Why is mass conserved in chemical reactions?

Category: Chemistry
Published: October 21, 2013
Updated: December 5, 2023

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Mass is not conserved in chemical reactions. The fundamental conservation law is the conservation of mass+energy. This means that the total mass plus energy before a reaction equals the total mass plus energy after the reaction. According to Einstein's famous equation, E = mc², mass can be transformed into energy and energy can be transformed into mass. This is not some exotic process, but in fact happens every time there is a reaction or a physical process that releases or absorbs energy. Mass is therefore never conserved in a chemical reaction because a little bit of it turns into energy (or a little bit of energy turns into mass). But mass+energy is always locally conserved. Energy cannot be created out of nothing. It can only be created by destroying the appropriate amount of mass according to E = mc². Between mass and energy, energy is the more fundamental property. In fact, modern physicists just consider mass as one particular type of energy which they call rest energy. For this reason, they don't usually call it the "Law of Conservation of Mass+Energy" but rather simply call it the "Law of Conservation of Energy" with the implication that this statement includes the mass type of energy. With this in mind, the equation E = mc² is really saying that the mass form of energy (i.e. rest energy) is indeed a type energy that can be converted to other forms of energy, and that other forms of energy can be converted to the mass form of energy.

Every chemical reaction involves conversion between energy and mass. Public Domain Image, source: Christopher S. Baird.

With that said, chemists have a good reason for pretending that mass is conserved in chemical reactions. For chemical reactions, only one billionth to one trillionth of the mass is converted to energy and vice versa. This means that your equipment would have to be able to accurately measure mass out to about ten significant figures in order to detect the change in mass of a system undergoing a chemical reaction. Most equipment can't do this. Therefore, for all practical purposes, we can pretend that mass is exactly conserved in chemical reactions. In other words, mass conservation is an extremely accurate approximation for chemical reactions.

The principle that mass is never truly conserved in chemical reactions is basic, established, mainstream physics; and has been for over a hundred years. However, from my experience, most people outside of physics either do not know this principle or refuse to believe it. Amazingly, over the years, I have received several ardent messages from highly-educated chemists trying to convince me that I am wrong and that mass really is conserved in chemical reactions. The approximation that mass is conserved in chemical reactions is so useful in practice and so ingrained in chemistry that even some of the most advanced chemists believe that it is exactly true at the fundamental level. However, it is common knowledge in physics that there is no fundamental law of conservation of mass, even in chemical reactions. For instance, a standard, mainstream, physics textbook used in lower-level undergraduate courses (Modern Physics by Felder and Felder) states:

It's a common mistake to think that although nuclear processes... involve changes in mass, chemical processes like burning something or dissolving salt in water don't. In fact both types of processes change a system's energy and thus its mass. The difference is that nuclear processes are governed by the strong nuclear force while chemical processes are governed by the electromagnetic force. The strong force, as you might guess, is stronger. So when oxygen and hydrogen combine to make water, the changes in their potential and kinetic energy change their total mass by less than one part in a billion. In nuclear reactions, however, these changes in mass can be on the order of 1%.

In nuclear reactions (which are changes to the nucleus of atoms), there is enough energy released or absorbed that the change in mass is significant and must be accounted for. In comparison, chemical reactions (which are changes to only the electrons in atoms) release or absorb very little energy so that the change in mass of the system is so small that it can be almost always ignored. As a perfectly reasonable and useful approximation, therefore, chemists can speak of the conservation of mass and use it to balance equations. But strictly speaking, the change in mass of the system during a chemical reaction, though small, is never zero. If the change in mass were exactly zero, there would be no where for the energy to come from. Chemistry textbooks like to speak of "chemical potential energy", "bond energy", and "internal energy" and talk as if the energy released in a reaction comes from the potential energy or from energy stored in the bonds. However, fundamentally, all internal forms of energy exist in the form of mass. There is not some magical bucket of potential energy in an atom or in chemical bonds from which a reaction can draw. There is just mass.

There are four general types of basic reactions:

1. The breaking of bonds, which always absorbs energy from the external environment and increases the system's mass.
2. The formation of bonds, which always releases energy to the external environment and decreases the system's mass.
3. The transformation of existing bonds which is really the excitation of the system to different states (which always absorbs energy and increases the system's mass) and de-excitation of the system to different states (which always releases energy and decreases the system's mass).
4. The creation of particles with mass (which always absorbs energy and increases the system's mass) and the annihilation of particles with mass (which always releases energy and decreases the system's mass).

Note that the phrase "external environment" in the statements above is used in a broad sense and refers to everything else besides the bond that is being changed or the particles that are being created and annihilated. In this sense, some other part of the atom or molecule could be the "external environment", i.e. the agent that is supplying energy to break or excite a bond or absorbing energy to create or de-excite a bond.

To be clear, when a chemical reaction absorbs energy and therefore gains mass, no new atoms are created. The total number of atoms before the reaction is the same as the total number of atoms after the reaction. Furthermore, the total number of atoms of each element does not change in a chemical reaction. In other words, the total number of oxygen atoms before a chemical reaction is the same as the total number of oxygen atoms after the reaction, and the same goes for every other element. If the total number of atoms of a particular element in a closed system changes, you have a nuclear reaction and not a chemical reaction, by definition. Although no new atoms are created and no existing atoms are destroyed in chemical reactions, the mass of the system still does indeed change. How is this possible?

The extra mass is held in the system as a whole. I know this is hard to visualize, but in addition to the particles of which a system is made, the states of these particles also contribute to the system's mass. Specifically, the more of an excited state that a particle is in, the more its state contributes to the system's mass. This concept is similar to the classical physics concept of potential energy, but we now know that the potential energy, and every other form of internal enegy, is fundamentally stored as mass. This is not just clever wording, baseless theoretical musings, or cunning book-keeping. Mass is a physically-measurable property. The changes in mass arising from reactions have been verified experimentally. Experimental evidence forces us to conclude that all internal forms of energy contribute to a system's mass, no matter how strange this sounds. The table below shows you a wide range of physical processes that convert mass to energy.

In everyday life, antimatter reactions mostly arise from radioactive decay. In fact, there is an extremely small number of antimatter reactions happening in your body right now, which are mostly arising from naturally radioactive potassium atoms. Hydrogen nuclear fusion is what releases energy in typical stars, in hydrogen nuclear bombs, and in fusion reactors. Uranium nuclear fission is what releases energy in uranium nuclear bombs and in uranium nuclear reactors. Methane combustion is what happens when you burn natural gas, such as in a natural gas power plant, in a stove, or in a household water heater. As you can see from this table, chemical reactions involve extremely small amounts of mass being created or destroyed, and therefore conservation of mass in chemical reactions is an excellent approximation.

Topics: chemical reaction, conservation of energy, conservation of mass, energy, mass, reaction