Which is a pure substance?
soda
gasoline
salt water
carbon dioxide

Carbon dioxide is a pure substance.

Explanation: A pure substance is defined when a substance has a single type of molecule. If more than 1 type of molecule is present in a substance, then it is considered as a mixture.

Soda is basically a mixture of water and carbon dioxide. More than 1 type of molecule is present.Gasoline is a mixture of may gases. More than 1 type of molecule is present.Salt water contains salt and water molecules, hence it is considered as a mixture.Carbon dioxide has only 1 type of molecule which is molecules. Hence, it is a pure substance.

Which of the following did you identify correctly

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.