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Its known to everyone that when a solid is heated up to its melting point it turns into a liquid. What happens when a liquid is heated? Simple, it tends towards becoming gaseous. While making omlettes I found that the yolk is in liquid state and putting it on a pan for heating results in the formation of what we call as the omlette which is a solid. How is this so?

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You've discovered organic chemistry! – dmckee Sep 8 '11 at 15:06
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Going along with dmckee's comment, the answers written so far mention the denaturation of individual proteins, but I believe that the proteins also become chemically cross-linked to each other, and that this is (one reason) why the process is irreversible. – Greg P Sep 8 '11 at 20:43
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Is this really a homework/educational question? It doesn't at all sound like one. – David Z Sep 8 '11 at 23:06
    
@David Zaslvasky- Well it is not a homework or educational question. Actually I could not find an appropriate tag for it and as my reputation is not enough to create a new tag hence I used the tag homework – Primeczar Sep 10 '11 at 8:20
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@dmckee ""You've discovered organic chemistry! "" Not at all, this is physics, no chemical reaction involved. And, of course not "organic" at all, biochemistry were appropriate if some chemistry were involved. The naming "organic chemistry" is traditional (early 19th century), nevertheless essentally wrong by our today knowledge, but it sticks. – Georg Sep 15 '11 at 16:09

Proteins that make up the egg are very complex molecules, far away from full thermodynamic equilibrium both when cold and when heated. Thus conclusions of thermodynamics of simple substances do not apply to egg proteins. Solidification of an egg into an omlette involves a process called denaturation.

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Proteins are long chains of amino acids. They are created in long chains, but interactions between various parts of the chain can result in a folded protein, in what we call its native state, see below:protein folded/unfolded

Interestingly when you apply heat to proteins the reverse happens: leaving the native state and returning to a long chain of amino acids (an un-native state), denaturation is what the process is called. Many of these long chains are created when you are cooking an egg and instead of folding back into the native state, random interactions form between different chains, usually interacting hydprophobic regions avoiding water that is accumulating around the hydrophilic regions of the peptide sequence. As the process continues, water molecules are incorporated into an ever-growing framework and a random three-dimensional structure emerges. Aggregation eventually results in very large masses and we have our gel -- or solid as you say. Same idea to making jello, really.

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if u want to involve some thermodynamics, a liquid yolk is having lower entropy, while a solid omelet is having higher!( contrary to the text book style teaching in which solids are having lower entropy). This is precisely because of the denatuation of protein. – Vineet Menon Sep 15 '11 at 4:57

Emulsions are part of a more generic two-phase systems of matter called colloids. Although the term colloid and emulsion are sometimes used interchangeably, emulsion tends to imply that both the continuous phase is dispersed as liquids.

An emulsion is a mixture of immiscible liquids in a more or less homogeneous. A liquid (dispersed phase) is dispersed in another (the continuous phase or dispersing phase).

An emulsifier (also known as an emulsifier) is a substance which stabilizes an emulsion, frequently a surfactant. Examples of food emulsifiers are egg yolk (where the main chemical emulsifier is lecithin), honey and mustard, where a variety of chemicals in the mucilage around the seed pod act as emulsifiers, proteins and low molecular weight emulsifiers are common.

For example, egg albumin, which is a colloid, has been obtained in crystalline form, which is why the heat is applied to a system which is apparently liquid, you get the texture you can see the omelet.

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are you saying that the egg albumin in an omelet is a colloidal crystal? – Chris Sep 8 '11 at 23:34

Indeed, when you heat an egg the proteins denature and become random coils. Now, these random coils blend together and depending on their density can form a viscoelastic fluid or a gel.

Interestingly, cooling the cooked egg does not reverse the process because the coils are entangled but the process can be reversed, e.g., by adding some chemical but it takes 3 hours and the egg will probably be inedible.

To understand a viscoelastic fluid, mix a spoon of starch in a (paper) cup of water. You can easily turn a spoon slowly in the liquid but the resistance increases as you increase the speed by which you turn the spoon. This is because the coils do not have enough time to rearrange.

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