Reversible and irreversible changes

We conducted an interesting investigation into reversible and irreversible changes today.  We placed zinc oxide in a test tube and heated it over a burner.  We noted that the white powder turned yellow when it became hot as oxygen was driven off; it returned to its original colour of white when it cooled down again.  Blue copper sulphate was then heated in the same way.  When it gained heat, it turned white and condensation was visible on the inside of the test tube; this was as a result of water molecules being driven from the powder.  When water was added to the test tube, the copper sulphate returned to the blue colour again.  We established that both of these changes were reversible changes, also known as physical changes.