Ask A Scientist© Biology Archive

 
Question:  We have been studying chemical and physical changes in 6th grade
science class and we were wondering whether hard boiling an egg would
be a chemical or a physical change?  Thanks for a reply!
Sandburg J High

Answer:  You decide.  Here's what's going on: the proteins in the fresh egg are
in the shape of tight little balls.  When you boil the egg, these proteins
unravel ("denature"), like balls of yarn unraveling into loose skeins.  The
strands of protein then get all tangled up with one another, so much so
that they are locked in place and can no longer move.  They also lock into
place the other liquid components of the egg, forming all together what's
called a "gel" instead of the liquid you started off with.  The gel acts
like a soft, rubbery solid because of the network of protein strands
holding it all together.  It's certainly true that when the protein
denatures some chemical bonds are broken, but the most important effect is
the tangling up process.
   The same thing happens when you boil your Jell-o, which is named that
way because of course it's a gel.  Other famous gels include contact
lenses, slug slime -- I am NOT making this up -- and the stuff inside your
eyeball.
Christopher Grayce