When you burn a log in a fireplace, at the end only ashes remain and everything seems to have vanished. This impression is misleading. Mass has not disappeared: it turned into gas (carbon dioxide, water vapor) that escaped into the air. If you could weigh everything — ashes + gas — you would recover exactly the starting mass.
Lavoisier's law
The French chemist Antoine Lavoisier, at the end of the 18th century, framed this fundamental principle:
"Nothing is lost, nothing is created, everything is transformed."
More precisely: in a chemical transformation, the total mass of the reactants (what reacts) equals the total mass of the products (what is formed). Mass is conserved.
How is mass conserved?
At the microscopic level, mass conservation comes from a simple fact: atoms are neither created nor destroyed in a chemical reaction. They are only rearranged. If you start with 4 hydrogen atoms and 2 oxygen atoms, you still have 4 hydrogen atoms and 2 oxygen atoms at the end — just arranged differently.
That is why a chemical equation must be balanced: count atoms of each element on the left and on the right, and make sure they are equal on both sides.
An example: burning methane
Natural gas is mainly methane (CH₄). When it burns:
CH₄ + 2 O₂ → CO₂ + 2 H₂O
Left: 1 carbon, 4 hydrogens, 4 oxygens. Right: 1 carbon, 4 hydrogens, 4 oxygens.
The equation is balanced: mass is conserved.
Why it matters
Mass conservation is the chemist's number one quantitative tool. It lets us predict how much product we get from a given amount of reactant — the basis of stoichiometry that you will meet in high school. Without this law, the chemical industry, pharmacy, and even cooking would be guesswork.