A chemical reaction transforms starting substances (reactants) into new substances (products). Although molecular identities change, fundamental laws dictate that certain properties must remain constant throughout the process. This concept of conservation is a bedrock principle in science. It ensures that specific quantities are neither created nor destroyed, even when matter is rearranged. Understanding what remains unchanged provides the framework for predicting the outcome of any chemical event.
The Conservation of Mass
The most widely recognized conserved quantity in chemistry is mass. The total mass of all reactants entering a chemical reaction must precisely equal the total mass of all products exiting the reaction in a closed system. This principle is known as the Law of Conservation of Mass.
The conservation of mass occurs because a chemical reaction is fundamentally a rearrangement of atoms, not a nuclear process. Atoms themselves are conserved; they are simply broken apart from their initial molecular structures and reformed into new product molecules. For instance, when hydrogen and oxygen gases react to form water, the number of hydrogen and oxygen atoms remains exactly the same before and after the reaction, only their bonding partners change.
This atomic accounting is the practical reason why chemical equations must be balanced. Balancing ensures that the count of each element’s atoms is identical on both the reactant and product sides. Following this rule allows scientists to accurately predict the amounts of product that will be formed from a given quantity of starting material.
The Conservation of Energy
Beyond mass, energy is also strictly conserved during a chemical transformation. The Law of Conservation of Energy, or the First Law of Thermodynamics, dictates that energy cannot be created or destroyed, only converted from one form to another. In chemical reactions, stored chemical potential energy transforms into forms like heat, light, or electrical energy.
The energy balance is determined by the difference between the energy required to break the bonds in the reactants and the energy released when new bonds form in the products. If the energy released during bond formation is greater than the energy consumed for bond breaking, the excess energy is expelled to the surroundings, defining the reaction as exothermic. Conversely, if more energy is needed to break the initial bonds than is released by the new ones, the reaction must absorb energy from the surroundings, classifying it as endothermic.
In an exothermic process, the products possess less stored chemical energy than the reactants, releasing the difference as heat. For an endothermic reaction, the absorbed energy is stored within the new chemical bonds, meaning the products have a higher chemical potential energy.
The Conservation of Electric Charge
The total electric charge is another property that must be conserved in every chemical reaction. The net electrical charge of the reactants must equal the net electrical charge of the products. This conservation principle is particularly evident in reactions involving ions, which are atoms or molecules that carry a positive or negative charge.
Any loss of electrons by one reactant, which results in a positive charge increase, must be perfectly balanced by the gain of those same electrons by another reactant, resulting in a negative charge increase. This process is the foundation of oxidation-reduction, or redox, reactions, which involve the transfer of electrons.
For example, if a positive ion reacts, the total positive charge on the reactant side must be accounted for on the product side. This is achieved either by a resulting ion of the same charge or by a combination of charges that sum to the initial value.