Re: Why does the yolk of a chicken egg expands when salt water is poored into?

hello Roberto......the reason that the yolk expands is due to the fact that 
water enters the egg yolk and the salt cannot escape.  this process is 
called osmosis and can be explained with the second law of thermodynamics. 
this law states that systems (in your case, the egg yolk and the water 
surrounding it) will tend towards a state of higher probability, or more 
generically put, it will become more "disordered".  for your system to 
create the maximum amount of disorder it would like to have the same 
concentration (# of molecules in a specified volume) of salt on the inside 
of the egg yolk as on the outside.   as i said before the salt cannot 
escape from the egg yolk, but the system would still like to have the same 
salt concentration inside of the yolk as the water surrounding it.  one way 
to decrease the concentration inside of the yolk is to increase the volume 
by addition of water (in this case there will be a smaller # of molecules 
per specified volume than you started with).  the water will enter the yolk 
(by a process called diffusion) until enough pressure builds up to keep the 
yolk from expanding further.  an example would be if you put salt water 
into the yolk so that the concentration was 1 gram of salt per milliliter 
of yolk.  water will enter and cause concentration to decrease to less 
than 1 gram of salt per milliliter of yolk (because the yolk has 
expanded and it's volume is now larger than when you first started).  i 
hope that this helps.......good luck with your project!