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Increased vibration causes the atoms in an object to spread out farther away. Thus, with added heat, and object goes form a solid to a liquid to a gas (and then to plasma, with enough heat). Why is it then, that when a heat something like cake batter, it solidifies?

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It is caused by physical and chemical reactions in the cake batter of two types:

1) When heated and mixed with water, starches present in the flour will undergo a process called gelatinization in which water is absorbed in the structure. This will cause the batter to 'set' from a liquid into a solid.

2) Additionally, reactions of the proteins from the egg that you added can also work to 'set' the batter. At first the proteins will unfold (denature) after which they start to form bigger proteins through cross-linking reactions that turn the liquid into a solid.

The baking soda in the batter turns into sodium carbonate, water and carbon dioxide. The latter will make your cake nice and fluffy.

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  • $\begingroup$ Mostly wrong! 1st there are numerous doughs which do not contain eggs, nevertheless they set on heating. For that the swelling of starch with water and heat is resposible. But both processes are physics, not chemistry! $\endgroup$ – Georg May 26 '13 at 10:15
  • $\begingroup$ @Georg Thanks for the remark. I was indeed still wondering how it was possible for cake batter without eggs to work out and starch gelatinization is indeed the answer. I'll edit it in $\endgroup$ – Michiel May 26 '13 at 12:04
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    $\begingroup$ @Georg wrong, the swelling of starch doesn't set the dough. It thickens it, but it doesn't make it "solid" - cook a pudding to see what happens. The eggless doughs set by denaturing of the gluten network, which arises when two wheat proteins bind in the present of water. As for cake, both the egg and the gluten are at work, and depending on the recipe, the egg may prevail. $\endgroup$ – rumtscho Oct 13 '14 at 19:57