The law of the Conservation of Matter is also called a law of conservation of the mass or Law of Lomonosov-Lavoisier, in honor of its creators. It postulates that the amount of matter before and after a transformation is always the same. It is one of the fundamental laws in all the natural sciences. That is to say: matter is not created or destroyed, it is transformed.
The matter is the general term that applies to everything that occupies space and has the attributes of gravity and inertia. It was developed independently by Mikhail Lomonosov in 1745 and by Antoine Lavoisier in 1785. This law is fundamental to an adequate understanding of chemistry. It is behind the usual description of chemical reactions by the chemical equation. Also of the gravimetric methods of analytical chemistry.
One caveat that must be taken into account is the existence of nuclear reactions, in which the mass itself is modified in a subtle way. In these cases in the sum of masses must be taken into account the equivalence between mass and energy.
Lavoisier demonstrated experimentally that the total mass of a closed system is constant, although in the system originates any type of chemical processes. In the past it was believed that matter could be destroyed and used as an example the burning of a piece of coal that after burning was reduced to ashes, with a mass well below the initial coal. Subsequently, the use of the balance showed that if the gases expelled during combustion were recovered, the system had the same mass in both the initial stage and the final stage of the chemical reaction. Therefore, the law was able to conclude that it is unlikely to create matter from nothing and neither is the inverse process possible, that is, to destroy the matter. However, it is possible from certain materials, transform them into others and if the system is closed, its mass will remain constant.
Mass
Mass is the measure that indicates the amount of matter a body has. A body corresponds to a portion of matter that can be found in a solid, liquid or gaseous state, which can be formed by materials of the same or different nature.
Differences between mass and weight
Mass and weight are generally used as synonyms, however, they are very different magnitudes. Weight is the force with which the earth draws a body towards the center of it. The weight depends on the mass of the body (generally, the greater the mass the greater the weight) and the distance it is from the center of the earth. The closer the body is to the center of the earth, the greater its weight will be.
For more information consult the bibliography or links that I leave here:
http://www.nuclear-power.net/laws-of-conservation/law-of-conservation-of-matter/
https://chem.libretexts.org/LibreTexts/University_of_South_Carolina_-Upstate/USC_Upstate%3A_Chemistry_of_Life(Mueller)/05%3A_Introduction_to_Chemical_Reactions/5.1%3A_The_Law_of_Conservation_of_Matter
https://en.wikipedia.org/wiki/Antoine_Lavoisier