two substances react and the mass is recorded. The mass of the products does not equal the mass of the reactants .Which is the best explaination for the different masses?

Mass can’t be destroyed. So since not all the mass was found in the products, that simply means that it was lost to the environment. For example, let’s say hydrogen and oxygen are the reactants and we get water in the product. Now that oxygen could have reacted with something else that we didn’t take into account when measuring the mass of the H2O released.

Tldr, some of the reactants were lost to the surroundings

The answer is !