Where is the equivalence of mass and energy important?
Changes where the loss of mass is significant, like the above examples, typically happen only in the extreme conditions of nuclear reactors or inside stars, like the Sun. However, any time we change the way particles are arranged, we are binding them in a different way, so very small amounts of binding energy are still being used. Therefore, the water vapor in the above example would weigh slightly more than the 15 grams of our original water because there is slightly less energy in the more “loosely bound” water vapor. Typically, this difference is so small as to be undetectable, so the principle of conservation of matter is good enough for everyday situations.
