Answer:
Gas
Step-by-step explanation:
The law of mass conservation states that the total mass of the reactants should be equal to the total mass of the products, that is, the mass should be conserved.
There are specific occasions when students might think that their findings do not follow the law. This is usually the case when some specific gas is evolved in a reaction.
The problem is that when gas evolves, if it's not soluble in a given solution, it escapes it into the surroundings and it seems at first that the mass of the products is actually lower than expected.
We would need to collect the gas evolved and calculate its mass in order to add it to the mass of the remaining products in order to verify the law in such cases.