A chemical reaction is a fundamental process where reactants transform into new substances called products. These products possess different chemical and physical properties than the starting materials. Understanding what happens to the total mass during this transformation is a central concept in chemistry. The total mass remains unchanged, a principle governing all ordinary chemical changes.
The Core Scientific Rule Governing Mass
The total mass of the substances involved in a chemical reaction does not change. This principle, known as the Law of Conservation of Mass, is a foundational rule in science. It states that mass is neither created nor destroyed during a standard chemical process. The implication is that the total mass of the reactants must exactly equal the total mass of the products. For instance, if you combine 10 grams of Substance A with 5 grams of Substance B, the resulting products must weigh precisely 15 grams. This equality requires chemists to “balance” chemical equations, ensuring the mass on both sides is accounted for.
How Atoms Rearrange Without Changing Total Mass
The constancy of mass during a chemical change is explained by looking at the process at the atomic level. A chemical reaction does not destroy or create atoms; instead, the atoms are only rearranged into different combinations. Reactants are made of molecules held together by chemical bonds. During the reaction, these existing bonds break, and the atoms separate briefly. The freed atoms then form new connections, creating the molecules of the products. If you take apart a structure made of 10 blocks and use those exact same 10 blocks to build a new, different structure, the total weight remains the same. Every single atom present at the start of the reaction is still present at the end, just connected in a new way. This preservation of the number and type of atoms is the reason why mass is conserved.
Why Mass Sometimes Seems to Change
Despite the strict rule of mass conservation, observing a chemical reaction can sometimes suggest that mass has been lost or gained. This apparent change is typically a result of the reaction occurring in an “open system,” where substances can enter or leave the immediate area being measured. The Law of Conservation of Mass applies universally, but accurate measurement requires a “closed system” that contains everything. If a reaction produces a gas, that gas can easily float away into the air, causing the measured mass of the remaining substances to decrease. For example, when an antacid tablet fizzes in water, the carbon dioxide gas escapes, and the beaker appears to weigh less afterward. Conversely, mass can seem to increase when a reactant is a gas taken from the air, like oxygen. When iron rusts, it combines with oxygen from the atmosphere, and the resulting rust weighs more than the original iron alone because the mass of the incorporated oxygen was not initially measured.