Question

If you could measure the mass of the substances inside the cup before and after the reaction, you would see that the mass decreased over a course of the reaction. Explain the difference.

Answer 1

Gases may have formed during the reaction, released into the air, thus lowering the mass after the reaction of the remaining products.

Answer 2

The difference is the mass of the gases that were formed during the reaction. If the cup is open, it is always the case that the gas would escape. Any reaction that forms a gas as a product is called a gas forming reaction.