lesson
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If you burn a 5-pound log of wood in a fire pit, you are left with just a small pile of ashes weighing less than a quarter-pound.
Where did the rest of the matter go — did it simply vanish from existence?
The Law of Conservation of Mass
In 1789, French chemist Antoine Lavoisier weighed sealed glass containers before and after chemical reactions and proved that mass is never created or destroyed.
The Law of Conservation of Mass states that the total mass of the reactants (starting substances) must equal the total mass of the products (final substances) because every single atom is accounted for.
📊A clean interactive scale diagram showing mass conservation. On the left scale pan is a sealed flask containing methane (CH4) and oxygen (O2) molecules with a total digital readout of 80.0 g. On the right scale pan is an identical flask after the reaction containing carbon dioxide (CO2) and water (H2O) molecules with an identical digital readout of 80.0 g. The scale beam is perfectly horizontal. Colorful atom key at the bottom: Carbon = black sphere, Hydrogen = white/light gray sphere, Oxygen = red sphere. A toggle switch allows students to break the seal, showing escaping gas molecules and the scale tipping.
To represent this atomic balance on paper, we write chemical equations — but how do we distinguish between the structure of a molecule and how many molecules we have?
Subscripts vs. Coefficients
A subscript is the small number written below an element symbol showing how many atoms of that element are bonded inside a single molecule, such as the 2 in H2O.
A coefficient is the whole number placed in front of a chemical formula that multiplies every atom in that entire molecule, such as the 2 in 2H2O (giving 4 hydrogen and 2 oxygen atoms).
📊A visual comparison diagram contrasting subscripts and coefficients. Section A shows H2O (1 water molecule) vs H2O2 (1 hydrogen peroxide molecule, a completely different toxic compound), emphasizing that CHANGING A SUBSCRIPT CHANGES THE CHEMICAL IDENTITY with a red warning badge. Section B shows H2O (1 water molecule: 2 H, 1 O) vs 2H2O (2 separate water molecules: 4 H, 2 O), showing that CHANGING A COEFFICIENT ONLY CHANGES THE QUANTITY with a green checkmark badge. Particle spheres are clearly labeled with element colors (H = blue, O = red).
Now that we know we can only adjust coefficients, what is the most reliable strategy to balance an equation without guessing blindly?