lesson
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If you burn a 10-gram piece of wood in an open campfire, you are left with less than a single gram of ash. Did nine grams of matter simply vanish from the universe?
In 1789, French chemist Antoine Lavoisier trapped chemical reactions inside sealed glass flasks, proving that when escaping gases are captured, the mass never changes.
The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction—the total mass of your reactants (starting substances) must equal the total mass of your products (substances produced).
📊A modern two-pan balance scale comparing a sealed reaction flask before and after a reaction. On the left pan: a sealed flask labeled 'Reactants (Before)' containing red and blue sphere molecules, showing digital scale reading '150.0 g'. On the right pan: the same sealed flask labeled 'Products (After)' containing bonded purple composite molecules, showing digital scale reading '150.0 g'. The balance beam is perfectly level. Include glowing badge: 'Mass of Reactants = Mass of Products'. Clean white card layout, dark navy text #1e2945, subtle borders #e6e6e6, responsive to 350px width.
How do chemists represent this atomic conservation on paper with clear, universally recognized symbols?
Anatomy of a Chemical Equation
A chemical equation uses chemical formulas and symbols to show how atoms reorganize during a reaction.
Every formula includes state symbols in parentheses: (s) for solid, (l) for pure liquid, (g) for gas, and (aq) for aqueous, meaning dissolved in water.
📊An interactive-looking annotated breakdown of the equation: '2 H2(g) + O2(g) -> 2 H2O(l)'. Callout lines point to: '2' labeled 'Coefficient (Number of molecules/moles)', subscript '2' labeled 'Subscript (Atoms bonded per molecule)', '(g)' & '(l)' labeled 'State Symbols (Phase of matter)', and '->' labeled 'Yields Arrow (Produces)'. Color-coded highlights: coefficients in vibrant blue (#0284c7), subscripts in amber (#d97706), state symbols in green (#16a34a). Responsive container, text #1e2945.
A coefficient is a whole number multiplier placed in front of a formula, while a subscript is the small number fixed inside a formula showing how many atoms are bonded together.
What is the absolute golden rule you must never violate when balancing these equations?
Coefficients vs. Subscripts
You can change coefficients freely to balance atom counts, but you must never change subscripts.