The question of why matter’s total quantity does not change during a transformation is fundamental to chemistry. A chemical reaction is a process where substances change their identity by breaking old connections and forming new ones. When a log burns or a battery generates power, the initial materials appear to vanish or change completely, but careful observation shows that nothing is truly lost. This consistency, where the amount of material remains the same, provides predictability to the study of material change.
The Principle: What the Law of Conservation of Mass States
The consistency of mass during a transformation is formally described by the Law of Conservation of Mass. This principle states that in any system closed off from its surroundings, mass is neither created nor destroyed during a chemical change. The total mass of all the starting materials, called reactants, must equal the total mass of all the final substances, known as products.
The condition of a closed system is important for accurate observation. If wood is burned in an open fireplace, the ash weighs less than the original wood, suggesting mass was lost. However, if that same wood were burned inside a sealed container, the total mass of the container and its contents would not change. The gases produced, which would have escaped in the open environment, are captured and accounted for, revealing the conservation of mass. This law was formalized in the late 18th century by the French chemist Antoine Lavoisier.
The Atomic-Level Reason: Atoms are Simply Reorganized
The reason mass is conserved lies in the nature of atoms, the smallest units of an element. In a chemical reaction, atoms are not destroyed or spontaneously generated. Instead, the reaction involves the rearrangement of these atoms as chemical bonds are broken and reformed into new combinations. This is similar to disassembling a structure made of LEGO bricks and using the exact same bricks to build a different structure.
Every atom of carbon, oxygen, or hydrogen that exists in the starting materials must also be present in the final products. Since the mass of any substance is the sum of the masses of the individual atoms it contains, and the atoms themselves are conserved, the total mass before and after the reaction must be equal.
Chemical reactions only involve the electrons that form the bonds between atoms, leaving the atom’s core, the nucleus, untouched. The mass of an atom is almost entirely concentrated in its nucleus, which is why the total mass remains unchanged during these electron-level rearrangements. While modern physics acknowledges that a minute amount of mass is converted to energy when chemical bonds form or break, this change is too small to be detected by standard chemical measurements. For practical purposes in chemistry, mass is conserved because the constituent atoms are merely shuffled into new molecular arrangements.
Practical Verification: Mass Conservation in Chemical Equations
Chemists use chemical equations as a direct, mathematical verification of the conservation of mass. An equation uses symbols to represent the reactants, the arrow indicating the transformation, and the products that are formed. For instance, hydrogen and oxygen gases combine to yield water vapor.
To satisfy the Law of Conservation of Mass, an equation must be balanced. Balancing ensures that the atom count for every element is identical on both sides of the reaction arrow. This is accomplished by placing numerical coefficients in front of the chemical formulas, which ensures the same number of atoms of each type enters the reaction as leaves it.
In the reaction forming water, two molecules of hydrogen gas combine with one molecule of oxygen gas to produce two molecules of water. Counting the atoms shows four hydrogen atoms and two oxygen atoms among the reactants, and four hydrogen atoms and two oxygen atoms among the products. This matching count provides quantitative proof that no atoms, and therefore no mass, have been gained or lost during the chemical process.