What is the difference between elements, compounds and mixtures in terms of particle theory?

Element is a single substance, while a compound is the chemical combination of two or more elements, and mixture is the physical combination of two or more elements that can easily be reversed to obtain it's previous state. 

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.