Reagents and products of a chemical reaction

A chemical reaction is always described using the same terminology, regardless of the phenomenon observed.

Basic terminology

• Reactants are the substances present at the start; these are the ones that will react and disappear (at least in part).
• Products are the new substances that appear at the end of the reaction.

The general formula for a chemical reaction is written as follows:

réactifs  ->  produits

The arrow is read as ‘give’ or ‘react to form’, never ‘equal’.

A fundamental principle: the conservation of matter

In the 18th century, the French chemist Antoine Lavoisier experimentally established, by carefully weighing reactants and products, one of the most important principles in all of chemistry:

‘Nothing is lost, nothing is created, everything is transformed. ”

In practical terms: the atoms present in the reactants never disappear and never multiply during a chemical reaction. They simply rearrange to form new substances. The total mass of the products is therefore always equal to the total mass of the reactants consumed.

Why this is sometimes counterintuitive

When a log burns, all that remains is a small pile of ash, much lighter than the original log: one might think that some mass has disappeared. In reality, a large proportion of the log’s matter has turned into gases (carbon dioxide, water vapour) which have escaped into the air. If you were to weigh all the products – ash and gas – you would find that the total mass is exactly equal to the mass of the log plus that of the oxygen consumed.