During an investigation a scientist heated 27 g of mercuric oxide till it decomposed to leave a dry product. the total mass of the product formed was 25 g. does the law of conservation of mass hold true in this case?

Yes.

Explanation:

The law of conservation of mass holds true, because the mass missing is in gas form. The fact that matter can change, doesn't mean they violate this law of conservation.

So, this law is always happening.

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.