What do the arrows stand for ?

Any photo? or something

Iwould you figure it out but you don't have a picture or in your question as to what the physical property you're looking for is

13) a 14) a15) b16) i don't know sorry17) a (maybe)

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.