Answer
A pot of boiling water is an open system because a burner supplies energy in the form of heat, and matter in the form of water vapor is lost as the water boils.
Step-by-step explanation
The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction.
The description of an example where an open system might result in conclusions that go against the above law is a pot of boiling water is an open system because a burner supplies energy in the form of heat, and matter in the form of water vapor is lost as the water boils.
Note: An open system can exchange both matter and energy with its surroundings while a closed system can exchange energy but not matter with its surroundings. However, the last type of system is an isolated system, and an isolated system exchanges neither energy nor matter with the surroundings.