The statement “Mass is conserved during a chemical reaction” means that in any closed system, the total quantity of matter remains unchanged before and after the chemical transformation. This fundamental concept, known as the Law of Conservation of Mass, dictates that mass can neither be created nor destroyed. Therefore, the mass of the starting substances (reactants) must exactly equal the mass of the final substances (products). This principle is a foundational rule of chemistry, allowing scientists to predict and quantify the outcomes of chemical processes.
The Principle of Atomic Rearrangement
The conservation of mass occurs because a chemical reaction involves only the rearrangement of atoms, the fundamental building blocks of matter. Atoms maintain their identity and mass throughout the process. The reaction simply breaks the chemical bonds holding the atoms together in the reactant molecules and forms new bonds to create the product molecules.
Every atom present in the reactants is accounted for in the products, just connected in a different structural pattern. For instance, when hydrogen and oxygen react to form water, the atoms are simply reconfigured into water molecules. The total count of each type of atom—carbon, hydrogen, oxygen, etc.—is identical on both sides of the chemical equation.
This microscopic understanding is the reason the macroscopic mass remains constant. The French chemist Antoine Lavoisier is credited with formalizing this law in the late 18th century, moving chemistry from alchemy to a modern science based on quantitative measurement. His work demonstrated that even in reactions like combustion, where matter seems to vanish, the total mass is conserved if all components, including gases, are captured and measured.
Experimental Verification Through Measurement
The Law of Conservation of Mass is verified by carefully measuring the total mass of the system before and after a reaction in a closed container. Researchers must isolate the reacting substances so that no matter can escape or enter the system, even if a gas is produced. If the reaction vessel is sealed, the mass reading will be exactly the same before the reactants are mixed as it is after the products have formed.
For example, a common verification experiment involves mixing solutions like barium chloride and sodium sulfate, which form a white solid precipitate. By weighing the sealed container with the separated reactants and then re-weighing it after the solutions are combined, the total mass is shown to be constant. This equality between the total mass of the reactants and the total mass of the products is the direct macroscopic evidence of the conservation principle.
The mathematical tool chemists use to uphold this principle is balancing chemical equations. Balancing ensures that the number of atoms of each element is identical on both sides of the equation, which is a direct representation of the conservation of mass at the atomic level. This systematic approach allows chemists to predict the precise amount of product that will be formed from a given amount of reactant.
Distinguishing Chemical and Nuclear Reactions
The concept of mass conservation holds true for chemical reactions because they only involve changes in the electrons that form bonds between atoms, leaving the dense atomic nucleus untouched. This is in sharp contrast to nuclear reactions, such as fission or fusion, which involve changes within the nucleus itself. Nuclear processes release or absorb enormous amounts of energy, which creates a detectable change in mass according to Albert Einstein’s equation, \(E=mc^2\).
The equation \(E=mc^2\) reveals that mass and energy are interchangeable; a small amount of mass can be converted into a large amount of energy, and vice versa. In a chemical reaction, the energy released or absorbed is so small that the resulting change in mass is less than one part in a billion, making it undetectable by conventional laboratory balances. Therefore, for practical purposes in chemistry, mass is considered conserved.
In nuclear reactions, however, the change in mass can be on the order of one percent, a significant and easily measured difference known as the mass defect. This means that the classic Law of Conservation of Mass must be modified to the Law of Conservation of Mass-Energy for nuclear processes. The total mass-energy of the system is always conserved, but mass itself is not strictly conserved when the nucleus is altered.