Mass is the amount of matter in an object, defined by the total collection of protons, neutrons, and electrons that make up its physical substance. When two or more substances are combined, a fundamental question arises: Does the total quantity of matter change, or does it remain exactly the same? This inquiry into the behavior of mass during interaction is answered by a foundational principle of physical science.
The Fundamental Rule of Mass Conservation
The scientific principle governing all non-nuclear interactions is the Law of Conservation of Mass (LCoM), which states that mass is neither created nor destroyed. In any physical or chemical change, the total mass of all substances involved must remain constant. This law was established by Antoine Lavoisier in the late 18th century, confirming that matter simply rearranges itself instead of disappearing.
The constancy of mass is rooted in the atomic structure of matter. Atoms are the basic building blocks, and in typical processes, the number and type of atoms before a change are exactly the same as the number and type after the change. Whether substances are physically mixed or chemically transformed, the fundamental particles are merely reorganized. The LCoM applies universally to all changes that do not involve nuclear fusion or fission, where mass is converted into energy.
Mass When Physically Combined or Dissolved
When objects are combined through physical mixing, the total mass is simply the sum of the individual masses. For example, if you mix sand and water, the mass of the resulting mixture will precisely equal the mass of the initial sand plus the mass of the initial water. This is the most straightforward application of the LCoM, as the individual components retain their identity and only their spatial arrangement changes.
A more complex form of physical combination is dissolution, such as dissolving table salt or sugar in water. Even though the solid disappears into the liquid, the total mass remains perfectly additive because the solute particles are still present. The sugar molecules, for instance, detach from the crystal structure and disperse among the water molecules, but no sugar molecules are created or destroyed. Therefore, if 200 grams of water has 5 grams of salt dissolved into it, the resulting saltwater solution will have a total mass of 205 grams.
Mass During Chemical Transformations
Mixing substances can also lead to a chemical transformation, where the starting materials react to form entirely new substances with different properties. A common example is the reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid), which produces new compounds including carbon dioxide gas. Despite the dramatic change, which often includes fizzing and bubbling, the LCoM still holds true: the total mass of the reactants equals the total mass of the products. The original atoms rearrange their chemical bonds to create the new molecules, but the count of each type of atom remains the same throughout the process.
To accurately verify this conservation of mass during a reaction, the experiment must be conducted in a closed system. A closed system is one where no matter, such as a gas, can enter or leave the reaction vessel. If the baking soda and vinegar reaction is performed in a sealed container, the mass measurement taken before the reaction will be identical to the mass measurement taken after the reaction is complete.
A reaction performed in an open system, such as a beaker, will appear to lose or gain mass. This perceived change is only a result of unmeasured components entering or leaving the system boundary, and does not violate the LCoM.
Explaining Apparent Changes in Mass
Observing a chemical reaction in an open container often creates the impression that mass has been lost or gained. The simplest reason for a perceived mass loss is the escape of a gaseous product, as seen when an effervescent tablet is dropped into water in an open glass. The gas bubbles out and diffuses away, meaning the scale only measures the remaining liquid and solid components, which is less than the starting mass. The matter has not disappeared, but merely changed form and left the immediate measured area.
Another scenario where mass appears to change is during combustion, like burning wood, which seemingly results in only a small pile of ash. While the mass of the ash is much less than the original wood, the total mass is conserved. This is because the wood has reacted with oxygen from the air to produce gases like carbon dioxide and water vapor. If one could measure the mass of the wood, the oxygen consumed, and the resulting ash and gases, the initial and final masses would be equal.
Conversely, some reactions appear to gain mass, such as the formation of rust on a piece of metal. When iron rusts, it combines with oxygen atoms from the atmosphere to form iron oxide, which is a heavier compound. The final rusted object weighs more than the original iron because the mass of the absorbed oxygen is now included in the measurement. These observations reinforce that any change in mass is accounted for by matter entering or leaving the system.