Agaseous substance turns into a solid. which best describes this change?

Deposition is the process by which a gas undergoes a change of state of matter to become a solid, without becoming a liquid in between. An example is water vapour turning straight to ice in subfreezing air against a glass.

The dispersed phase is liquid and dispersed medium in milk is liquid

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.