In which part of the digestive system does a CHEMICAL change of the food first occur?

A: mouth. Chemical digestion begins in the mouth when enzymes in saliva begin to break down carbohydrates. Most chemical changes in digestion occur in the small intestine. Large molecules of food are broken down into smaller molecules that can be absorbed by our cells.

You would be correct, boiling water does disinfect contamination.    the property of disinfecting means to kill  germs, bacteria and other pathogens or contamination  that might cause an infection or cause a person to become sickly. a couple methods of disinfecting are using soap, other cleaning supplies or by simply "boiling" the contaminate.  the reason why boiling water disinfects certain attributes is because some cells cannot survive the heat and pressure.