Topic 1 Law of Conservation of Mass

 Topic 1

Law of Conservation of Mass 

Under certain conditions, the reactants undergo a chemical reaction to generate new substances, such as the combustion of magnesium strips to generate magnesium oxide, and the electrolysis of water to generate hydrogen and oxygen. So what is the relationship between the mass of reactants and products? Chemists have been interested in this question since the very beginning.

1774 , Lavoisier used precise quantitative experiments to study the relationship between the quality of various substances in the decomposition and synthesis of mercury oxide. He heated and decomposed 45.0 parts of mercuric oxide by mass, and got exactly 41.5 parts by mass of mercury and 3.5 parts by mass of oxygen. Is this a coincidence?

A large number of experiments have proved that the sum of the masses of the substances participating in the chemical reaction is equal to the sum of the masses of the substances produced after the reaction. This law is called the law of conservation of mass .

Why is the sum of the masses of each substance equal before and after a chemical reaction occurs? This is because the process of a chemical reaction is the process in which the atoms of the substances (reactants) participating in the reaction recombine to form other substances (products). In a chemical reaction, the types of atoms do not change before and after the reaction, the number does not increase or decrease, and the mass of the atoms does not change.

When learning chemistry, it is often necessary to describe the reactions between various substances. How to express chemical reactions easily?

This kind of formula that expresses a chemical reaction with a chemical formula is called a chemical equation. This formula not only shows the reactants, products and reaction conditions, but at the same time, the relative molecular mass (or relative atomic mass) can also express the mass relationship between substances, that is, the mass ratio between substances.

That is to say: carbon and oxygen react to produce carbon dioxide under the condition of ignition; every 12 mass parts of carbon reacts completely with 32 mass parts of oxygen to produce 44 mass parts of carbon dioxide. clearly express  the relative quantitative relationship between the reactants, products and various particles in the reaction (that is, the relationship between the "quality" and "quantity" of the chemical reaction).

You should know after finishing this topic

1.The law of conservation of mass means that the sum of the masses of the substances participating in the chemical reaction is equal to the sum of the masses of the substances produced after the reaction.

2.The process of a chemical reaction is the process in which the atoms of the substances (reactants) participating in the reaction recombine to form other substances (products). In a chemical reaction, the types of atoms before and after the reaction do not change, the number does not increase or decrease, and the mass of the atoms does not change, so the sum of the masses of the substances before and after the chemical reaction must be equal.

3.Information provided by chemical equations includes:

Which substances participate in the reaction (reactants); by what conditions the reaction occurs;

What substances (products) are produced by the reaction; the relative quantity of each particle participating in the reaction; mass conservation before and after the reaction; and so on.