What was Antoine-Laurent Lavoisier's most important contribution to chemistry in 1789?

Correct — A. In 1789 Lavoisier published his Traité élémentaire de chimie, the book that set out the law of conservation of mass: in a chemical reaction matter is neither created nor destroyed, so the mass of the products equals the mass of the reactants.

He got there by weighing reactants and products in sealed vessels, where no gas could escape unrecorded. Closing the system and putting it on a balance is what turned chemistry into a quantitative science.

Why the others are wrong

• (b)
  Law of Multiple Proportions — The law of multiple proportions is John Dalton's, from the early 1800s — when two elements form more than one compound, the masses of one that combine with a fixed mass of the other stand in small whole-number ratios.
• (c)
  Law of Definite Proportions — The law of definite proportions, or constant composition, is Joseph Proust's — a given compound always contains the same elements in the same fixed proportion by mass, whatever its source.
• (d)
  Law of Conservation of Energy — Conservation of energy is physics and comes half a century later, established in the 1840s by Mayer, Joule and Helmholtz as the first law of thermodynamics.

Before Lavoisier, chemists explained burning with phlogiston, a weightless substance a burning body was supposed to give off.

The balance killed that idea: Lavoisier showed combustion is a substance combining with oxygen, and that the burnt product is heavier by exactly the mass of oxygen taken up.

Conservation of mass follows directly. It is why a chemical equation has to be balanced — the same atoms stand on both sides and only their arrangement changes.

Key facts

• Lavoisier published Traité élémentaire de chimie in 1789, treated as the first modern chemistry textbook.
• Law of conservation of mass: in a chemical reaction the total mass of the products equals the total mass of the reactants.
• Lavoisier's oxygen theory of combustion replaced the phlogiston theory.
• The law of definite proportions is Proust's and the law of multiple proportions is Dalton's.

Common traps

• Reading conservation of energy as the chemistry law because both begin with the same word
• Crediting Lavoisier with the atomic theory, which is Dalton's
• Thinking mass conservation fails when a gas escapes — it only looks that way in an open vessel

No directly related past PYQ was found.