Is Mass Really Conserved?

Today a question came up regarding conservation. Specifically, we were discussing if mass is completely conserved during a chemical reaction.

The answer, oddly enough, is both yes and no depending on your frame of reference and scale you’re willing to look at.

Using classical physics and chemistry, we know that if we take the reactants of a chemical reaction and compare that to the products of a chemical reaction, we’ll have the same amount on both sides. This is what the Law of Conservation of Mass tells us is true. Generally, that’s what our scales tell us too.




However, all is not as it seems…

Simple thought experiment: Imagine a mass contained in a see-through container on a scale with infinite precision. This container contains the hydrocarbon of your choice. The temperature is the ignition temperature for that hydrocarbon.

Unfreeze time… now.

You witness the hydrocarbon ignite and undergo complete combustion. Electron bonds break apart. Energy shells lower. Photons are released as light from the fire. You interpret vibrating air particles as the crackling of the fire.

You look at your infinitely precise scale. What do you see?

There is a different value than when you started. There are several fewer femto-grams than when you started. That’s 0.000000000000001 grams.

This has been empirically proven. It is not just a thought experiment. For almost every reasonable situation, the Law of Conservation of Mass holds true. However, on the atomic level it tends to break down ever so slightly. The energy released will cause a change in mass. Thus, we must invoke the Law of Conservation of Mass and Energy. This will account for that infinitesimally small mass defect that we currently lack the technology to adequately measure.

One could argue that the difference in mass is due to atomic-level processes and, as a result, isn’t strictly due to chemical reactions. The term “chemical reaction” classically refers to reactions that involve only the electrons of an atom. Nuclear reactions cover the rest. However, that does not alter the fact that, given the best scale developed by human technology, you would see a small difference in weight if you burned a log in your hermetically sealed theoretical fireplace.

Alright, so how does this happen exactly? Check out our post on atomic binding energy for more!

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