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Topic: why do solids and liquids expand when heated?  (Read 5815 times)

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Offline confusedstud

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why do solids and liquids expand when heated?
« on: October 19, 2012, 08:35:17 AM »
When they are heated, their kinetic energy increases and thus they vibate more about their own fixed positions. However, their potential energy which relates to the distance between each molecule should remain the same right? So what allows them to expand? I don't see how the increased vibration would mean the solid expands since the magnitude of vibration has to remain the same (PE is still the same).

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Offline nuclearalcemist2

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Re: why do solids and liquids expand when heated?
« Reply #1 on: October 19, 2012, 10:10:34 AM »
KE increases. Vibrational amplitude increases. So, distance between the atoms increasing right?

Offline Arkcon

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Re: why do solids and liquids expand when heated?
« Reply #2 on: October 19, 2012, 10:16:17 AM »
Just what nuclearalcemist2: said.  Consider also this, if you add enough kinetic energy, liquids become a gas.  Then the volume expands to fit the container.  Why is that?  Also consider the inverse, the cooler you get something the less space it takes up, except for water (and a few others.)  Now why is that?  You may have simply confused yourself with the potential energy, how exactly is that germane to molecular spaces?
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Offline Vidya

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Re: why do solids and liquids expand when heated?
« Reply #3 on: October 19, 2012, 11:39:00 AM »
As the kinetic energy increases on heating this results in weakening of the ionic attractions or intermolecular forces of attractions and molecules or ions are more free to move .Think about it !

Offline confusedstud

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Re: why do solids and liquids expand when heated?
« Reply #4 on: October 19, 2012, 11:53:16 AM »
Hi, but in my physics class the only time a substance has its particles move further from each other is when there is a change in state. Since only the kinetic energy increase, how can the vibrational amplitude increase which causes the increase in volume (expansion)? Q=mcΔθ is for the kinetic energy component while Q=ml is the potential energy component and they cannot happen together right?

Offline Arkcon

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Re: why do solids and liquids expand when heated?
« Reply #5 on: October 19, 2012, 04:19:08 PM »
Your physics class is at least wrong about the volume change that occurs when water freezes into ice.
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Online Borek

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Re: why do solids and liquids expand when heated?
« Reply #6 on: October 19, 2012, 04:39:38 PM »
http://en.wikipedia.org/wiki/Negative_thermal_expansion

I remember reading that Schott kitchen stove tops are made from the material that is a mixture of two substances - one with a positive, and one with a negative expansion coefficient, combined in such proportions that the resulting thermal expansion coefficient is zero (just like invar). That's what gives it its resistance to the rapid changes of temperature.
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Offline confusedstud

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Re: why do solids and liquids expand when heated?
« Reply #7 on: October 20, 2012, 02:16:57 AM »
but when simply heating with no change in state what allows the potential energy to increase? since the solid expands.

Water is an exception for this i guess.

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