Final answer:
The property of matter conserved in chemical reactions is mass, following the law of conservation of mass, which states that the mass of reactants equals the mass of products.
Step-by-step explanation:
The property of matter that is conserved in chemical reactions is mass. According to the law of conservation of mass, no matter is lost or created during a chemical reaction. This means that the mass of the reactants equals the mass of the products. For example, when magnesium metal reacts with oxygen to form magnesium oxide, if the reaction is carried out in a closed system, the mass of the reactants (magnesium and oxygen) will equal the mass of the product (magnesium oxide). This principle applies universally to all chemical reactions, reflecting the fact that atoms are neither destroyed nor created during chemical transformations.