Law of Conservation of Mass

Mass is neither created nor destroyed in chemical reactions.

Short definition

Mass is neither created nor destroyed in chemical reactions.

Full explanation

Law of Conservation of Mass in the context of chemical reactions equations: Mass is neither created nor destroyed in chemical reactions. In practice this means relating macroscopic observation (mass, pressure, color), particle-level picture (atoms, electrons, ions moving and rearranging), and symbolic representation (formulas, equations, units). For Law of Conservation of Mass, students connect the definition to linked lessons: do not just memorize the phrase — predict what changes when conditions change and check with dimensional analysis.

Notation and units

• Notation: Σm_react=Σm_prod
• Common units: SI or derived, temperature in kelvins when in gas/energy contexts.
• Symbolic hint: See related formula if applicable.

Example

Example for law of conservation of mass: In a worked problem where this term appears, substitute measured values with correct units and check that the dimensional result matches the intended quantity. For instance, calculating a law of conservation of mass often uses the form $n=m/M$ or $c=n/V$ or $PV=nRT$ with $T$ in kelvins — illustrating how law of conservation of mass leads to a numerically different but dimensionally correct result when units are handled.

Confused with

• Law of Definite Proportions — distinction: Law of Conservation of Mass is mass is neither created nor destroyed in chemical reactions. while Law of Definite Proportions has a different defining property. See cross-link in glossary.

Related resources

• Topic: chemical-reactions-equations
• Formula library — when a specific equation involves this term it is linked.
• Constants — if a constant is required, it is sourced and versioned (NIST 2022).

Sources

• NIST Physical Constants 2022 (nist-2022, CODATA 2022). Accessed 2026-08-21.

Last reviewed: 2026-08-21 — Review due: 2027-08-21 — Author: Chemistry Fundamentals Editorial Team — Reviewer: Editorial Review Board (transparent placeholder).

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