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Author Topic: Freezing/Melting point of gold.  (Read 19545 times)

Offline Broca

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Freezing/Melting point of gold.
« on: 09/06/2004 19:19:01 »
Hi, I am here with a 5th grade class learning about matter and energy. We are reading that the freezing point of gold anf the melting point of gold is the same. Can you please explain to us why or how this can be.
Thanks!


 

Offline Rokitansky

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Re: Freezing/Melting point of gold.
« Reply #1 on: 09/06/2004 20:18:07 »
??? What you don`t understand ?

Gold is solid on a room temperature. If we heet it enough to it`s melting point it will melt when temperature exceeds the melting point. If now we get the temperature lower, on the same point (melting=freezing) it will turn back in solid state.
 

Offline Broca

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Re: Freezing/Melting point of gold.
« Reply #2 on: 09/06/2004 23:56:21 »
OMG I feel so stupid...<laughing> ok ok  go ahead point at me and laugh then. <hanging my head in shame>  :-)
 

Offline gsmollin

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Re: Freezing/Melting point of gold.
« Reply #3 on: 10/06/2004 05:29:24 »
There is a latent heat of fusion between the melting and freezing points. Now the temperature, at equilibrium, is the same, BUT, since energy won't move into or out of the material at equilibrium, there does have to be some temperature drop across the material to change its state from liquid to solid or solid to liquid. So there is a practical difference in temperature between the melting point and the freezing point, even though the equilibrium temperature is always given as one point.
 

Offline Ylide

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Re: Freezing/Melting point of gold.
« Reply #4 on: 10/06/2004 05:48:24 »
There are some substances which have significant differences between freezing and melting points, too.  Typical nutrient agar, the material used in Petri plates to grow bacteria, melts around 100 C but will not resolidify until it gets down to 40 C.  That stuff is a mixture of different materials, though.  Pure compounds tend to behave a little "better" and have freezing and melting points closer together.



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Offline tweener

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Re: Freezing/Melting point of gold.
« Reply #5 on: 11/06/2004 03:46:41 »
quote:
Originally posted by Broca

OMG I feel so stupid...<laughing> ok ok  go ahead point at me and laugh then. <hanging my head in shame>  :-)



It's OK, we all have our moments.  A couple of weeks ago, it took my wife and I both several hours to figure out why an outlet would run the garbage disposal when we turned on the switch, and not the dishwasher.  Duh, the outlet was off when the switch was off and we didn't want to leave the disposal running the whole time the dishwasher was running, so we left the switch off then.  We both have degrees in electrical engineering which is what makes it reeeeealllllly embarassing.[:I]

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Offline gsmollin

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Re: Freezing/Melting point of gold.
« Reply #6 on: 11/06/2004 19:00:32 »
You're and EE... and you married one? Geez, I gave up on women engineers in college after 3 mis-adventures with them.
 

Offline neilep

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Re: Freezing/Melting point of gold.
« Reply #7 on: 11/06/2004 22:58:39 »
...I have a similar situation.....before my wifey cooks anything it's edible (say a carrot or a stick of celery or other vegetable)...then when she has cooked it, it's inedible !!....I know I know...it's completely unrelated to the temperature thing....and in fact it's complete nonsense isn't it ?...but then...I give good nonsense !!!...I suppose I'm talking about the differences between stability and instability and you guys are talking about heat differenetials...........oh....I don't know why I bother either  !!!

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Offline OmnipotentOne

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Re: Freezing/Melting point of gold.
« Reply #8 on: 17/06/2004 00:22:24 »
OH I remember in 6th grade our science teacher gave us this substance called "Ubleck" We had to specify if it was either a solid or a liquid.  The problem was, when we touched it it turned from a play-doh like solid to a watered down syrup feel.  everyone was fascinated by this, and although I argued that MAYBE the melting and freezing point were close together, I was ridiculed for such a far out answer(eventually the teacher said I was right!) still a traumatizing experience *tear

I've never read into it, but isnt plasma the stage inbetween solid and liquid, I really have no idea.

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Offline neilep

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Re: Freezing/Melting point of gold.
« Reply #9 on: 17/06/2004 03:31:32 »
quote:
Originally posted by cannabinoid

There are some substances which have significant differences between freezing and melting points, too.  Typical nutrient agar, the material used in Petri plates to grow bacteria, melts around 100 C but will not resolidify until it gets down to 40 C.  That stuff is a mixture of different materials, though.  Pure compounds tend to behave a little "better" and have freezing and melting points closer together.



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Hi Jay,

I'm just trying to get my head around this....

It melts at 100 C and won't solidify until it reaches only 40 C........but when it's being heated, it is still solid until it reaches 100 C, even though it has passed 40 C, which when cooling down it would still be a liquid.....so in other words these substances can exist as two different states at the same temeperature eh ?....but only because of due process eh ?

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Offline Ylide

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Re: Freezing/Melting point of gold.
« Reply #10 on: 17/06/2004 03:44:26 »
Precisely.  You could have a beaker of solid agarose gel at 60 C while it's heating up and another beaker of liquid agarose at 60 C that is cooling down.  Weird huh?  I don't really know the chemistry behind it, might make for a fun nerdly night of researching.



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Offline neilep

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Re: Freezing/Melting point of gold.
« Reply #11 on: 17/06/2004 03:56:54 »
Well stick a rock up my bum and call me a peach !!..Yep..that's well weird !!...

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Offline gsmollin

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Re: Freezing/Melting point of gold.
« Reply #12 on: 19/06/2004 16:39:45 »
This effect is caused by hysteresis in the phase diagram. It is interesting that it can occur here. I am mainly familiar with hysteresis in ferromagnetism and mechanics. In both cases the hysteretic behavior is caused by friction. In the former case it is friction to the realignment of magnetic domains, and in the later it is the difference between static and dynamic friction. There must be some form of friction that holds the agar together; the chemical bonds require more energy to break than they do to stay in the liquid phase afterwards.
 

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Re: Freezing/Melting point of gold.
« Reply #12 on: 19/06/2004 16:39:45 »

 

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