Re: Are all physical changes reversible?

In the case of baking, there are three major changes occuring.  The first is the 
formation of the air pockets in the cake, the second is what I will call the 
"setting" of the batter, and the third is the loss of water.

1)  When the baking soda or powder is mixed with the water-based batter, CO2 is 
formed.  So, this is the general acid-base reaction we see in alka-seltzer, etc. 
 This is a chemical change.  What happens next is that the CO2 is less soluble 
in the batter as it is heated (use cold soda keeping its fizz better analogy).  
As the CO2 becomes less soluble, it rises through the batter, forming the tiny 
air pockets that cause a cake to rise.  This is what I would refer to as a 
physical change.  The reason it is not reversible in this instance is that the 
CO2 is gone.  (We could, however, trap the CO2 again in cool water once it 
passes through the vent of the oven.)

2)  The other major change in baking is the denaturation of proteins and/or the 
crystallization of sugars (the latter is more important in pie fillings and 
candies, etc.).  The proteins become denatured as they are heated.  In other 
words, they vibrate so much that they break the hydrogen-bonds, ionic bonds, 
disulfide bridges, and hydrophobic interactions that hold proteins in the active 
3-D shape.  As this happens, the proteins become less and less soluble in the 
solution.  This is what happens when an egg is fried.  I would call these 
chemical interactions, though I am sure it could be argued effectively that 3 of 
the 4 interactions are physical.

3)  The batter would still be gooey if we did not evaporate off some of the 
water.  I think this is probably the physical change that the site refers to.  
As the batter is heated, it loses water.  Once again - this is not reversible in 
the application because the water evaporates.  If we lose too much, the cake is 
hard.  If we lose too little, the cake is runny in the center.  People at Betty 
Crocker and similar companies spend a great deal of time calculating and then 
testing just how long it will take to evaporate the correct amount of water with 
the best rising characteristics caused by loss of CO2.

I hope this answers your question!