Understanding the Law of Conservation of Mass

The title, “Understanding the Law of Conservation of Mass,” sounds very scientific and complex.  However, in reality, it’s really very simple.  The law of conservation of mass just says this, “Mass is neither created nor destroyed in any ordinary chemical reaction.”  Now the first part of this law is easy to understand because if something wasn’t there in the first place, how could it be there in the second place?  And actually the second part is fairly easy too because even if you destroy something, it’s clear that it doesn’t disappear because there are pieces of it all over the place.

The person who is credited with coming up with this law is a French chemist by the name of Antoine Lavoisier (although the early Roman poet, Lucretius, had already said, “things cannot be born from nothing”).  You would think that scientists would be able to come up with something more scientific than a poet.  However, to the scientists’ credit, they did make it sound more scientific by saying, “The mass of substances produced by a chemical reaction is always equal to the mass of the reacting substances.”

If you want to be scientific yourself, you can do an experiment to demonstrate this law with stuff you have in the kitchen.  Just fill a cup halfway with vinegar and another cup halfway with baking soda.  Place both cups carefully in a plastic bag, being careful not to spill the content of either cup.  Weigh the bag.  Seal the bag (the bag must be sealed completely), and then pour the vinegar into the baking soda without opening the bag.  A gas will form which will inflate the bag.  Then weigh the bag again.  When you are done, pat yourself on the back because now you are scientific, and you have proved the law of conservation of mass.

Before you get carried away, though, let me say that this law only works in a closed system (in other words, a system where no energy can get out).  That’s why I told you to seal the bag for your experiment.  Otherwise, the gas would have gotten out of the bag, and you would have thought I was making all of this up as it would have seemed that matter had disappeared.  In nature, of course, things aren’t sealed in a bag, and so it is easy to get the impression that matter is being destroyed when, in essence, it’s just escaping as energy.

By the way, just in case this law doesn’t sound scientific enough to you, it can be used to explain what happens on an atomic level during a reaction, namely that every atom that was there at the start of the reaction is there at the end of the reaction.  Just slip that into your next conversation, and everyone will be impressed.

Anyway, after this law was discovered, other conservation laws followed, i.e. the law of conservation of electrical charge, the law of conservation of parity.  But Einstein (who as you know was very scientific), came along and said, “let’s just put this all together,” which made perfect sense as in actuality matter and energy can be transformed into one another.  Thus, the law of conservation of mass-energy was born.  Now if that’s not scientific, what is?