Some plant species release chemicals into the soil that end up killing nearby plants. this is an example of what? competition, cooperation, mutualism, working together

This is an example of competition because the plant is releasing the chemical in order to kill off the other nearby plants that may be competing and fighting for the water and nutrients in the soil in order to survive. The plant species releasing the chemical will kill the nearby plants, rather than benefit them. Therefore, it cannot be mutualism. The plant species is also not working together with the nearby plants by killing them. It also cannot be cooperation or working together. 

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.