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sudo

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Do different phases of the same substance have the same specific heat capacities? Can someone please explain this?
 

chiddler

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Yes. It takes more energy to convert water to steam than it takes to convert ice to water. With the former, you break Hbonds and really go against entropy and is thus more energetic. With the latter, don't remember how ice to water exactly happens but it takes less energy.
 

milski

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Chiddler is correct but he/she is talking about heat of fusion/evaporation.

The answer is still the same for specific heat capacities - they do vary between different phases of the same substance. They actually vary with temperature within the same substance but that's rarely significant unless you're heating them to really high temperatures.

What the specific heat capacity will be depends on how many ways a molecule of the substance can absorb energy. In general, the more degrees of freedom a molecule has, the more energy it can absorb before the temperature being risen. That means that typically it will be higher at higher temperatures but that's a very general trend and you can probably find exceptions. The detailed reasons of why and how exactly it changes are more than what you need for MCAT.
 
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pfaction

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I'll throw this in from TBR; liquids have the highest heat capacity because in liquids to gases you're breaking intramolecular bonds so you're going to need a lot of energy, compared to solid->liquid where you're just weakening bonds.
 

milski

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I'll throw this in from TBR; liquids have the highest heat capacity because in liquids to gases you're breaking intramolecular bonds so you're going to need a lot of energy, compared to solid->liquid where you're just weakening bonds.
This is related to heat of fusion (energy needed to change state at constant temp), not specific heat capacity (energy needed to raise the temperature by a certain amount without changing state).
 

milski

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Whoops, carry on then.

Wait, so they have the same heat capacities or they don't have the same?

They do not have the same capacities but the reasoning is very different. For example, you certainly are not breaking any type of H-bonds or other intermolecular forces. They're either already broken (gas) or staying in tact (solid).
 
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