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Different Liquids Or Different Forms Of One Liquid?


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#1 hazelm

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Posted 27 June 2017 - 07:28 AM

From Science Daily: - a Stockholm University Study:  https://www.scienced...70626190610.htm

 

 

Water exists as two different liquids at low temperatures where  ice crystallization is slow.  "...two phases of the liquid with large differences in structure and density"  (paraphrased quotes from report)

 

Question:  If both are H2O (not mentioned by the report) and if the differences are in form only (structure and density) why aren't they still just one and the same liquid? 

 



#2 exchemist

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Posted 27 June 2017 - 08:29 AM

From Science Daily: - a Stockholm University Study:  https://www.scienced...70626190610.htm

 

 

Water exists as two different liquids at low temperatures where  ice crystallization is slow.  "...two phases of the liquid with large differences in structure and density"  (paraphrased quotes from report)

 

Question:  If both are H2O (not mentioned by the report) and if the differences are in form only (structure and density) why aren't they still just one and the same liquid? 

According to the abstract of the PNAS article, which is all I could access,these seem to have been experiments done on amorphous ice at very low temperatures (110-130K or so, i.e. below minus 100C.) Amorphous ice means ice that is not formed into a regular crystal structure but is instead like a glass, with randomised arrangements of molecules, as you find in a liquid, but "frozen in".

 

Amorphous ice apparently exists in higher and lower density forms. The researchers show that diffusion occurs in both, indicating that both these forms can be regarded as glassy liquid phases of extremely high viscosity, rather than true crystalline solids, and they can see the transition occurring between these two phases.  

 

But to you and I, it seems that both phases would look like just ice. You see something like this with a block of bitumen. It looks solid and you can shatter it with a hammer, but if you leave it a while, it slowly flows and spreads out under its own weight, because it is actually a very viscous liquid rather than a true solid.  

 

At least, that is what I get from reading the article quickly. 

 

As to the differences in structure of the two forms, no details are given in the abstract. But I do know that water below about 4C does start to expand, as semistable portions of ice structure start to develop. I presume that something like this may be responsible. 

 

The paradox of water is that there is a balance between the normal tendency of all liquids to minimise energy by van der Waals forces pulling the molecules together on the one hand and, on the other, the special energy reduction mechanism available to water by virtue of its ability to form "hydrogen bonds" with its nearest neighbours. In normal ice the hydrogen bonds win out, resulting in a less dense structure than in liquid water, which is why ice floats. 

 

I think we would need to see the whole article to understand this properly, though.


Edited by exchemist, 27 June 2017 - 08:31 AM.


#3 hazelm

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Posted 27 June 2017 - 08:51 AM

So they are not really talking about two different liquids - as they say they are - but of two different forms/structures of one and the same liquid:  H2O.  Yes?  Can you use the same chemical formula to identify two different liquids? 

 

Thank you for the analysis.  Very clear.



#4 exchemist

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Posted 27 June 2017 - 09:26 AM

So they are not really talking about two different liquids - as they say they are - but of two different forms/structures of one and the same liquid:  H2O.  Yes?  Can you use the same chemical formula to identify two different liquids? 

 

Thank you for the analysis.  Very clear.

Well this is the thing about science magazine articles. Often they somewhat oversimplify and make things seem more dramatic. It's unusual to have two liquid phases of one substance, but very common to have more than one solid phase, due to the different arrangements molecules can often take up to form a crystal.