Lavoisier: Conservation of Mass 1785

Proving the conservation of mass was a difficult problem in science.  In the late 18th century Antoine Lavoisier (1743-1794) made a series of controlled scientific experiments that measured the mass or weight of liquids that were converted into gases by heating.  His discovery was that the weight of objects containing both the liquid and the trapped gas that caught the vapors of the liquid were equal to each other.  In other words, no mass (measured in weight) was lost in the conversion of liquids to gas.

Here are 2 short videos that introduce Lavoisier, his experiments and his discoveries:

Source Youtube:  https://www.youtube.com/watch?v=x9iZq3ZxbO8

The prnciple of Conservation of Mass is described here at the BBC-GCSE Bitesize website:

Atoms are the smallest particle of an element. A chemical reaction involves atoms in the reactants being rearranged to form the products. No material is lost or gained. A precipitation reaction is a good way of showing this conservation of mass.Conservation of massAtoms are the smallest particles of an element that can take part in a chemical reaction. During any chemical reaction no particles are created or destroyed: the atoms are simply rearranged from the reactants to the products. The products may have different properties to the reactants.Mass is never lost or gained in chemical reactions. We say that mass is always conserved. In other words, the total mass of products at the end of the reaction is equal to the total mass of the reactants at the beginning.This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example:

Source:  GCSE Bitesize  BBC http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/materials_from_earth/conservation_of_massrev1.shtm

Key words:

Conservation:  حِفْظ 
Mass:  وَزْن