The Law of Conservation of Mass

When two chemical solutions are combined within a sealed container, a specific physical law governs the relationship between the starting materials and the resulting substances.
The total mass of the reactants (the substances present at the start of the reaction) is precisely equal to the total mass of the products (the substances formed by the reaction).
Under these conditions, a strict balance is maintained where no mass is gained from external sources and no mass is lost to the environment.
This observation leads to the fundamental conclusion that mass is conserved throughout the duration of the chemical reaction.

A common question arises regarding whether this law applies to all reactions, especially those that appear to lose matter, such as a campfire.
In the case of a campfire, wood burns down to a pile of fluffy ashes which clearly possesses significantly less mass than the original wood. Historically, this led to confusion among observers.
Scientists eventually determined that earlier observations failed to account for reactions involving gases, which also possess mass even if they are not easily visible.
When an experiment is specifically designed to trap all reactants and all products—explicitly including any gases created during the chemical change—the results always demonstrate that mass is conserved.
The perceived loss of mass in an open system like a campfire is simply due to the release of gaseous products into the surrounding atmosphere, rather than the destruction of matter itself.

The formal definition of the law states: In any given chemical reaction, the total mass of the reactants equals the total mass of the products.
Mathematically, this can be expressed as: $\text{Total Mass}_{\text{Reactants}} = \text{Total Mass}_{\text{Products}}$
The underlying reason for this constancy is found at the atomic level. During a reaction, the atoms contained within the reactants are not destroyed or newly created; instead, they are simply rearranged to form the products.
Because the atoms are merely reshuffled, the symbol $\therefore$ (therefore) indicates that all atoms originally present in the reactants must still be present in the products at the conclusion of the reaction.

The conceptual understanding of mass conservation serves as the foundation for broader chemical principles.
The next phase of study involves examining how the Law of Conservation of Mass relates specifically to the construction and balancing of chemical equations.