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When you add heat to a liquid (or a fluid), can it be solidified? If not, why in the world does an egg's stuffs become solid (or at least no more a liquid) when you 'boil' it in water?

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I think the answer contains chemistry... – Bernhard Apr 21 '13 at 12:09
really, what is it then? – newera Apr 21 '13 at 12:57

A simple material will not undergo a liquid to solid transition as the temperature is raised. When you see this it means somthing more complicated than a simple phase transition is going on.

In the example of egg white, what you are seeing is denaturation of the protein albumin. The heat causes the protein to lose its tertiary structure then form cross links, and it's the cross linked structure that turns everything solid. You can tell it's not a phase transition because it isn't reversible if you lower the temperature gain.

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The explanation of egg turning solid which is a chemical change has well been explained by John. Interestingly, if we can trust the following article from Physics World, A liquid solution of α-cyclodextrine (αCD), water and 4-methylpyridine (4MP) turns from liquid to solid when heated to 45-75 degrees C. The interesting part to note here is the reversible nature of the physical change as explained in the linked article.

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I'm sure there's a polysaccharide that forms a gel at high temperature but a liquid at low temperature. However I came across it back in the late 80s and I now can't remember the details. Googling hasn't helped, so I may simply be misremembering. – John Rennie Apr 21 '13 at 17:53

The liquids are either dissolved or in a colloidal form. When the liquid is boiled off, the solute or particulate matter comes together to make a solid. This is also achieved by the changing of the molecular structure. This procedure is also called denaturation. This is how liquids can be heated into solids.

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