What methods do scientists use to determine if a change was physical or chemical?

A physical change will not change the chemical make-up of the material. the change will not produce a new substance.

a chemical change will have a reaction that produces heat, light, a change in color, releases a gas, or rust

The answer is d.potassium

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.