't we covert matter into energy everytime we set a log on fire? I mean the fire consumes the log, giving off heat energy right?
Combustion is not a matter to energy conversion but merely a vigerous oxidation reaction. It releases the energy stored in the chemical bonds of the wood, but all of the matter is still around in some form.
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Old 01-12-2005, 01:00 PM   #32 (permalink)
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I am unsure about hte salt off the top of my head, however with the second question it should read something to the degree of per cubic meter. In which case everything but water as far as we can tell would agree with the statement. However, water being as it is, expands upon becoming a solid therefore when water is a liquid it "weighs" (or contains more of itself) inside a cubic meter. However if the question is really as simple as it is stated then the only answer is nothing.
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Old 01-12-2005, 01:11 PM   #33 (permalink)
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I am speaking on no authority here, but I believe that the first question is correct and that the saline solution will have broken apart the water molecules therefore needing less energy to seperate them into a vapor. I know one of the complex parts about sperating water is their shifting hydrogen bonds. As a matter of fact it is nigh impossible for a computer to model even five water molecules interactions in real time.
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Old 01-12-2005, 01:49 PM   #34 (permalink)
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Damn you Phage. I had a nice short essay about E=MC^2 and how it meant that turning an amount of solid into a gas resulted in something that weighed more.

Then I read more of the thread, and you had already done it. Better than my essay too!

Humbug! Ruined my fun. I do have one thing to add:

The amount of mass (and hence weight) added by taking a solid and turning it into a gas is so small I personally doubt it would be measureable.

If you took a cube 10 m on each side, weighing about 1000 tonnes, and turned it into water vapour, the resulting gas would have about 36 miligrams more mass than the original ice cube.

As an aside, the energy in a 1 megaton (H-bomb size) nuclear explosion comes to about 47 grams of mass. The energy in the tsunami in SE asia massed a quarter of a kilogram. The energy in the earthquake that triggered it massed between 10 and 100 kg.

Another fun thing: if you took a cube of water, 30 km on each side, and turned it completely into energy, you would have about enough energy to push the earth into the sun.
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Old 01-12-2005, 03:22 PM   #35 (permalink)
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Quote:
As a matter of fact it is nigh impossible for a computer to model even five water molecules interactions in real time.
In what way?
I ask because I've modeled upwards of several million gold, nickel, and copper atoms in nanoindentation simulations. You'd be surprised at what a modern supercomputer can do.
EDIT: I can't type.
EDIT2: nor can I spell
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Old 01-12-2005, 09:32 PM   #36 (permalink)
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Quote:
Originally Posted by Slavakion

[i]In which of these states is matter heaviest: solid, liquid or gas?
Maybe they meant to say most dense? That'd be pretty legit then.
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Old 01-13-2005, 01:27 AM   #37 (permalink)
Insane
 
Quote:
Originally Posted by Yakk
If you took a cube 10 m on each side, weighing about 1000 tonnes, and turned it into water vapour, the resulting gas would have about 36 miligrams more mass than the original ice cube.
WHAT??? Why would you convert mass into energy with a simple phase change? I'd really like to see your calculations.
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Old 01-13-2005, 01:30 AM   #38 (permalink)
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Quote:
Originally Posted by fckm
In what way?
I ask because I've modeled upwards of several million gold, nickel, and copper atoms in nanoindentation simulations. You'd be surprised at what a modern supercomputer can do.
EDIT: I can't type.
EDIT2: nor can I spell

It's possible he means in a fully thorough, quantum mechanics sort of way. I know that even for something as simple as a hydrogen atom, or a helium molecule, the math is pretty involved, so i can only imagine what it's like for one, much less 5 water molecules
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Old 01-13-2005, 11:30 AM   #39 (permalink)
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Location: Ontario, Canada
Quote:
Originally Posted by Amano
WHAT??? Why would you convert mass into energy with a simple phase change? I'd really like to see your calculations.
I calculated how much energy it would take to turn a 10 m cube of ice into water vapour. Somewhere around 700 calories/gram.

I then divided by c^2, and got the mass of that energy. Remember, all energy has mass.

Thus, if you take a block of ice, and turn it into water vapour, the resulting water vapour will mass more (and hence be heavier) than the original block of ice, but only by a very small amount.
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Old 01-13-2005, 02:02 PM   #40 (permalink)
Insane
 
Quote:
Originally Posted by Yakk
I calculated how much energy it would take to turn a 10 m cube of ice into water vapour. Somewhere around 700 calories/gram.

I then divided by c^2, and got the mass of that energy. Remember, all energy has mass.

Thus, if you take a block of ice, and turn it into water vapour, the resulting water vapour will mass more (and hence be heavier) than the original block of ice, but only by a very small amount.
That is based on the assumption that all the energy is contributed to an increase in mass, which is wrong. Energy does not get converted to mass in a simple phase change. Any energy you put into the system goes into the thermal energy in the water to increase temperature and boil it. But that energy does NOT go into a mass increase.

I still maintain that all energy does NOT have a mass. Take a 1kg block and drop it from a height of 1m. When it hits the ground it will have approximately 9.8Joules of energy, but the energy does not go into increasing the mass.

Likewise, an exothermic reaction releases energy due to a difference in Gibbs free energy. The difference in energy in the chemical bonds that make up the molecules of the reactants and products is responsible for this. But no mass is converted into energy or vice versa.

I'll agree to the statement that a given amount of energy can be associated with a given mass, according to E=mc^2. But energy does not have mass.

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