Here's why:

* Law of Conservation of Mass: This fundamental law in chemistry states that matter cannot be created or destroyed in a chemical reaction. It can only be transformed from one form to another.

* Chemical Reactions: When chemicals react, they rearrange their atoms to form new substances. The total mass of the reactants (the starting chemicals) must equal the total mass of the products (the substances formed).

In your example:

* You start with 5g of one chemical and 10g of another, for a total of 15g.

* Even if a chemical reaction occurs, the total mass of the product(s) must still be 15g.

It's important to note:

* While the mass remains constant, the volume and density of the product might be different from the reactants.

* The mass of a product might seem to increase or decrease slightly due to factors like gas release or absorption, but these are typically small changes that don't violate the law of conservation of mass.