passage: Chemical changes produce new substances .
  For this reason , they often can not be undone .
  Some chemical changes can be reversed , sort of .
  To do so requires another chemical change to take place .
  For example , you can undo the tarnish on copper pennies by placing them in vinegar .
  The acid in the vinegar reacts with the tarnish .
  This is a chemical change that makes the pennies bright and shiny again .
  You can try this yourself at home to see how well it works .
  Other chemical changes can not be reversed at all or may be difficult to do .
  Rusting is a chemical change .
  If metal rusts , the best you can do is to sand off the rust to get down to the shiny metal .
  Although the metal may now be shiny , the rust was removed .
  The rust was not changed back into the original metal .
  Some chemical reactions occur in only one direction .
  These reactions are called irreversible reactions .
  For example , you can not change a fried egg back into a raw egg .
  Can you think of some other irreversible reactions related to cooking ?
  Would you like a piece of cake ?
query_and_answer: What are some examples of irreversible reactions ? || mixing water and oil
evidences: Some chemical reactions occur in only one direction . These reactions are called irreversible reactions . For example , you can not change a fried egg back into a raw egg . Would you like a piece of cake ?