Under what conditions is mass conserved?

A. Whenever the system is isolated or closed.
B. Only when the change is a chemical one.
C. Always.
D. Only when the change is a physical one.

I was thinking A.

C. Matter (mass) cannot be created or destroyed, it can only change.

You are correct, option A. Whenever the system is isolated or closed, mass is conserved.

To understand why, let's break down the options and explain how to arrive at the correct answer:

A. Whenever the system is isolated or closed: An isolated or closed system refers to a system that does not exchange mass or energy with its surroundings. In such a system, mass is conserved because there are no external influences that can add or remove mass from the system.

B. Only when the change is a chemical one: This option is incorrect because the conservation of mass applies to all types of changes, not just chemical reactions. Both physical and chemical changes can conserve mass.

C. Always: This option is incorrect because mass conservation does not apply in all scenarios. Mass can be converted into energy according to Einstein's famous equation E=mc². In nuclear reactions, a small fraction of mass is converted into energy, violating the principle of mass conservation.

D. Only when the change is a physical one: This option is incorrect because the conservation of mass applies to both physical and chemical changes. In physical changes, there may not be any change in the chemical composition, but mass is still conserved.

Therefore, option A, which states that mass is conserved whenever the system is isolated or closed, is the correct choice.