Mass is neither created nor destroyed in a chemical reaction, so total mass of reactants equals that of products.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
In a closed system, the total mass of the reactants equals the total mass of the products in any chemical reaction. Lavoisier established this principle through careful quantitative experiments in the late eighteenth century. The law holds because atoms are rearranged during a reaction and are neither created nor destroyed. It is the reason chemical equations must be balanced, with the same number of each kind of atom on both sides. Apparent mass losses occur only when a gas escapes from an open vessel, and the mass is still conserved in the whole system.
Key ideas: Total mass of reactants equals total mass of products in a closed system (Lavoisier, 1789). To use it, add up reactant masses and subtract the known products to find an unknown. Example: 12 g of carbon plus 32 g of oxygen gives 44 g of CO2. It works because atoms are rearranged, not created or destroyed.
Common mistakes and connections: A classic mistake is ignoring gases, so burning a candle or rusting iron seems to lose or gain mass in an open system. Strictly, mass-energy is conserved and nuclear reactions convert small mass to energy, which chemical reactions do not measurably do. It is the basis for balancing equations and stoichiometry.
Burning 4.03 g of hydrogen with 32.00 g of oxygen gives exactly 36.03 g of water, so total mass is the same before and after the reaction. The interactive lesson shows this as a live animation or chart.
Burning 12 g of carbon with 32 g of oxygen in a sealed vessel gives exactly 44 g of carbon dioxide.
Mass is neither created nor destroyed in a chemical reaction, so total mass of reactants equals that of products.
Law of Conservation of Mass is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Pure Substance, Physical Change, Chemical Change.
The next topics in Atomic Structure & Periodicity are Law of Definite Proportions and Law of Multiple Proportions.
Yes. The interactive lesson includes a self-check question on Law of Conservation of Mass, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Law of Conservation of Mass is topic 9 of 115 in Atomic Structure & Periodicity.