Law of conservation of mass

Practice Question Solving
1

Hello! I’m having a tough time grasping the concept of this and here’s the specific question I’m doing it says

In a chemical reaction:
A- there is always emission of energy in the form of heat
B- the total mass of the products is not necessarily equal to the total mass of the reactants
C- the total number of product molecules must be equal to the total number of reactant molecules
D- the total mass of the product is always equal to the total mass of the reactants
E - you can have transformation of one element into another

The answer is D. But, I can’t quite understand it because I fail to wrap my head around the mass of the reactants ALWAYS being equal to the mass of the products. I personally picked B. I put it into chatGPT and it said this was the law of conservation of mass in a CLOSED SYSTEM which does make sense but I feel like the question is a bit too vague? please give me your input and thanks for reading

2

Hi.
You have a certain number of atoms in your reactants irrespective of how these atoms are combined to each other to make different molecules. The same number of atoms are valid for your products with the same argument. This is why we balance the chemical reactions i.e. based on the law of mass conservation. (Certain number of atoms have certain amount of mass)

The system even follows the law of energy conservation - not part of this question - if we consider our system as the univers or in a colorimeter/closed system.

If you instead have a nuclear reaction, this would be another story, and you cannot use the same law. It will then follow the quantum theory and laws.