If a little ____ is added to water, the water boils at a temperature higher than 100^oC.

Adding salt (sodium chloride, NaCl) to water elevates its boiling point through boiling point elevation, a colligative property of solutions. The salt ions (Na+ and Cl-) disrupt the formation of water vapor at the surface, requiring more energy (higher temperature) for the water to boil. The boiling point increases proportionally to the concentration of dissolved particles. Sugar also raises boiling point but salt is the classic example. Oil doesn't dissolve in water, and chlorine is a gas.